Geomorphology
- Interior Structure of the Earth
Like all terrestrial planets, the Earth’s interior is differentiated. This means that its internal structure consists of layers, arranged like the skin of an onion. Peel back one, and you find another, distinguished from the last by its chemical and geological properties, as well as vast differences in temperature and pressure.
- Layers of the Earth
- The Earth can be divided into one of two ways – mechanically or chemically. Mechanically – or rheologically, meaning the study of liquid states – it can be divided into the lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core. But chemically or by composition, which is the more popular of the two, it can be divided into the crust, the mantle (which can be subdivided into the upper and lower mantle), and the core – which can also be subdivided into the outer core, and inner core.
Compositional layers of the Earth:
Core, mantle, and crust are divisions based on composition. The crust makes up less than 1 percent of Earth by mass, consisting of oceanic crust and continental crust is often more felsic rock. The mantle is hot and represents about 68 percent of Earth’s mass. Finally, the core is mostly iron metal. The core makes up about 31% of the Earth.
Crust:
- It is the outermost solid part of the earth, normally about 8-40 kms thick.
- It is brittle in nature.
- Nearly 1% of the earth’s volume and 5% of earth’s mass are made of the crust.
- The thickness of the crust under the oceanic and continental areas are different. Oceanic crust is thinner (about 5kms) as compared to the continental crust (about 30kms).
- Major constituent elements of crust are Silica (Si) and Aluminium (Al) and thus, it is often termed as SIAL(Sometimes SIAL is used to refer Lithosphere, which is the region comprising the crust and uppermost solid mantle, also).
- The mean density of the materials in the crust is 3g/cm3.
- The discontinuity between the hydrosphere and crustis termed as the Conrad Discontinuity.
Mantle:
- The portion of the interior beyond the crust is called as the mantle.
- The discontinuity between the crust and mantleis called as the Mohorovich Discontinuity or Moho discontinuity.
- The mantle is about 2900kms in thickness.
- Nearly 84% of the earth’s volume and 67% of the earth’s mass is occupied by the mantle.
- The major constituent elements of the mantle are Silicon and Magnesium and hence it is also termed as SIMA.
- The density of the layer is higher than the crust and varies from 3.3 – 5.4g/cm3.
- The uppermost solid part of the mantle and the entire crust constitute the Lithosphere.
- The asthenosphere (in between 80-200km) is a highly viscous, mechanically weak and ductile, deforming region of the upper mantle which lies just below the lithosphere.
- The asthenosphere is the main source of magma and it is the layer over which the lithospheric plates/ continental plates move (plate tectonics).
- The discontinuity between the upper mantle and the lower mantleis known as Repetti Discontinuity.
- The portion of the mantle which is just below the lithosphere and asthenosphere, but above the core is called as Mesosphere.
Core:
- It is the innermost layer surrounding the earth’s centre.
- The core is separated from the mantle by Guttenberg’s Discontinuity.
- It is composed mainly of iron (Fe) and nickel (Ni) and hence it is also called as NIFE.
- The core constitutes nearly 15% of earth’s volume and 32.5% of earth’s mass.
- The core is the densest layer of the earth with its density ranges between 9.5-14.5g/cm3.
- The Core consists of two sub-layers: the inner core and the outer core.
- The inner core is in solid state and the outer core is in the liquid state (or semi-liquid).
- The discontinuity between the upper core and the lower core is called as Lehmann Discontinuity.
- Barysphere is sometimes used to refer the core of the earth or sometimes the whole interior.
Mechanical Layers of the Earth:
The structure of the Earth can also be defined and divided based on how the insides of the planet behavior. Thereby, the mechanical layers correspond to the physical or mechanical properties of these layers.
Below are brief overviews of the five mechanical layers of the Earth:
Lithosphere:
- The lithosphere is the outermost layer of the Earth that consists of the entire crust and the top-most portion of the mantle.
- The average thickness is ~70km, but ranges widely: It can be very thin, only a few km thick under oceanic crust or mid-ocean ridges, or very thick, 150+ km under continental crust, particularly mountain belts.
- Depth- 0-100 km
- Furthermore, they are divided into pieces called tectonic plates.
- The movements of these plates are responsible for mountain-building, oceanic trench formation, earthquakes, and volcanic eruption.
Asthenosphere:
- The asthenosphere includes the soft layer of the mantle on which the lithosphere moves.
- Depth- 100km to 350 km .
- It is made of solid silicate materials, but the high temperature allows it to flow on very long timescales.
- The lithosphere-asthenosphere boundary is where geophysicists mark the difference in ductility between the two layers.
Mesosphere:
- The mesosphere is the layer below the asthenosphere but above the outer core. It is essentially the lower mantle.
- Average depth-350-2900km
- Despite its high temperature, the intense pressure in this region restricts the movements of the molecules of the silicate material despite being under high temperature, thus making it extremely rigid.
Outer Core:
- The outer core extends from the bottom of the mesosphere or the lower mantle and surrounds the inner core.
- Composed of iron and nickel, the extreme temperature allows these metals to remain in their liquid phases.
- It is the only layer of the Earth that is a true liquid.
- Furthermore, its movement is responsible for generating the magnetic field.
Inner Core:
- The inner core is also made of iron and some nickel.
- However, unlike the outer core, it is a solid ball.
- The solidity is due to the intense pressure from the upper layers.
- Hence, although it is as hot as the surface of the Sun, there is speculation that the inner core is slowly growing as the liquid outer core at the boundary with the inner core cools and solidifies due to the gradual interior cooling.
Temperature, Pressure and Density of the Earth’s Interior
Temperature
- A rise in temperature with increase in depth is observed in mines and deep wells.
- These evidence along with molten lava erupted from the earth’s interior supports that the temperature increases towards the centre of the earth.
- The different observations show that the rate of increase of temperature is not uniform from the surface towards the earth’s centre. It is faster at some places and slower at other places.
- In the beginning, this rate of increase of temperature is at an average rate of 1C for every 32m increase in depth.
- While in the upper 100kms, the increase in temperature is at the rate of 12C per km and in the next 300kms, it is 20C per km. But going further deep, this rate reduces to mere 10C per km.
- Thus, it is assumed that the rate of increase of temperature beneath the surface is decreasingtowards the centre (do not confuse rate of increase of temperature with increase of temperature. Temperature is always increasing from the earth’s surface towards the centre).
- The temperature at the centre is estimated to lie somewhere between 3000C and 5000C, may be that much higher due to the chemical reactions under high-pressure conditions.
- Even in such a high temperature also, the materials at the centre of the earth are in solid state because of the heavy pressure of the overlying materials.
Pressure
- Just like the temperature, the pressure is also increasing from the surface towards the centreof the earth.
- It is due to the huge weight of the overlying materials like rocks.
- It is estimated that in the deeper portions, the pressure is tremendously high which will be nearly 3 to 4 million times more than the pressure of the atmosphere at sea level.
- At high temperature, the materials beneath will melt towards the centre part of the earth but due to heavy pressure, these molten materials acquire the properties of a solid and are probably in a plastic state.
Density
- Due to increase in pressure and presence of heavier materials like Nickel and Iron towards the centre, the density of earth’s layers also gets on increasing towards the centre.
- The average density of the layers gets on increasing from crust to core and it is nearly 14.5g/cm3 at the very centre.
Sources of information for the study of Earth’s Interior
Direct Sources of information about the Earth’s Interior
- Deep earth mining and drilling reveal the nature of rocks deep down the surface.
- But as mining and drilling are not practically possible beyond a certain depth, they don’t reveal much information about the earth’s interior.
- Mponeng gold mine(deepest mine in the world) and TauTona gold mine(second deepest mine in the world) in South Africa are deepest mines reaching to a depth of only 3.9 km.
- And the deepest drilling is only about 12 km deep hole bored by the Soviet Union in the 1970s over the Kola Peninsula.
- Volcanic eruption forms another source of obtaining direct information.
Indirect Sources of information about the Earth’s Interior:
- By analysing the rate of change of temperature and pressurefrom the surface towards the interior.
- Meteors, as they belong to the same type of materials earth is made of.
- Gravitation, which is greater near poles and less at the equator.
- Gravity anomaly, which is the change in gravity value according to the mass of material, gives us information about the materials in the earth’s interior.
- Magnetic sources. Magnetic surveys also provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.
- Seismic Waves: the shadow zones of body waves (Primary and secondary waves) give us information about the state of materials in the interior.
Relevance of Seismology in studying the interior of the Earth
Seismic waves are the waves of energy that travel through the Earth as a result of an earthquake and can tell a lot about the internal structure of the Earth because these waves travel at different speeds in different materials.
There are two types of waves that travel through the Earth:p-waves and s-waves.
- P- waves are faster and they can travel through both solids and liquids.
- S-waves are slower and cannot travel through liquids.For both kinds of waves, the speed at which the wave travels also depends on the properties of the material through which it is traveling.
Thus, if there is an earthquake somewhere, the first waves that arrive are P-waves. In essence, the gap in P-wave and S-wave arrival gives a first estimate of the distance to the earthquake.
Relevance:
- Scientists are able to learn about Earth’s internal structure by measuring the arrival of seismic waves at stations around the world. For example, they know that Earth’s outer core is liquid because s-waves are not able to pass through it.
- Seismic waves travel in curved paths through the Earth (because of the increasing pressure, materials are more dense towards the core, travel velocity of seismic waves increases).
- Refraction of seismic waves causes them to curve away from a direct path.
- Reflection causes them to glance off certain surfaces (e.g. core mantle boundary) when they hit it at too shallow of an angle.
- The result of this behavior, in combination with the fact that S-waves cannot travel through liquids is the appearance of seismic shadows, opposite of the actual earthquake site.
- When an earthquake occurs there is a “shadow zone” on the opposite side of the earth where no s-waves arrive.
- Similarly earth has a solid inner core because some p-waves are reflected off the boundary between the inner core and the outer core.
- By measuring the time it takes for seismic waves to travel along many different paths through the earth, they figure out the velocity structure of the earth. Abrupt changes in velocity with depth correspond to boundaries between different layers of the Earth composed of different materials.
- environment and the life within it.
- We can learn to minimize our risks from earthquakes, volcanoes, slope failures, and damaging storms.
- We can learn how and why Earth’s climate has changed in the past, and use that knowledge to understand both natural and human-caused climate change.
- We can recognize how our activities have altered the environment in many ways and the climate in increasingly serious ways, and how to avoid more severe changes in the future.
- We can use our knowledge of Earth to understand other planets in our solar system, as well as those around distant stars.
Rocks
Rocks
Rocks
Rocks
Rocks
- Landform Development
A landform is a feature on the Earth’s surface that is part of the terrain.
Mountains, hills, plateaus, and plains are the four major types of landforms. Minor landforms include buttes, canyons, valleys, and basins.
Tectonic plate movement under the Earth can create landforms by pushing up mountains and hills. Erosion by water and wind can wear down land and create landforms like valleys and canyons. Both processes happen over a long period of time, sometimes millions of years.
Classification of landforms:
In terms of origin, oceans and continents have certain differences. Generally, the materials which constitute the ocean bottom are harder and heavier than those which constitute the continents. Though the interior of the earth is still in a hot and molten state, it is still undergoing contraction.
The contractions could be slow or sudden. Whether slow or sudden, the contractions are continuously altering the form of the earth’s surface. Such changes on the earth’s surface are therefore caused by the actions of internal force.
Since the very beginning of the earth, its surface has been continuously subjected to change by the action of river, glaciers, winds, sea waves, earthquakes, etc. Such changes are, therefore, caused by the actions of external force.
Based on the order of relief development landforms can be classified into:
First order, second order and third order landforms
- Landforms of First Order: By the actions of internal forces anticlines and synclines were formed and in course of time these have been identified as continents and oceans. That is why, they are called as Landforms of First Order or Primary landforms.
The continental landforms consist of Americas, Eurasia, Africa, Australia and Antarctica. The total area is nearly 148 million sq km, i.e., 28 per cent of the earth’s surface and average height is 830 metres.
- Landforms of Second Order:
The plateaus, mountains, plains and extensive deserts of the continents are the example of the landforms of second order on the continents.
- Landform of Third Order:
Various features which are generally smaller parts of second order landforms or which form on the second order landforms are known as landforms of third order. There are innumerable such landforms over the continents and at the sea floor.
Peaks, cols, cirques, gorge, morains, alluvial fans, floodplains, ox-bow lakes, levees, deltas, ocean islands, volcanoes and ridges are some of the many features of third order landforms.
The landforms that are found on the surface of the Earth can also be grouped into 4 of the following categories:
(a) Structural Landforms – landforms that are created by the solidification of large quantities of magma or by massive movements due or rock because of plate tectonics. This includes landforms like: shield, fold mountains, rift valleys, and volcanoes.
(b) Weathering Landforms – landforms that are created by the physical, chemical or biological decomposition of rock through weathering. Weathering produces landforms where rocks and sediments are decomposed and disintegrated. This includes landforms with some of the following geomorphic features: karst, patterned ground, and soil profiles.
(c) Erosional Landforms – landforms formed from the removal of weathered and eroded surface materials by wind, water, glaciers, and gravity. This includes landforms with some of the following geomorphic features: river valleys, glacial valleys, and coastal cliffs.
(d) Depositional Landforms – landforms formed from the deposition of weathered and eroded surface materials. On occasion, these deposits can be compressed, altered by pressure, heat and chemical processes to become sedimentary rocks. This includes landforms with some of the following geomorphic features: beaches, deltas, flood plains, and glacial moraines.
Many landforms show the influence of several of the above processes. These landforms are called polygenetic. Processes acting on landforms can also change over time, and a single landscape can undergo several cycles of development. This type of landscape development is called polycyclic.
- First order relief (Theories)
First order relief (Theories)
Relief is simply the difference in elevation between higher point and lower point on the earth’s surface. The highest point of the earth is the peak of the Mount Everest and the lower point is Mariana trench in Pacific Ocean. The difference in elevation of the earth’s surface is due to endogenic and exogenic process operating in the earth’s crust. Relief is arranged in order according to time, process and the ways are formed (shaping or reshaping).
First order relief would be global scale contrasts between continents and ocean basins, between, say, Africa and the Indian Ocean or North America and the Pacific Basin.
First order relief features are tectonic plates and are the largest in special extent. There are two types of plates; continental plates and Oceanic plates. These are differentiated by their rock and mineral composition. Continental plates are lighter in density and are composed of granitic rock materials rich in silica and aluminum. The oceanic plates are made up of dense, basaltic rock composed of silica and magnesium.
The formation of First order reliefs can be explained by the following theories:
- Continental Drift Theory
- Sea Floor Spreading
- Plate Tectonics Theory
Continental Drift Theory was put forward by the German scientist Alfred Wegner in 1915.
According to the Continental Drift Theory, part of the crust are capable of horizontal movement round the globe causing the continents to slowly change their positions in relation to one another.
The fact that South America is a mirror image of Africa is presented as a proof of the continental drift theory (see video below for an animation showing the migration of both of these continents).
For hundreds of millions of years, all the land of Earth was joined together in one large mass or super continent. Scientists call it Pangaea (meaning “all lands” in Greek). Then about 200 million years ago the land began to drift apart. It broke into two pieces, and scientists have called the continent in the north Laurasia and the continent in the south Gondwanaland (named by Eduard Suess, an Austrian geologist).The two large continents continued to break apart into the smaller continents that exist today. Scientists call this movement ‘continental drift’.
Forces responsible for drifting of continents (According to Alfred Wegner)
According to Wegener, the drift was in two directions:
- Towards the equator due to the interaction of forces of gravity, pole-fleeing force (due to centrifugal force caused by earth’s rotation) and buoyancy (ship floats in water due to buoyant force offered by water)
- Westwards due to tidal currents because of the earth’s motion (earth rotates from west to east, so tidal currents act from east to west, according to Wegener).
- Wegener suggested that tidal force (gravitational pull of the moon and to a lesser extent, the sun) also played a major role.
- The polar-fleeing force relates to the rotation of the earth. Earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth (greater centrifugal force at the equator).
- Centrifugal force increases as we move from poles towards the equator. This increase in centrifugal force has led to pole fleeing, according to Wegener.
- Tidal force is due to the attraction of the moon and the sun that develops tides in oceanic waters (tides explained in detail in oceanography).
- According to Wegener, these forces would become effective when applied over many million years, and the drift is continuing.
The evidences in support of the continental drift theory:
Jigsaw Fit:
The similarity in outline of the coastlines of eastern South America and West Africa had been noted for some time. The best fit is obtained if the coastlines are matched at a depth of 1,000 meters below current sea level
Geological Fit:
When the geology of eastern South America and West Africa was mapped it revealed that ancient rock outcrops (cratons) over 2,000 million years old were continuous from one continent to the other.
Tectonic Fit:
- Fragments of an old fold mountain belt between 450 and 400 million years ago are found on widely separated continents today.
- Pieces of the Caledonian fold mountain belt are found in Greenland, Canada, Ireland, England, Scotland and Scandinavia. When these land masses are re-assembled the mountain, belt forms a continuous linear feature.
Glacial Deposits:
- Today, glacial deposits formed during the Permo-Carboniferous glaciation (about 300 million years ago) are found in Antarctica, Africa, South America, India and Australia.
- If the continents haven’t moved, then this would suggest an ice sheet extended from the South Pole to the equator at this time – which is unlikely as the UK at this time was also close to the equator and has extensive coal and limestone deposits.
- If the continents of the southern hemisphere are re-assembled near the South Pole, then the Permo-Carboniferous ice sheet assumes a much more reasonable size
Fossil Evidence:
- There are many examples of fossils found on separate continents and nowhere else, suggesting the continents were once joined. If Continental Drift had not occurred, the alternative explanations would be:
- The species evolved independently on separate continents – contradicting Darwin’s theory of evolution.
- They swam to the other continent/s in breeding pairs to establish a second population.
Criticism faced by Continental Drift Theory:
- Wegener failed to explain why the drift began only in Mesozoic era and not before.
- The theory doesn’t consider oceans.
- Proofs heavily depend on assumptions that are generalist.
- Forces like buoyancy, tidal currents and gravity are too weak to be able to move continents.
- Modern theories (Plate Tectonics) accept the existence of Pangaea and related landmasses but give a very different explanation to the causes of drift
A map of the ocean floor shows a variety of topographic features: flat plains, long mountain chains, and deep trenches. Mid-ocean ridges are part of chain of mountains some 84,000 km long. The Mid-Atlantic Ridge is the longest mountain chain on Earth. These ridges are spreading centers or divergent plate boundaries where the upwelling of magma from the mantle creates new ocean floor.
Deep-sea trenches are long, narrow basins which extend 8-11 km below sea level. Trenches develop adjacent to subduction zones, where oceanic lithosphere slides back into the mantle.
Hypothesis
Sea-floor spreading — In the early 1960s, Princeton geologist Harry Hess proposed the hypothesis of sea-floor spreading, in which basaltic magma from the mantle rises to create new ocean floor at mid-ocean ridges. On each side of the ridge, sea floor moves from the ridge towards the deep-sea trenches, where it is subducted and recycled back into the mantle.
A test of the hypothesis of sea-floor spreading was provided by studies of the Earth’s magnetism.
Evidences
Age of the sea floor:
- The age of the sea-floor also supports sea-floor spreading.
- If sea-floor spreading operates, the youngest oceanic crust should be found at the ridges and progressively older crust should be found in moving away from the ridges towards the continents. This is the case.
- The oldest known ocean floor is dated at about 200 million years, indicating that older ocean floor has been destroyed through subduction at deep-sea trenches.
Magnetic anomalies:
- Magnetic surveys over the ocean floor in the 1960s revealed symmetrical patterns of magnetic “bands,” (zebra stripes)anomalies parallel to midoceanic rifts .
- The same patterns in relation to midoceanic rifts are present in different oceans.
- The magnetic anomalies coincide with the episodes of magnetic reversals that have been documented from studies on land, indicating that the andesitic rocks that form new oceanic crust in the tensional setting of the rift valley record the earth’s magnetic field as they cool.
- A rock has a normal (positive) polarity when its paleomagnetic field is the same as the earth’s field today.
- The positive magnetism adds to the earth’s magnetic field and creates a higher magnetic measurement at that location.
- Rocks are negatively polarizedwhen the earth’s field is reversed, which reduces the earth’s net field strength.
- Since the ages of these anomalies are known from dating the paleomagnetic reversals on land, the rate of movement of the ocean floor can be calculated.
- The fact that new ocean crust moves away from the midoceanic ridge at speeds that range from 2 to 10 centimeters per year has also been documented using satellite measurements and radar. For example, if it is known that a segment of sea floor that formed 10.0 million years ago is now 50 kilometers (5.0 million cm) away from the crest of the ridge, it can be calculated that it traveled that distance at about 2 centimeters per year.
- By using the calculated ages for episodes of paleomagnetic reversal, scientists can construct sea floor age maps, which confirm that the youngest oceanic crust is presently being formed at midoceanic ridges and that the oldest is about 150 to 200 million years old, or late Jurassic in age.
- This older material is the farthest from the spreading centers and is the next crust to be subducted.
- Sea floor age maps have been proven correct by the age dates calculated from hundreds of rock samples gathered from the ocean floor.
Seismic studies:
- More proof for sea floor spreading comes from seismic studies indicating that earthquakes occur along the rift valley of a midoceanic ridge and the cross‐cutting fractures that offset it.
- Rift valley earthquakes occur only along transform faults, those portions of the fracture zone located between the offset sections of a ridge and rift valley.
- Because of the way in which the sea floor spreads (that is, away from both sides of a midoceanic ridge), transform faults are the only areas along the fracture zone in which sections of the oceanic crust pass one another in opposite directions.
- The concentration of earthquakes in the transform‐fault sections of the fracture zones further supports the concept of ocean crust moving away from a midoceanic ridge.
Modern plate tectonic theory:
- By the 1960s, the theories of continental drift and sea floor spreading were supported by reliable scientific data and combined to develop modern‐day plate tectonic theory.
- The theory maintains that the crust and uppermost mantle, or lithosphere, is segmented into a number of solid, rigid slabs called lithospheric plates.
- These slabs move slowly over the asthenosphere, the 200‐kilometer‐thick zone of more plastic mantle material that underlies the plates.
- New oceanic crust is created at the crests of the midoceanic ridges and pushed laterally away by new accumulations of crust. It begins to cool as it moves away from the high heat flows at the ridge.
- By the time it is subducted at the convergent boundary with another plate, it is cold and dense enough that it begins to sink back into the mantle.
- Subduction is also probably a function of a down‐turning mantle convection current below the converging plates.
Theory:
- Plate tectonics(from the Late Latin tectonicus, from the Greek: τεκτονικός “pertaining to building”)is a scientific theory describing the large-scale motion of 7 large plates and the movements of a larger number of smaller plates of the Earth‘s lithosphere, over the last hundreds of millions of years.
- The theoretical model builds on the concept of continental drift developed during the first few decades of the 20th century. The geo scientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.
- The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken up into tectonic plates. The Earth’s lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates.
- Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform.
- Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The relative movement of the plates typically ranges from zero to 100 mm annually.
- Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust.
- Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading.
- In this way, the total surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual shrinking (contraction) or gradual expansion of the globe.
- Tectonic plates are able to move because the Earth’s lithosphere has greater strength than the underlying asthenoshere.
- Lateral density variations in the mantle result in convection.
- Plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge (due to variations in topography and density of the crust, which result in differences in gravitational forces) and drag, with downward suction, at the subduction zones.
- Another explanation lies in the different forces generated by tidal forces of the Sun and Moon.
- The relative importance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.
Types of Plate Boundaries
A divergent boundary
- A divergent boundaryoccurs when two tectonic plates move away from each other.
- Along these boundaries, lava spews from long fissures and geysers spurt superheated water.
- Frequent earthquakes strike along the rift. Beneath the rift, magma—molten rock—rises from the mantle.
- It oozes up into the gap and hardens into solid rock, forming new crust on the torn edges of the plates.
- Magma from the mantle solidifies into basalt, a dark, dense rock that underlies the ocean floor.
- Thus at divergent boundaries, oceanic crust, made of basalt, is created.
Convergent boundary
- When two plates come together, it is known as a convergent boundary.
- The impact of the two colliding plates buckles the edge of one or both plates up into a rugged mountain range, and sometimes bends the other down into a deep seafloor trench.
- A chain of volcanoes often forms parallel to the boundary, to the mountain range, and to the trench.
- Powerful earthquakes shake a wide area on both sides of the boundary.
- If one of the colliding plates is topped with oceanic crust, it is forced down into the mantle where it begins to melt.
- Magma rises into and through the other plate, solidifying into new crust. Magma formed from melting plates solidifies into granite, a light colored, low-density rock that makes up the continents.
- Thus at convergent boundaries, continental crust, made of granite, is created, and oceanic crust is destroyed.
Transform plate boundary
- Two plates sliding past each other forms a transform plate boundary.
- Natural or human-made structures that cross a transform boundary are offset—split into pieces and carried in opposite directions.
- Rocks that line the boundary are pulverized as the plates grind along, creating a linear fault valley or undersea canyon.
- As the plates alternately jam and jump against each other, earthquakes rattle through a wide boundary zone.
- In contrast to convergent and divergent boundaries, no magma is formed.
- Thus, crust is cracked and broken at transform margins, but is not created or destroyed.
Latest findings made in understanding Plate Tectonics:-
- Axial seamount = It refers to a live recording of volcano mountain. The volcano rising from Juan de fuca ridge demonstrates it. It supports the divergent movement.
- After 2012 Sumatra Indonesia earthquake in Indian ocean the Indo Australian plate broken into many plate. It was mainly due to slipping of plate in interpolated and hence the activation of Barren volcano happened.
- Zealandia:-It’s a new continent. It broke from Antarctica 100 million years and from Australia 80 million yrs ago. Its formation supports movement of plates.
- Heat from the base of the mantle contributes significantly to the strength of the flow of heat in the mantle and to the resultant plate tectonics. Buoyancy is created by heat rising up from deep within the Earth’s core.
How plate tectonics is an improvement over continental drift theory?
- Plate tectonic explains the mechanism of the motion of the tectonic plates while continental drift theory left this question completely unanswered.
- Tectonic plates have been constantly moving over the globe throughout the history of the earth. It is not the continent that moves as believed by Wegener. Continents are part of a plate and what moves is the plate.
- Wegener had thought of all the continents to have initially existed as a super continent in the form of Pangaea. However, later discoveries reveal that the continental masses, resting on the plates, have been wandering all through the geological period, and Pangaea was a result of converging of different continental masses that were parts of one or the other plates.
- At the time that Wegener proposed his theory of continental drift, most scientists believed that the earth was a solid, motionless body. However, concepts of sea floor spreading and the unified theory of plate tectonics have emphasised that both the surface of the earth and the interior are not static and motionless but are dynamic.
- Sea floor spreading:-
- The mobile rock beneath the rigid plates is believed to be moving in a circular manner. The heated material rises to the surface, spreads and begins to cool, and then sinks back into deeper depths. This cycle is repeated over and over to generate what scientists call a convection cell or convective flow
- The ultimate proof of this was the discovery of “magnetic stripes”on the seafloor later in the 1960s: the magnetic domains in oceanic rocks recorded reversal of Earth’s magnetic field over time. The pattern was symmetric to the ridge, supporting the idea of symmetric seafloor spreading. The idea of subduction zoneswas born
- With plate tectonics we have a theory that explains Wegener’s observations and how lithosphere can be produced and consumed so that Earth does not change its size
- Sea floor spreading:-
- Wegener’s continental drift theory lacked was a propelling mechanism. Other scientists wanted to know what was moving these continents around. Unfortunately, Wegener could not provide a convincing answer. The technological advances necessitated by the Second World War made possible the accumulation of significant evidence now underlying modern plate tectonic theory.
- The following two forces are too small to bring in change :-
- Pole-fleeing or centrifugal force:
- The spinning of Earth on its own axis creates a centrifugal force i.e. force oriented away from the axis of rotation towards the equator. Wegener believed the centrifugal force of the planet caused the super continent to break apart and pushed continents away from the Poles toward the equator. Therefore, He called this drifting mechanism as the “pole-fleeing or centrifugal force”
- Tidal force:-
- Wegener tried to attribute the westward drift of the Americas to lunar-solar drag i.e. by invoking tidal force that is the gravitational forces of the sun and the moon .He also admitted that it is probable that pole- fleeing or centrifugal force and tidal force are responsible for the journey of continents. Wegener failed to devise a sound mechanism for the movement of the continents. For Wegener the drifting mechanism was the most difficult question to solve.
- Plate tectonics is the grand unifying theory of geosciences that explains
- Movement of continents
- Earthquakes, volcanism most major features on Earth’s surface, including mountain building, formation of new lithosphere ,consumption of old lithosphere, mid-ocean ridges
- Pole-fleeing or centrifugal force:
- Second Order
Second Order
Continental features that are classified in the second order of relief include continental masses, mountain masses, plateaus, plains and lowlands. A few examples are the Himalayas, Alps, Rocky Mountains, Andes, Tibetan plateau, plateau of Anatolia (Turkey), Indo-Gangetic plains, Siberian lowlands and the plains of Mississippi. The great rock cores (shields) that form the heart of each continental mass arc of this order.
In the ocean basins, the second order of relief includes continental rises, slopes, abyssal plains, mid-ocean ridges, submarine canyons, and subduction trenches.
Mountains can be defined as a large natural elevation of the earth’s surface rising abruptly from the surrounding level.
Plateau is extensive area of flat upland usually bounded by an escarpment (i.e., steep slope) on all sides but sometimes enclosed by mountains. The essential criteria for plateaus are low relative relief and some altitude.
In geography, a plain refers to a flat area with little or no changes in elevation. It is one of the world’s major landforms.
Other Land forms
- Internal forces & their impact
Internal forces & their impact
Forces working within the earth’s surface are called endogenic forces.
Endogenic forces are the pressure within the earth, also known as internal forces. Such internal forces contribute to vertical and horizontal motions and lead to subsidence, land upliftment, volcanism, faulting, folding, earthquakes, etc.Sudden geomorphic movements occur mostly at the lithospheric plate margins (tectonic plate margins).The plate margins are highly unstable regions due to pressure created by pushing and pulling of magma in the mantle (convectional currents).These movements cause considerable deformation over a short period of time.
Volcanism and Earthquakes are caused as a result of sudden movements due to earth’s endogenic forces.
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. The process is called Volcanism and has been ongoing on Earth since the initial stages of its evolution over 4 billion years ago.
Volcanoes are Earth’s geologic architects. They’ve created more than 80 percent of our planet’s surface, laying the foundation that has allowed life to thrive. Their explosive force crafts mountains as well as craters. Lava rivers spread into bleak landscapes. But as time ticks by, the elements break down these volcanic rocks, liberating nutrients from their stony prisons and creating remarkably fertile soils that have allowed civilizations to flourish.
There are volcanoes on every continent, even Antarctica. Some 1,500 volcanoes are still considered potentially active around the world today; 161 of those—over 10 percent—sit within the boundaries of the United States.
But each volcano is different. Some burst to life in explosive eruptions, like the 1991 eruption of Mount Pinatubo, and others burp rivers of lava in what’s known as an effusive eruption, like the 2018 activity of Hawaii’s Kilauea volcano. These differences are due to the chemistry driving the molten activity. Effusive eruptions are more common when the magma is less viscous, or runny, which allows gas to escape and the magma to flow down the volcano’s slopes. Explosive eruptions, however, happen when viscous molten rock traps the gasses, building pressure until it violently breaks free.
- The majority of volcanoes in the world form along the boundaries of Earth’s tectonic plates—massive expanses of our planet’s lithosphere that continually shift, bumping into one another.
- When tectonic plates collide, one often plunges deep below the other in what’s known as a subduction zone.
- As the descending landmass sinks deep into the Earth, temperatures and pressures climb, releasing water from the rocks.
- The water slightly reduces the melting point of the overlying rock, forming magma that can work its way to the surface—the spark of life to reawaken a slumbering volcano.
- Not all volcanoes are related to subduction,
- Another way volcanoes can form is what’s known as hotspot volcanism.
- In this situation, a zone of magmatic activity—or a hotspot—in the middle of a tectonic plate can push up through the crust to form a volcano.
- Although the hotspot itself is thought to be largely stationary, the tectonic plates continue their slow march, building a line of volcanoes or islands on the surface. This mechanism is thought to be behind the Hawaii volcanic chain.
- Some 75 percent of the world’s active volcanoes are positioned around the ring of fire, a 25,000-mile long, horseshoe-shaped zone that stretches from the southern tip of South America across the West Coast of North America, through the Bering Sea to Japan, and on to New Zealand.
- This region is where the edges of the Pacific and Nazca plates butt up against an array of other tectonic plates. Importantly, however, the volcanoes of the ring aren’t geologically connected. In other words, a volcanic eruption in Indonesia is not related to one in Alaska, and it could not stir the infamous Yellowstone supervolcano.
The Ring of Fire is a string of volcanoes and sites of seismic activity, or earthquakes, around the edges of the Pacific Ocean. Roughly 90% of all earthquakes occur along the Ring of Fire, and the ring is dotted with 75% of all active volcanoes on Earth.
The Ring of Fire isn’t quite a circular ring. It is shaped more like a 40,000-kilometer (25,000-mile) horseshoe. A string of 452 volcanoes stretches from the southern tip of South America, up along the coast of North America, across the Bering Strait, down through Japan, and into New Zealand. Several active and dormant volcanoes in Antarctica, however, “close” the ring.
Plate Boundaries
The Ring of Fire is the result of plate tectonics. Tectonic plates are huge slabs of the Earth’s crust, which fit together like pieces of a puzzle. The plates are not fixed but are constantly moving atop a layer of solid and molten rock called the mantle. Sometimes these plates collide, move apart, or slide next to each other. Most tectonic activity in the Ring of Fire occurs in these geologically active zones.
Convergent Boundaries
A convergent plate boundary is formed by tectonic plates crashing into each other. Convergent boundaries are often subduction zones, where the heavier plate slips under the lighter plate, creating a deep trench. This subduction changes the dense mantle material into buoyant magma, which rises through the crust to the Earth’s surface. Over millions of years, the rising magma creates a series of active volcanoes known as a volcanic arc.
If you were to drain the water out of the Pacific Ocean, you would see a series of deep ocean trenches that run parallel to corresponding volcanic arcs along the Ring of Fire. These arcs create both islands and continental mountain ranges.
The Aleutian Islands in the U.S. state of Alaska, for example, run parallel to the Aleutian Trench. Both geographic features continue to form as the Pacific Plate subducts beneath the North American Plate. The Aleutian Trench reaches a maximum depth of 7,679 meters (25,194 feet). The Aleutian Islands have 27 of the United States’ 65 historically active volcanoes.
The Andes Mountains of South America run parallel to the Peru-Chile Trench, created as the Nazca Plate subducts beneath the South American Plate. The Andes Mountains include the world’s highest active volcano, Nevados Ojos del Salado, which rises to 6,879 meters (over 22,500 feet) along the Chile-Argentina border. Many volcanoes in Antarctica are so geologically linked to the South American part of the Ring of Fire that some geologists refer to the region as the “Antarctandes.”
Divergent Boundaries
A divergent boundary is formed by tectonic plates pulling apart from each other. Divergent boundaries are the site of seafloor spreading and rift valleys. Seafloor spreading is the process of magma welling up in the rift as the old crust pulls itself in opposite directions. Cold seawater cools the magma, creating new crust. The upward movement and eventual cooling of this magma has created high ridges on the ocean floor over millions of years.
The East Pacific Rise is a site of major seafloor spreading in the Ring of Fire. The East Pacific Rise is located on the divergent boundary of the Pacific Plate and the Cocos Plate (west of Central America), the Nazca Plate (west of South America), and the Antarctic Plate. In addition to volcanic activity, the rise also has a number of hydrothermal vents.
Transform Boundaries
A transform boundary is formed as tectonic plates slide horizontally past each other.
Parts of these plates get stuck at the places where they touch. Stress builds in those areas as the rest of the plates continue to move. This stress causes the rock to break or slip, suddenly lurching the plates forward and causing earthquakes.
These areas of breakage or slippage are called faults. The majority of Earth’s faults can be found along transform boundaries in the Ring of Fire.
The San Andreas Fault, stretching along the central west coast of North America, is one of the most active faults on the Ring of Fire.
It lies on the transform boundary between the North American Plate, which is moving south, and the Pacific Plate, which is moving north.
Measuring about 1,287 kilometers (800 miles) long and 16 kilometers (10 miles) deep, the fault cuts through the western part of the U.S. state of California. Movement along the fault caused the 1906 San Francisco earthquake, which destroyed nearly 500 city blocks. The earthquake and accompanying fires killed roughly 3,000 people and left half of the city’s residents homeless.
Hot SpotsThe Ring of Fire is also home to hot spots, areas deep within the Earth’s mantle from which heat rises. This heat facilitates the melting of rock in the brittle, upper portion of the mantle. The melted rock, known as magma, often pushes through cracks in the crust to form volcanoes.
Hot spots are not generally associated with the interaction or movement of Earth’s tectonic plates. For this reason, many geologists do not consider hot spot volcanoes part of the Ring of Fire.
Mount Erebus, the most southern active volcano on Earth, sits over the eruptive zone of the Erebus hot spot in Antarctica. This glacier-covered volcano has a lava lake at its summit and has been consistently erupting since it was first discovered in 1841.
Active Volcanoes in the Ring of Fire
- Most of the active volcanoes on The Ring of Fire are found on its western edge, from the Kamchatka Peninsula in Russia, through the islands of Japan and Southeast Asia, to New Zealand.
- Mount Ruapehu in New Zealand is one of the more active volcanoes in the Ring of Fire, with yearly minor eruptions, and major eruptions occurring about every 50 years. It stands 2,797 meters (9,177 feet) high. Mount Ruapehu is part of the Taupo Volcanic Arc, where the dense Pacific Plate is subducting beneath the Australian Plate.
- Krakatau, perhaps better known as Krakatoa, is an island volcano in Indonesia. Krakatoa erupts less often than Mount Ruapehu, but much more spectacularly. Beneath Krakatoa, the denser Australian Plate is being subducted beneath the Eurasian Plate. An infamous eruption in 1883 destroyed the entire island, sending volcanic gas, volcanic ash, and rocks as high as 80 kilometers (50 miles) in the air. A new island volcano, Anak Krakatau, has been forming with minor eruptions ever since.
- Mount Fuji, Japan’s tallest and most famous mountain, is an active volcano in the Ring of Fire. Mount Fuji last erupted in 1707, but recent earthquake activity in eastern Japan may have put the volcano in a “critical state.” Mount Fuji sits at a “triple junction,” where three tectonic plates (the Amur Plate, Okhotsk Plate, and Philippine Plate) interact.
- The Ring of Fire’s eastern half also has a number of active volcanic areas, including the Aleutian Islands, the Cascade Mountains in the western U.S., the Trans-Mexican Volcanic Belt, and the Andes Mountains.
- Mount St. Helens, in the U.S. state of Washington, is an active volcano in the Cascade Mountains. Below Mount St. Helens, the Juan de Fuca plate is being subducted beneath the North American Plate. Mount St. Helens lies on a particularly weak section of crust, which makes it more prone to eruptions. Its historic 1980 eruption lasted 9 hours and covered nearby areas in tons of volcanic ash.
- Popocatépetl is one of the most dangerous volcanoes in the Ring of Fire. The mountain is one of Mexico’s most active volcanoes, with 15 recorded eruptions since 1519. The volcano lies on the Trans-Mexican Volcanic Belt, which is the result of the small Cocos Plate subducting beneath the North American Plate. Located close to the urban areas of Mexico City and Puebla, Popocatépetl poses a risk to the more than 20 million people that live close enough to be threatened by a destructive eruption.
Volcanic landforms are divided into extrusive and intrusive landforms based on weather magma cools within the crust or above the crust. Rocks formed by either plutonic (cooling of magma within the crust) or volcanic (cooling of lava above the surface) are called ‘Igneous rocks’.
Destructive effects of volcanism:
- Volcanism can be a greatly damaging natural disaster. The damage is caused by advancing lava which engulfs whole cities. Habitats and landscapes are destroyed by lava flows.
- Showers of cinders and bombs can cause damage to life.
- Violent earthquakes associated with volcanic activity and mud flows of volcanic ash saturated by heavy rain can bury nearby places.
- Sometimes ash can precipitate under the influence of rain and completely cover the surrounding regions.
- Health concerns after a volcaniceruption include infectious disease, respiratory illness, burns, injuries from falls, and vehicle accidents related to the slippery, hazy conditions caused by ash.
- Further effects are the deterioration of water quality, fewer periods of rain, crop damages, and the destruction of vegetation.
- In coastal areas, seismic sea waves called tsunamis are an additional danger which are generated by submarine earth faults where volcanism is active.
Positive effects of volcanism:
- Volcanism creates new landforms like islands, plateaus,volcanic mountains etc. For example: Deccan plateau, Mt. Vesuvius.
- The volcanic ash and dust are very fertile for farms and orchards.
- Volcanic rocks yield very fertile soil upon weathering and decomposition.
- Although steep volcano slopes prevent extensive agriculture, forestry operations on them provide valuable timber resources.
- Mineral resources, particularly metallic ores are brought to the surface by volcanoes. Sometimes copper and other ores fill the gas bubble cavities. The famed Kimberlite rock of South Africa, source of diamonds is the pipe of an ancient volcano.
- Lava rock is extensively used as a source of crushed rock for concrete aggregate or rail road ballast, and other engineering purposes
- In the vicinity of active volcanoes, waters in the depth are heated from contact with hot magma giving rise to springs and geysers. The heat from the earth’s interior in areas of volcanic activity is used to generate geothermal electricity.
- Countries producing geothermal power include USA, Russia, Japan, Italy, New Zealand and Mexico.
- At many places volcanic landforms attract heavy tourist traffic. At several places national parks have been set up centred around volcanoes.
- One of the most effective ways of reducing the risk of a volcanic eruption is having an evacuation plan. This involves ensuring evacuation strategies are in place along with emergency shelter and food supplies being planned for.
- Based on monitoring data, exclusion zones can be set up to ensure people are evacuated from areas likely to be affected before an eruption.
- Local people can also be educated about actions they can take to reduce the risk of loss of life or injury.
- People are taught that if they are unable to be evacuated what they should do to protect themselves, e.g. go indoors to avoid falling ash and rock.
Earthquakes
- Earthquakes are caused due to release of energy. The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks. Rocks along a fault tend to move in opposite directions. As the overlying rock strata press them, the friction locks them together.
- However, their tendency to move apart at some point of time overcomes the friction. As a result, the blocks get deformed and eventually, they slide past one another abruptly. This causes a release of energy, and the energy waves travel in all directions.
This release of energy, along a fault line, may be due to several factors. They may be categorised as:
Natural causes
Tectonic earthquakes
- The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet.
- The Earth’s crust consists of seven large lithospheric plates and numerous smaller plates and the edges of the plates are called the plate boundaries. These plates move towards each other (a convergent boundary), apart (a divergent boundary) or past each other (a transform boundary).
- The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Earthquakes are caused by a sudden release of stress along these faults in the earth’s crust.
- As seen in below figure, most of the earthquakes take place along the plate boundaries. A rather more susceptible region around the pacific plate is called ‘ring of fire’ due to very high frequency of earthquakes in the region.
- The continuous motion of tectonic plates causes a steady build-up of pressure in the rock strata on both sides of a fault. It continues until the stress is sufficiently great that it is released in a sudden, jerky movement. The resulting waves of seismic energy propagate through the ground and over its surface, causing the shaking we perceive as earthquakes.
- There are mainly 3 types of faults along the plate boundaries as shown in figure
Volcanic earthquakes
- Volcanic earthquakes are caused by slip on a fault near a volcano. Volcanoes are often found in areas of crustal weakness and the mass of the volcano its self adds to the regional strain.
- They occur as a result of regional strain exerted in an area of weak faults. They can also be generated from changes of pressure under the volcano caused by the injection or removal of magma (molten rock) from the volcanic system.
- After the withdrawal of magma from a system, an empty space is left to be filled. The result is a collapse of surrounding rock to fill the void, also creating earthquakes.
- They are generally not as powerful as tectonic quakes and often occur relatively near the surface. Consequently, they are usually only felt in the vicinity of the hypocentre.
Anthropogenic causes or induced seismicity
- Induced seismicity refers to typically minor earthquakes and tremors that are caused by human activity that alters the stresses and strains on the Earth’s crust. Most induced seismicity is of a low magnitude.
- In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
- Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
- The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes.
Earthquakes can strike any location at any time, but history shows they occur in the same general patterns year after year, principally in three large zones of the earth:
- Circum-Pacific seismic belt: The world’s greatest earthquake belt is found along the rim of the Pacific Ocean, where about 81 per cent of our planet’s largest earthquakes occur. It is also known as “Ring of Fire”.
- Alpide earthquake belt: It extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. This belt accounts for about 17 percent of the world’s largest earthquakes.
- Submerged mid-Atlantic Ridge: The ridge marks where two tectonic plates are spreading apart (a divergent plate boundary).
- The earthquake is a natural hazard. As discussed above, not all the parts of the globe necessarily experience major shocks. The quakes of high magnitude, i.e. 8+ are quite rare; they occur once in 1-2 years whereas those of ‘tiny’ types occur almost every minute.
- Release of energy during earthquake generates waves which are called Earthquake Waves. Earthquake waves are basically of two types — body waves and surface waves.
- Body waves: They are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves. The body waves interact with the surface rocks and generate new set of waves called surface waves.
- Surface waves: These waves move along the surface. The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities.
There are two types of body waves. They are called P and S-waves.
- P-waves or ‘primary waves’ move faster and are the first to arrive at the surface. The P-waves are similar to sound waves. They travel through gaseous, liquid and solid materials.
- P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation. As a result, it creates density differences in the material leading to stretching and squeezing of the material.
- S-waves or secondary waves arrive at the surface with some time lag. They can travel only through solid materials. This characteristic of the S-waves is quite important. It has helped scientists to understand the structure of the interior of the earth.
- The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass. Surface waves are considered to be the most damaging waves.
Shadow zones
- There exist some specific areas where the waves are not reported by seismograph. Such a zone is called the ‘shadow zone’. The study of different events reveals that for each earthquake, there exists an altogether different shadow zone.
- Figure 6 shows the shadow zones of P and S-waves. It was observed that seismographs located at any distance within 105° from the epicentre, recorded the arrival of both P and S-waves. However, the seismographs located beyond 145° from epicentre record the arrival of P-waves, but not that of S-waves.
- Thus, a zone between 105° and 145° from epicentre was identified as the shadow zone for both the types of waves.
- The entire zone beyond 105° does not receive S-waves.
- The shadow zone of S-wave is much larger than that of the P-waves.
- The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicentre. The shadow zone of S-waves is not only larger in extent but it is also a little over 40 per cent of the earth surface.
- The earthquake events are scaled either according to the magnitude or intensity of the shock.
- The magnitude scale is known as the Richter scale. The magnitude indicates energy released during the quake. It is expressed in absolute numbers 0-10.
- The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale indicates the visible damage caused by the event. The range of intensity scale is from 1-12.
Earthquakes have all encompassing disastrous effects on the area of their occurrence. Some of the important ones are listed in Table
On ground On manmade structures On water Fissures Settlements Cracking, Slidings Waves, Hydro-Dynamic Pressure Landslides, Liquefaction Overturning, Buckling, Collapse Tsunami Earth Pressure and Possible Chain-effects Possible Chain-effects Possible Chain-effects National Geophysical Laboratory, Geological Survey of India has divided India into the following five earthquake zones:
- Very high damage risk zone
- High damage risk zone
- Moderate damage risk zone
- Low damage risk zone
- Very low damage risk zone
- The major reason for the high frequency and intensity of the earthquakes is that the Indian plate is driving into Eurasia at a rate of approximately 47 mm/year.
- Himalayan belt: Collision between Indo-Austral plate with Eurasian plate and Burma Plate with Java Sumatra plate. This collision causes lots of strain in underlying rocks’ energy of which is released in the form of earthquakes.
- Andaman and Nicobar Islands: Seafloor displacement and underwater volcanoes which disturb the equilibrium of earth’s surface.
- Deccan Plateau: some earth scientists have come up with a theory of the emergence of a fault line and energy build-up along the fault line of the river Bhima (Krishna) near Latur and Osmanabad (Maharashtra).
- Increasing population and unscientific land use for construction make India a high-risk land for earthquakes.
- Folding
- When a body of rock, especially sedimentary rock, is squeezed from the sides by tectonic forces, it is likely to fracture and/or become faulted if it is cold and brittle, or become folded if it is warm enough to behave in a plastic manner.
A fold is an undulating structure (wave-like) that forms when rocks or a part of the earth’s crust is folded (deformed by bending) under compressional stress. The folds are made up of multiple strata (rock layers).
The folds that are upwardly convex are called as anticlines. The core (centre) of an anticline fold consists of the older strata, and the strata are progressively younger outwards.
In contrast, the folds that are downwardly convex are called synclines. The core of a syncline fold consists of the younger strata, and the strata are progressively older outwards.
- An upward fold is called an anticline, while a downward fold is called a syncline.
- In many areas it’s common to find a series of anticlines and synclines , although some sequences of rocks are folded into a single anticline or syncline.
- A plane drawn through the crest of a fold in a series of beds is called the axial planeof the fold.
- The sloping beds on either side of an axial plane are limbs.
- An anticline or syncline is described as symmetricalif the angles between each of limb and the axial plane are generally similar, and asymmetrical if they are not.
- If the axial plane is sufficiently tilted that the beds on one side have been tilted past vertical, the fold is known as an overturned anticline or syncline.
- A very tight fold, in which the limbs are parallel or nearly parallel to one another is called an isoclinalfold
- Isoclinal folds that have been overturned to the extent that their limbs are nearly horizontal are called recumbent folds.
- Folds can be of any size, and it’s very common to have smaller folds within larger folds .
- Large folds can have wavelengths of tens of kilometres, and very small ones might be visible only under a microscope.
- Anticlines are not necessarily, or even typically, expressed as ridges in the terrain, nor synclines as valleys.
- Folded rocks get eroded just like all other rocks and the topography that results is typically controlled mostly by the resistance of different layers to erosion.
- A symmetrical foldis one in which the axial plane is vertical.
- An asymmetrical foldis one in which the axial plane is inclined.
- An overturned foldhas a highly inclined axial plane such that the strata on one limb are overturned.
- A recumbent foldhas an essentially horizontal axial plane.
- An isoclinal foldhas limbs that are essentially parallel to each other and thus approximately parallel to the axial plane.
Folds are classified into two main types namely anticlines or up-folds and synclines or down-folds.
- Anticline Folds:
An anticline consists of beds bent upwards with limbs dipping away from each other.
- Syncline Fold
A syncline consists of beds bent downwards with limbs dipping towards each other.
- Symmetrical Fold and Asymmetrical Fold:
A symmetrical fold is a fold whose axial plane is vertical and the limbs dip equally. The axial plane in this case divides the fold into two equal halves. If the two limbs dip at different angles the fold is an Asymmetrical fold.
- Monocline:
This is a fold in which only one limb is bent. This is a case when a rock-bed bends abruptly and resumes the original attitude at the lower level.
- Plunging Fold or Pitching Fold:
This is a fold whose axis is at some angle with the horizontal. The inclination of the fold axis with the horizontal is called plunge of the fold.
- Isocline or Carinate Fold:
This is a fold whose limbs dip at the same angle in the same direction. The two limbs in this case are parallel. The axial plane may be vertical, inclined or horizontal.
- Overturned Fold:
This is a fold whose limbs dip unequally in the same direction.
- Recumbent Fold:
This is a fold whose limbs are bent back on themselves almost horizontally.
- Zigzag Fold or Chevron Fold:
This is a fold having a sharp angular crest or trough.
10. Supratenuous Fold:
This is a fold whose beds are thinner at the crest and thicker at the trough. Such folds are formed due to contemporaneous sedimentation, compaction over irregular surfaces uplift folding, sinking etc.
These folds are produced by tangential pressures which lift up the beds slowly and vertically at the crests. The thick troughs are formed due to sinking and large accumulation of sediments.
- Dome Fold or Quaquaversal Fold or Pericline:
Dome fold consists of a set of rock beds lifted centrally giving the feature of a dome. The area of rock bed lifted may be circular or oval shaped. In a vertical section through the summit, the fold exhibits an anticlinal feature. For this reason this fold is also called a compound anticline. After the domes are eroded, the younger rocks appear surrounding the older rocks.
- Basin Fold or Centrocline:
Basin fold consists of a set of rock beds which are sunk down centrally giving the feature of a basin. The area of the rock bed sunk may be circular or oval shaped. In a vertical section taken centrally the fold exhibits a synclinal feature. For this reason this fold is also called a compound syncline.
Fold mountains are created where two or more of Earth’s tectonic plates are pushed together. At these colliding, compressing boundaries, rocks and debris are warped and folded into rocky outcrops, hills, mountains, and entire mountain ranges.
Fold mountains are often associated with continental crust. They are created at convergent plate boundaries, sometimes called continental collision zones or compression zones. Convergent plate boundaries are sites of collisions, where tectonic plates crash into each other. Compression describes a set of stresses directed at one point in a rock or rock formation.
At a compression zone, tectonic activity forces crustal compression at the leading edge of the crust formation. For this reason, most fold mountains are found on the edge or former edge of continental plate boundaries. Rocks on the edge of continental crust are often weaker and less stable than rocks found in the continental interior. This can make them more susceptible to folding and warping.
Most fold mountains are composed primarily of sedimentary rock and metamorphic rock formed under high pressure and relatively low temperatures. Many fold mountains are also formed where an underlying layer of ductile minerals, such as salt, is present.
Some examples of Fold mountains are The Himalayas, the Rockies, The Alps, the Aravallis,etc.
- Fold mountains are formed when sedimentary rock strata in geosynclines are subjected to compressive forces.
- They are the loftiest mountains, and they are generally concentrated along continental margins.
- Fold mountains belong to the group of youngest mountains of the earth.
- The presence of fossils suggests that the sedimentary rocks of these folded mountains were formed after accumulation and consolidation of silts and sediments in a marine environment.
- Fold mountains extend for great lengths whereas their width is considerably small.
- Generally, fold mountains have a concave slope on one side and a convex slope on the other.
- Fold mountains are mostly found along continental margins facing oceans (C-O Convergence).
- Fold mountains are characterized by granite intrusions (formed when magma crystallises and solidifies underground to form intrusions) on a massive scale.
- Recurrent seismicity is a common feature in folded mountain belts.
- High heat flow often finds expression in volcanic activity (Himalayas is an exception, because of C-C convergence).
- These mountains are by far the most widespread and also the most important.
- They also contain rich mineral resources such as tin, copper, gold etc.
- Faulting
- Rocks are very slowly, but continuously moving and changing shape. Under high temperature and pressure conditions common deep within Earth, rocks can bend and flow. In the cooler parts of Earth, rocks are colder and brittle and respond to large stresses by fracturing.
Rocks are very slowly, but continuously moving and changing shape. Under high temperature and pressure conditions common deep within Earth, rocks can bend and flow. In the cooler parts of Earth, rocks are colder and brittle and respond to large stresses by fracturing.
A fault is a crack across which the rocks have been offset.
- They range in size from micrometers to thousands of kilometers in length and tens of kilometers in depth, but they are generally much thinner than they are long or deep.
- In addition to variation in size and orientation, different faults can accommodate different styles of rock deformation, such as compression and extension.
Not all faults intersect Earth’s surface, and most earthquakes do not rupture the surface. When a fault does intersect the surface, objects may be offset or the ground may get cracked, or raised, or lowered. We call a rupture of the surface by a fault a fault scarp and identifying scarps is an important task for assessing the seismic hazards in any region.
Although the number of observations of deep fault structure is small, the available exposed faults provide some information on the deep structure of a fault.
A fault “zone” consists of several smaller regions defined by the style and amount of deformation within them.
Structure of an exposed section of a vertical strike-slip fault zone (after Chester et al., Journal of Geophysical Research, 1993). The center of the fault is the most deformed and is where most of the offset or slip between the surrounding rock occurs.
The region can be quite small, about as wide as a pencil is long, and it is identified by the finely ground rocks called cataclasite ( the ground up material found closer to the surface, gouge).
From all the slipping and grinding, the gouge is composed of very fine-grained material that resembles clay.
Surrounding the central zone is a region several meters across that contains abundant fractures.
Outside that region is another that contains distinguishable fractures, but much less dense than the preceding region. Last is the competent “host” rock that marks the end of the fault zone.
Active, Inactive, and Reactivated Faults
Active faults are structures along which one expects displacement to occur. By definition, since a shallow earthquake is a process that produces displacement across a fault, all shallow earthquakes occur on active faults.
Inactive faults are structures that one can identify, but which do not have earthquakes. Because of the complexity of earthquake activity, judging a fault to be inactive can be tricky, but often we can measure the last time substantial offset occurred across a fault. If a fault has been inactive for millions of years, it’s certainly safe to call it inactive. However, some faults only have large earthquakes once in thousands of years, and we need to evaluate carefully their hazard potential.
Reactivated faults form when movement along formerly inactive faults can help to alleviate strain within the crust or upper mantle. Deformation in the New Madrid seismic zone in the central United States is a good example of fault reactivation. Structure formed about 500 Ma ago are responding to a new forces and relieving strain in the mid-continent.
Faulting is a complex process and the variety of faults that exists is large. We will consider a simplified but general fault classification based on the geometry of faulting, which we describe by specifying three angular measurements: dip, strike, and slip.
Dip:
- In Earth, faults take on a range of orientations from vertical to horizontal.
- Dip is the angle that describes the steepness of the fault surface. T
- his angle is measured from Earth’s surface, or a plane parallel to Earth’s surface.
- The dip of a horizontal fault is zero (usually specified in degrees: 0°), and the dip of a vertical fault is 90°.
- Some old mining terms are used to label the rock “blocks” above and below a fault. If you were tunneling through a fault, the material beneath the fault would be by your feet, the other material would be hanging above you head. The material resting on the fault is called the hanging wall, the material beneath the fault is called the foot wall.
Strike:
- The strike is an angle used to specify the orientation of the fault and measured clockwise from north.
- For example, a strike of 0° or 180° indicates a fault that is oriented in a north-south direction, 90° or 270° indicates east-west oriented structure.
- To remove the ambiguity, always specify the strike such that when you “look” in the strike direction, the fault dips to you right.
- If the fault is perfectly vertical you have to describe the situation as a special case.
- If a fault curves, the strike varies along the fault, but this is seldom causes a communication problem if you are careful to specify the location (such as latitude and longitude) of the measurement.
Slip:
- Dip and strike describe the orientation of the fault, we also have to describe the direction of motion across the fault.
- That is, which way did one side of the fault move with respect to the other.
- The parameter that describes this motion is called the slip.
- The slip has two components, a “magnitude” which tells us how far the rocks moved, and a direction (it’s a vector).
- We usually specify the magnitude and direction separately.
- The magnitude of slip is simply how far the two sides of the fault moved relative to one another; it’s a distance usually a few centimeters for small earthquakes and meters for large events.
- The direction of slip is measured on the fault surface, and like the strike and dip, it is specified as an angle.
- Specifically the slip direction is the direction that the hanging wall moved relative to the footwall.
- If the hanging wall moves to the right, the slip direction is 0°; if it moves up, the slip angle is 90°, if it moves to the left, the slip angle is 180°, and if it moves down, the slip angle is 270° or -90°.
Hanging wall movement determines the geometric classification of faulting. One can distinguish between “dip-slip” and “strike-slip” hanging-wall movements.
Dip slip- Dip-slip – movement occurs when the hanging wall moved predominantly up or down relative to the footwall.
- If the motion was down, the fault is called a normal fault, if the movement was up, the fault is called a reverse fault.
- Downward movement is “normal” because we normally would expect the hanging wall to slide downward along the foot wall because of the pull of gravity.
- Moving the hanging wall up an inclined fault requires work to overcome friction on the fault and the downward pull of gravity.
Strike slip–When the hanging wall moves horizontally, it’s a strike-slip
- If the hanging wall moves to the left, the earthquake is called right-lateral, if it moves to the right, it’s called a left-lateral fault.
- The way to keep these terms straight is to imagine that you are standing on one side of the fault and an earthquake occurs.
- If objects on the other side of the fault move to your left, it’s a left-lateral fault, if they move to your right, it’s a right-lateral fault.
Oblique slip-When the hanging wall motion is neither dominantly vertical nor horizontal, the motion is called oblique-slip. Although oblique faulting isn’t unusual, it is less common than the normal, reverse, and strike-slip movement.
The style of faulting is an indicator of rock deformation and reflects the type of forces pushing or pulling on the region.
Near Earth’s surface, the orientation of these forces are usually oriented such that one is vertical and the other two are horizontal. The precise direction of the horizontal forces varies from place to place as does the size of each force.
The style of faulting that is a reflection of the relative size of the different forces – in particular is the relative size of the vertical to the horizontal forces.
There are three cases to consider, the vertical force can be the smallest, the largest, or the intermediate (neither smallest or largest). If the vertical force is the largest, we get normal faulting, if it is the smallest, we get reverse faulting. When the vertical force is the intermediate force, we get strike-slip faulting.
Normal faulting is indicative of a region that is stretching, and on the continents, normal faulting usually occurs in regions with relatively high elevation such as plateaus.
Reverse faulting reflects compressive forces squeezing a region and they are common in uplifting mountain ranges and along the coast of many regions bordering the Pacific Ocean. The largest earthquakes are generally low-angle (shallow dipping) reverse faults associated with “subduction” plate boundaries.
Strike-slip faulting indicates neither extension nor compression, but identifies regions where rocks are sliding past each other. The San Andreas fault system is a famous example of strike-slip deformation – part of coastal California is sliding to the northwest relative to the rest of North America – Los Angeles is slowly moving towards San Francisco.
As one might expect, the distribution of faulting styles is not random, but varies systematically across Earth and was one of the most important observations in constructing the plate tectonic model which explains so much of what we observe happening in the shallow part of Earth.
Fault Type: Normal Faulting Reverse Faulting Transform Faulting Deformation Style: Extension Compression Translation Force Orientation: Vertical Force Is Largest Vertical Force Is Smallest Vertical Force Is Intermediate Faulting is essentially a process of rupturing and displacement along the plane of rupture.
Its effect may involve:
- Changes in the elevation of the ground,
- Omission of some strata where they are normally expected,
iii. Repetition of some strata in a given direction against the normal order of superposition, and,
- Displacements and shifts in the continuity of the same rocks in certain regions.
In faults of some magnitude, it needs lot of fieldwork involving extensive mapping on the exposed outcrops and also geophysical measurements for establishing contacts of different types of rocks. It is only from the study of geological maps that the existence of faults at the first place and their effects on the rocks may get established with some certainty.
Further, the features produced due to faulting on the ground are subject to modifications by the subaerial processes of weathering and erosion with the passage of time. Hence what we describe today as the effects of faults may be, in fact, the effects of faults as modified by erosion and weathering.
Effect of Faulting On Topography:
One of the main effects of the faults on topography is that they very often result in the development of distinct types of steep slopes which are aptly called fault scarps. Three types of fault associated scarps are often recognized– fault scarps, fault-line scarps and composite-fault scarps.
In fault scarps, the relief is developed due to downward slip along the fault surface.
In the fault-line scarps, however, the slope relief is produced due to process of unequal erosion along the fault line with the passage of time.
When a given slope is believed to be the result of both of these processes, the scarp is of a composite type.
Besides fault scarps, faulting is also responsible for development of Block Mountains like horsts and deep elongated valleys called the grabens and the rift valleys.
Faults are also known to cause deflection in the course of streams.
Similarly, in certain regions, a number of springs may come into being along a fault line that happens to cut across an aquifer. These aligned springs may often prove to be an important evidence of faulting in the region.
- Isostacy
Isostasy (Greek isos “equal,” stasis “stand still”) is a term in geology, geophysics, and geodesy to describe the state of mass balance (equilibrium) between the Earth’s crust and upper mantle. It describes a condition to which the mantle tends to balance the mass of the crust in the absence of external forces.
- The term isostasywas proposed in 1889 by the American geologist C. Dutton, but the first idea of mass balancing of the Earth’s upper layer goes back to Leonardo da Vinci (1452–1519).
- The term means that the Earth’s topographic mass is balanced (mass conservation) in one way or another, so that at a certain depth the pressure is hydrostatic.
- Isostasy is an alternative view of Archimedes’ principle of hydrostatic equilibrium.
- Isostasy is a fundamental concept in the Geology.
- It is the idea that the lighter crust must be floating on the denser underlying mantle.
- It is invoked to explain how different topographic heights can exists on the Earth’s surface.
- Isostatic equilibrium is an ideal state where the crust and mantle would settle into in absence of disturbing forces.
- The waxing and waning of ice sheets, erosion, sedimentation, and extrusive volcanism are examples of processes that perturb isostasy.
- The physical properties of the lithosphere (the rocky shell that forms Earth’s exterior) are affected by the way the mantle and crust respond to these perturbations.
- Therefore, understanding the dynamics of isostasy helps us figure out more complex phenomena such as mountain building, sedimentary basin formation, the break-up of continents and the formation of new ocean basins.
There are two main ideas, developed in the mid-19th century, on the way isostasy acts to support mountain masses.
- In Pratt’s theory, there are lateral changes in rock density across the lithosphere. Assuming that the mantle below is uniformly dense, the less dense crustal blocks float higher to become mountains, whereas the more dense blocks form basins and lowlands.
- On the other hand, Airy’s theory assumes that across the lithosphere, the rock density is approximately the same, but the crustal blocks have different thicknesses. Therefore, mountains that shoot up higher also extend deeper roots into the denser material below.
Both theories rely on the presumed existence of a denser fluid or plastic layer on which the rocky lithosphere floats. This layer is now called the asthenosphere, and was verified in the mid-20th century to be present everywhere on Earth due to analysis of earthquakes – seismic waves, whose speed decrease with the softness of the medium, pass relatively slowly through the asthenosphere.
Both theories predict a relative deficiency of mass under high mountains, but Airy’s theory is now known to be a better explanation of mountains within continental regions, whereas Pratt’s theory essentially explains the difference between continents and oceans, since the continent crust is largely of granitic compostion which is less dense than the basaltic ocean basin.
Difference between Airy and Pratt’s views on Isostasy
Views of Airy Views of Pratt Uniform density of crustal material. Varying density of crustal material. Varying depth up to which root penetrates. crustal material reaches. Uniform depth up to which crustal material reaches. Deeper root below the mountain and smaller beneath plain. No root formation, but a level of Compensation. Resulting Geological Processes from Isostasy
The laws of buoyancy act on continents just as they would on icebergs and rafts.
An iceberg will rise further out of the water when the top is melted, and a raft will sink deeper when loads are added. However, the adjustment time for continents is much slower, due to the viscosity of the asthenosphere. This results in many dynamic geological processes that are observed today. The following paragraphs illustrate some of these examples.
- The formation of ice sheets could cause the Earth’s surface to sink. In areas which had ice sheets in the last ice age, the land is now “rebounding” upwards since the heavy ice has melted and the load on the lithosphere is reduced.
- Evidence from geological features include former sea-cliffs and associated wave-cut platforms that are found hundreds of meters above the sea level today.
- In the Baltic and in Canada, the amount and rate of uplift can be measured. In fact, due to the slowness of rebound, much of the land is still rising.
- Isostatic uplift also compensates for the erosion of mountains.
- When large amounts of material are carried away from a region, the land will rebound upwards to be eroded further.
- Due to drainage patterns, the erosion and removal of material is more prominent at plateau edges.
- Isostatic uplift may raise the edge higher than it used to be, so the ridge tops can be at an elevation considerably higher than the plateau itself.
- This mechanism is especially probable in mountain ranges bounding plateaus, such as the Himalayas and Kunlun Mountains bounding the Tibetan Plateau .
- Interestingly, given enough time and reaction kinetics, due to chemical transformations, the thick crustal root underneath mountains can become denser and founder into the mantle.
- The removal of the dense root can happen by the convection of the underlying asthenosphere or by delamination.
- After the root has detached, the asthenosphere rises and isostatic equilibrium leads to more mountain building at that region.
- For instance, this is thought to be the reason behind the late Cenozoic uplift of the Sierra Nevada in California.
- In fact, seismic data provide images of crust-mantle interactions during the supposed active foundering of the dense root beneath the southern Sierra Nevada.
- It appears that dense matter flowed asymmetrically into a mantle drip beneath the adjacent Great Valley.
- At the top of this drip, a V-shaped cone of crust is being dragged down tens of kilometers into the center of the mantle drip, leading to the disappearance of the Mohorovicic discontinuity (the boundary between crust and mantle) in seismic images.
- Likewise, at the northern Sierra Nevada, there is also a seismic “hole” known as the Redding anomaly, lending to the assumption that lithospheric foundering occurred there as well.
In conclusion, isostasy is yet another example of a deceptively simple idea in physics that provides crucial and sweeping explanatory power for other sciences.
- External forces & their impact
Weathering is breaking down rocks, soil, and minerals as well as wood and artificial materials by contacting the atmosphere, water, and biological organisms of the Earth. Weathering takes place in situ, i.e. in the same place, with little or no movement. It should therefore not be confused with erosion involving the movement of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity, and then transported and deposited elsewhere.
Erosion is the geological process in which earthen materials are worn away and transported by natural forces such as wind or water.
Erosion is the opposite of deposition, the geological process in which earthen materials are deposited, or built up, on a landform.
Most erosion is performed by liquid water, wind, or ice (usually in the form of a glacier). If the wind is dusty, or water or glacial ice is muddy, erosion is taking place. The brown color indicates that bits of rock and soil are suspended in the fluid (air or water) and being transported from one place to another. This transported material is called sediment.
Physical erosion describes the process of rocks changing their physical properties without changing their basic chemical composition. Physical erosion often causes rocks to get smaller or smoother. Rocks eroded through physical erosion often form clastic sediments. Clastic sediments are composed of fragments of older rocks that have been transported from their place of origin.
Plant growth can also contribute to physical erosion in a process called bioerosion. Plants break up earthen materials as they take root, and can create cracks and crevices in rocks they encounter.
Ice and liquid water can also contribute to physical erosion as their movement forces rocks to crash together or crack apart. Some rocks shatter and crumble, while others are worn away. River rocks are often much smoother than rocks found elsewhere, for instance, because they have been eroded by constant contact with other river rocks.
Erosion is the geological process in which earthen materials are worn away and transported by natural forces such as wind or water.
Erosion is the opposite of deposition, the geological process in which earthen materials are deposited, or built up, on a landform.
Most erosion is performed by liquid water, wind, or ice (usually in the form of a glacier). If the wind is dusty, or water or glacial ice is muddy, erosion is taking place. The brown color indicates that bits of rock and soil are suspended in the fluid (air or water) and being transported from one place to another. This transported material is called sediment.
Physical erosion describes the process of rocks changing their physical properties without changing their basic chemical composition. Physical erosion often causes rocks to get smaller or smoother. Rocks eroded through physical erosion often form clastic sediments. Clastic sediments are composed of fragments of older rocks that have been transported from their place of origin.
Plant growth can also contribute to physical erosion in a process called bioerosion. Plants break up earthen materials as they take root, and can create cracks and crevices in rocks they encounter.
Ice and liquid water can also contribute to physical erosion as their movement forces rocks to crash together or crack apart. Some rocks shatter and crumble, while others are worn away. River rocks are often much smoother than rocks found elsewhere, for instance, because they have been eroded by constant contact with other river rocks.
Erosion is the physical removal and transport of material by mobile agents such as water, wind or ice.
The three common agents of erosion are:
- Water
- Wind
- Ice
These agents are mobile at the Earth’s surface and are responsible for the transport of sediment.
Erosion and mass wasting appear to be similar processes but have distinctly different causes. The movement of sediment by erosion requires mobile agents such a water, wind and ice. That is, the sediment is transported by the movement of the agents. Mass wasting (commonly referred to as landslides) involves the transfer of rock and soil downslope under the influence of gravity. Gravity is the key factor in mass wasting and the movement of material does not require a mobile agent.
Erosion (transport of sediment) usually ends with the deposition of sediments (and soil). Deposition occurs when the forces responsible for erosion are no longer sufficient to transport the sediment.
There are a wide variety of landscapes on the Earth’s surface where the deposition of sediments occur as the result of fluvial (rivers), aeolian (wind) and glacial (ice) erosion.
Erosion by Water
Water is the most efficient and effective agent for erosion. Erosion by water occurs in three different geologic settings:
- Streams and Rivers – erosion commonly occurs along the bank of the stream. A stream is any size channelized body of running water (small creeks to giant rivers).
- Coastlines – erosion that occurs on coastlines is due to the action of ocean currents, waves, and tides.
- Underground– erosion that occurs at the sub-surface level due to underground water
The landforms created as a result of degradational action (erosion and transportation) or aggradational work (deposition) of running water are called fluvial landforms.
Fluvial erosional landforms
Fluvial Erosional Landforms are landforms created by the erosional activity of rivers.Various aspects of fluvial erosive action include:
- Hydration:the force of running water wearing down rocks.
- Corrosion:chemical action that leads to weathering.
- Attrition:river load particles striking, colliding against each other and breaking down in the process.
- Corrasion or abrasion:solid river load striking against rocks and wearing them down.
- Downcutting (vertical erosion):the erosion of the base of a stream (downcutting leads to valley deepening).
- Lateral erosion:the erosion of the walls of a stream (leads to valley widening).
- Headward erosion:erosion at the origin of a stream channel, which causes the origin to move back away from the direction of the stream flow, and so causes the stream channel to lengthen.
The following are some of the major landforms formed as a result of fluvial erosion:
River Valley
- The extended depression on the ground through which a stream flows is called a river valley.
- At different stages of the erosional cycle, the valley acquires different profiles.
- At a young stage, the valley is deep, narrow with steep wall-like sides and a convex slope.The erosional action here is characterized by predominantly vertical downcuttingThe profile of valley here is typically ‘V’ shaped.
- A deep and narrow ‘V’ shaped valley is also referred to as gorgeand may result due to downcutting erosion or because of the recession of a waterfall (the position of the waterfall receding due to erosive action).
- Most Himalayan rivers pass through deep gorges (at times more than 500 metres deep) before they descend to the plains.
- An extended form of the gorge is called a The Grand Canyon of the Colorado River in Arizona (USA) runs for 483 km and has a depth of 2.88 km.
- A tributary valley lies above the main valley and is separated from it by a steep slope down which the stream may flow as a waterfall or a series of rapids.
- As the cycle attains maturity, the lateral erosion (erosion of the walls of a stream) becomes prominent and the valley floor flattens out (attains a ‘V’ to ‘U’ shape).
- The valley profile now becomes typically ‘U’ shaped with a broad base and a concave slope.
Waterfalls:
- A waterfall is simply the fall of an enormous volume of water from a great height.
- They are mostly seen inthe youth stage of the river.
- Relative resistance of rocks, the relative difference in topographic reliefs, fall in the sea level and related rejuvenation, earth movements are responsible for the formation of waterfalls.
- Angel Fallsin Venezuela is the world’s highest waterfall, with a height of 979 metres and a plunge of 807 metres.
- Tugela Falls(948 m) in the Drakensberg mountains, South Africa is the world’s second highest waterfall.
Potholes
- The kettle-like small depressions in the rocky beds of the river valleys are called potholes which are usually cylindrical in shape.
- Potholes are generally formed in coarse-grained rocks such as sandstones and granites.
- Potholing or pothole drilling is the mechanism through which the grinding tools (fragments of rocks e.g., boulders and angular rock fragments) when caught in the water eddies or whirling water start dancing in circular manner and grind and drill the rock beds of the velleys like drilling machine and thus form small holes which are gradually enlarged by the repetition of the said mechanism.
- The potholes go on increasing in both diameter (and perimeter) and depth.
- The diameter of pot holes ranges from a few centimetres to several metres. The depth of potholes is far more than their diameters.
- Potholes of much bigger size are called plunge pools. In fact, plunge pools are generally formed at the base of waterfalls due to pounding of rocks by gushing water of the falls (waterfalls).
- Many of the river valleys are studded with numerous potholes. For example, in Chotanagpur highlands where the rivers have been rejuvenated due to upliftment effected during Tertiary period.
- The basaltic bed of the Gaur river near Bhadbhada (east of Jabalpur, M.P.) presents a magnificent view of numerous potholes of various dimension. Pothole drilling is the effective mechanism of valley deepening.
Terraces
- The narrow flat surfaces on either side of the valley floor are called river terraces which represent the level of former valley floors and the remnants of former (older) flood plains.
- Sometimes, the river valleys are frequented by several terraces on either side wherein they are arranged in step-like forms.
- River terraces are generally formed due to dissection of fluvial sediments of flood plains deposited along a valley floor.
- There are much variations in terraces as regards their morphology, structure and mode of origin. River terraces are classified in various ways.
- For example, terraces are divided into: Paired terraces and Unpaired terraces on the basis of nature of erosion.
- Paired terraces are formed due to rapid rate of vertical erosion resulting into the occurrence of terraces on both the sides of the river valley almost at the same level.
- It may be pointed out that paired terraces mean occurrence of terraces on both the sides of valley at the same height.
- Unpaired terraces are formed due to concamitant vertical erosion (valley deepening) and lateral movement of the channel.
Structural Benches:
- The step-like flat surfaces on either side of the present lowest valley floors are called terraces.
- The benches or terraces formed due to differential erosion of alternate bands of hard and soft rock beds are called structural benches or terraces because of lithological control in the rate of erosion and consequent development of benches.
Gullys and rills
- Gulley is a water-worn channel, which is particularly common in semi-arid areas.
- It is formed when water from overland-flows down a slope, especially following heavy rainfall, is concentrated into rills, which merge and enlarge into a gulley.
- The ravines of Chambal Valleyin Central India and the Chos of Hoshiarpur in Punjab are examples of gulleys.
Meanders
- A meander is defined as a pronounced curve or loop in the course of a river channel.
- The outer bend of the loop in a meander is characterized by intensive erosion and vertical cliffs and is called the cliff-slope side. This side has a concave slope.
- The inner side of the loop is characterized by deposition, a gentle convex slope, and is called the slip-off side.
- The meanders developed during first cycle of erosion by a stream are called simple meanders. These are formed by lateral erosion.
- These meanders may be wavy, horse-shoe type or oxbow type.
- Incised meanders are the representative features of rejuvenation and are developed through vertical erosion leading to valley incision or deepening.
- The narrow and deep meanders formed due to accelerated rate of valley incision caused by rejuvenation (either due to upliftment of land area or fall in sea level) inside simple meanders (having wide and shallow valleys) developed by lateral erosion during first stage of cycle of erosion are called incised meanders.
- Simple meanders develop over loose geomaterials (such as alluvium) as well as over resistant bedrocks but incised meanders are always dug out in bedrocks.
- Five terms are in use to indicate incised meanders which are developed due to vertical erosion (downcutting or valley incision) of bedrock viz.: Incised meanders, Entrenched meanders, Intrenched meanders, Inclosed meanders and Ingrown meanders.
- Inclosed and incised meanders represent those meanders of deep and narrow valleys which are inclosed by rocky walls.
- In fact, incised meanders mean the formation of meanders in older meanders through downcutting of valley floors.
Oxbow lakes:
- Sometimes, because of intensive erosion action, the outer curve of a meander gets accentuated to such an extent that the inner ends of the loop come close enough to get disconnected from the main channel and exist as independent water bodies called as oxbow lakes.
- These water bodies are converted into swamps in due course of time.
- In the Indo-Gangetic plains, southwards shifting of Ganga has left many oxbow lakes to the north of the present course of the Ganga.
Peneplains:
- Peneplains represent low featureless plain having undulating surface and remnants of convexo-con- cave residual hills. These are, in fact, the end products of normal cycle of erosion.
- This refers to an undulating featureless plain punctuated with low-lying residual hills of resistant rocks. It is considered to be an end product of an erosional cycle.
- Fluvial erosion, in the course of geologic time, reduces the land almost to base level (sea level), leaving so little gradient that essentially no more erosion could occur.
- These are frequented with low residual hills known as monadnocks (named by W.M. Davis after Monadnock hills of New England region, USA) which are left out due to less erosion of relatively resistant rocks.
- The end product of normal or fluvial cycle of erosion has been variously named by different geomorphologists. e.g., peneplain (W.M. Davis), endrumpf ((W. Penck), panplain (C.H. Crickmay), pediplain (L.C. King), etchplain (Pugh and Thomas), panfan (A.C. Lawson) etc.
Fluvial depositional landforms
- Rivers deposit sediments in different parts of their courses and thus form three major types of landforms which are called constructional landforms such as alluvial fans cones, natural levees and deltas.
- The depositional action of a stream is influenced by stream velocity and the volume of river load.
- The decrease in stream velocity reduces the transporting power of the streams which are forced to leave some load to settle down.
- Increase in river load is effected through accelerated rate of erosion in the source catchment areas consequent upon deforestation.
Various landforms resulting from fluvial deposition are as follows:
Alluvial fans and cones
- Alluvial fans and cones due to accumulation of materials are always formed at the base of foothills where there is abrupt drop (decrease) in the channel gradient.
- The transporting capacity of the streams decreases enormously at the foothill zones while they leave the mountains and enter the plain topography because of substantial decrease in their velocity consequent upon decrease in channel gradient.
- Consequently, load consisting of finer to coarser and big-sized materials coming from upstream is deposited at the point of break in slope or foothill zone and thus alluvial fans are formed.
- There is sorting of materials in the alluvial fans. The size of sediments decreases outward from the apex (which is towards the hills) of the fans towards their outer margins (distal side).
- The shapes of alluvial fans are usually semi-circular or arcuate, the appex of which is located at the mouth of narrow opening through which the stream comes out of the hills and enter the surface of low height and gentle slope.
- Alluvial fans and cones are more or less similar except difference in their gradients. Alluvial fans have gentler slopes than the cones.
- Sometimes, a series of alluvial fans are formed along the piedmont zone. They grow in size and are ultimately coalesced to form an extensive fan which is called compound alluvial fan. The most extensive compound alluvial fans form undulating and sloping alluvial plain in front of peidmont zone. Such plain is called piedmont alluvial plain.
Natural Levees:
- The narrow belt of ridges of low height built by the deposition of sediments by the spill water of the stream on its either bank is called natural levee or natural embankment.
- Not all the streams build natural levees.
- Levees are formed due to deposition of sediments during flood periods when the water overtops the river banks and spreads over adjoining flood plains.
- Long ridges of low height are formed parallel to the river valleys. Average height of natural levees is within 10 metres.
- Natural levees limit the lateral spread of river water except during severe and widespread floods.
- Natural levees are more or less stabilized landforms which attract human settlements.
- Sometimes, natural levees are also used for agricultural purposes because water table of groundwater is very high.
- Generally, natural levees help in checking the floods but when breached they cause severe catastrophic floods inflicting heavy loss of human health and wealth.
- Since the channel is more or less confined within the natural levees and hence there is continuous sedimentation which causes gradual rise of the river beds (valley floor).
- Consequently, sometimes the bed of the stream becomes higher than the adjoining flood plain.
- Breach of natural Levees in such situation causes sudden catastrophic floods because the river water gushes in the flood plains and settlements with high velocity beyond imagination. Such cases of breaches of natural levees and consequent severe floods are very often reported from the Yellow river (formerly Hwang Ho) of China. This was the reason that the Hwang Ho was called “Sorrow of China”.
Delta
- The depositional feature of almost triangular shape at the mouth of a river debouching either in lake or a sea is called delta.
- The word delta, derived from Greek letter, was first used by Greek historian Herodotous (485-425BC) for the triangular depositional feature at the mouth of the Nile River.
- Whether small or large, almost every river forms delta.
- The size of delta of major and small rivers all over the world varies from a few square kilometres to thousands of square kilometres (e.g. Ganga delta in India and Bangladesh).
- Conditions for Delta Formation: The ideal favourable conditions for the formation and growth of delta include:
- Suitable place in the form of shallow sea and lake shores.
- Long courses of the rivers (i.e. long rivers so that they bring enough amount of sediments).
- Medium size of sediments (because if the sediments are very fine, they would be carried in the sea in suspension for longer distances and if they are very coarse, they would soon settle down at the sea bottom, and hence no delta would be formed).
- Relatively calm or sheltered sea at the mouths of the rivers (so that ocean currents, strong waves or high tidal waves do not interfere with the natural process of gradual sedimentation and delta formation).
- Large amount of sediment supply.
- Accelerated rate of erosion in the catchment area of the concerned river.
- Almost stable condition of sea coast and oceanic bottom (because sea coast subjected to frequent emergence or submergence caused by tectonic movements does not allow regular sedimentation and thus disfavours delta formation) etc.
Delta Formation:
- The formation of delta starts with the deposition of sediments if the aforesaid favourable conditions are available.
- The sedimentation takes place regularly at the mouth of the river, on the sides of stream channel, in the bed of the river and in front of river mouth where the river debouches in the sea.
- Thus, an extensive fan is formed which slopes towards the sea. Several such fans are formed at the mouth of the river.
- These fans gradually grow towards the sea. Ultimately these fans are coalesced and a delta is formed.
- These deposits obstruct the free flow of main river and hence it is divided into several branches.
- This process of segmentation of main stream is known as bifurcation.
- Thus, the main channel is bifurcated into numerous small and narrow sub-channels which are called distributaries and the stream with numerous distributaries is called braided stream.
Classification of Deltas:
Deltas are divided into following six types on the basis of shape and growth: 1. Arcuate Delta 2. Bird-Foot Delta 3. Estuarine Delta 4. Truncated Delta 5. Growing Delta 6. Blocked Delta.
Type 1: Arcuate Delta:
- Such deltas are like an arc of a circle or a bow and are of lobate form in appearance wherein middle portion has maximum extent towards the sea whereas they narrow down towards their margins.
- Such deltas are formed when the river water is as dense as the sea water.
- The arcuate or semi-circular shape is also given to such deltas by sea waves and oceanic currents.
- The Nile Delta is the best example of arcuate deltas , which is also called as Nile type of delta.
- Arcuate deltas are formed of coarser materials including gravels, sands and silt. The main river is bifurcated into numerous channels known as distributaries.
- Such deltas are very often formed in the regions of semi-arid climate.
- Significant examples of arcuate delta include Ganga delta, Rhine delta, Niger delta, Yellow (Hwang Ho) delta, Irrawaddy delta, Volga delta, Indus delta, Danub Delta, Meekong Delta, Po delta, Rhone Delta, Leena delta etc.
- Arcuate delta is an example of growing delta as it grows towards the sea every year but the annual rate of growth varies from one delta to another. This process of seaward growth of deltas is called progradation.
Type 2 : Bird-Foot Delta:
- Bird-foot deltas resembling the shape of foot of a bird are formed due to deposition of finer materials which are kept in suspension in the river water which is lighter than the sea water.
- The rivers with high velocity carry suspended finer load to greater distances inside the oceanic water.
- The fine materials after coming in contact with saline oceanic water settle down on either side of the main channel and thus a linear delta is formed.
- It is interesting to note that the distributaries of the main channel also form linear segments of delta.
- These linear bars of sediments on either side of the distributaries of the main channel resemble the fingers of human hand.
- Such delta is, thus, also called finger delta. The Mississippi delta exhibits the best example of bird-foot delta.
Type 3 :Estuarine Delta:
- The deltas formed due to filling of estuaries of rivers are called estuarine deltas.
- Those mouths of the rivers are called estuaries which are submerged under marine water and sea waves and oceanic currents remove the sediments brought by the rivers.
- There is continuous struggle between the rivers and sea waves wherein the former deposit sediments while the latter remove them.
- Whenever rivers succeed in depositing sediments at their submerged mouths, long and narrow deltas are formed.Such deltas are called estuarine deltas.
- The deltas of Narmada and Tapi (formerly Tapti) rivers of India are the examples of estuarine deltas.
- The other significant examples of estuarine deltas include Mackenzie delta, Vistuala delta, Elb delta, Ob delta, Seine delta, Hudson delta etc.
Type 4: Truncated Delta:
- Sea waves and ocean currents modify and even destroy deltas deposited by the river through their erosional work. Thus, eroded and dissected deltas are called truncated deltas.
Type 5: Blocked Delta:
- Blocked deltas are those whose seaward growth is blocked by sea waves and ocean currents through their erosional activities.
- The progradation of deltas may also be hampered due to sudden decrease in the supply of sediments consequent upon climatic change or management of catchment areas of concerned rivers.
Type 6: Abandoned Delta:
- When the rivers shift their mouths in the seas and oceans, new deltas are formed, while the previous deltas are left unnourished. Such deltas are called abandoned deltas.
- The Yellow (formerly Hwang Ho) river of China has changed its mouths several times and thus has formed several deltas.
- For example, the present delta of the Yellow river is to the north of Shantung Peninsula while the previous delta was deposited to the south of the peninsula. The western part of the Ganga delta, which is drained by the Hoogli River is an example of abandoned delta.
Coastal erosion and landforms thus formed
- The sea performs the function of erosion and deposition through sea waves, aided by currents, tides and storms in coastal areas.
- The erosive work of the sea depends upon
- size and strength of waves
- seaward slope
- height of the shore between low and high tides
- composition of rocks
- depth of water
- The wave exerts a pressure to the magnitude of 3000 to 30,000 kilograms per square kilometre.
- This wave pressure compresses the air trapped inside rock fissures, joints, faults, etc. forcing it to expand and rupture the rocks along weak points.
- This is how rocks get worn down under wave action.
- Waves also use rock debris as instruments of erosion. These rock fragments carried by waves themselves get worn down by striking against the coast or against one another.
- The solvent or chemical action of waves is another mode of erosion, but it is pronounced only in case of soluble rocks like limestone and chalk.
The marine landforms can be studied under erosional and depositional categories.
Erosional Landforms
Chasms:
These are narrow, deep indentations carved out through vertical planes of weakness in the rocks by wave action. With time, further headward erosion is hindered by the chasm mouth, which itself keeps widening till a bay is formed.
Wave-Cut Platform:
When the sea waves strike against a cliff, the cliff gets eroded gradually and retreats. The waves level out the shore region to carve out a horizontal plane or a wave-cut platform. The bottom of the cliff suffers the maximum intensive erosion by waves and, as a result, a notch appears at this position.
Sea Cliff:
It is the seaward limit of coast which is marked by a steep scarp.
Sea Caves:
Differential erosion by sea waves through a rock with varying resistance across its structure produces arched pockets in rocks. These are called sea caves.
Sea Arches:
When the waves attack a rock- form from two opposite sides, the differential erosion produces bridge-like structures or sea arches.
Stacks/Skarries/ Chimney Rock:
When a portion of the sea arch collapses, the remaining column-like structure is called a stack, skarry or chimney rock.
Hanging Valleys:
If the fluvial erosion by streams flowing down the coast is not able to keep pace with the retreat of the cliff, the rivers appear to be hanging over the sea. These river valleys are called hanging valleys.
Blow Holes or Spouting Horns:
A narrow fissure through the roof of a sea arch is called a blow hole or a spouting horn because the wave action compresses and squeezes out the air from the sea caves through blow holes making a peculiar noise.
Plane of Marine Erosion/Peneplain:
The eroded plain left behind by marine action is called a plain of marine erosion, and if the level difference between this plain and the sea level is not much, the agents of weathering convert it into a peneplain.
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