GEOMORPHOLOGY

GEOMORPHOLOGY

 

Topic Covered

 

  • Interior of the Earth
    • Direct & Indirect Methods
    • Earthquakes & Seismic Waves
    • Layered Structure
  • Minerals & Rock System- Igneous, Sedimentary & Metamorphic
  • Continental Drift Theory
  • Convection Current Theory
  • Ocean Floor Mapping & Sea Floor Spreading Theory
  • Plate Tectonics Theory
  • Plate Interactions
    • Divergence
      • O/O (MOR)
      • C/C (Rift Valley)
    • Transverse (Fault)
    • Convergence-
      • O/O (Island Arc)
      • C/C (Fold Mountains)
      • O/C (Volcanic Mountains)

 

 

Interior Of The Earth

The knowledge of earth’s interior is very important to understand the various geophysical phenomenons like earthquake, volcanism, etc. It also helps in mineral exploration. Geologists have used two main types of evidence to learn about Earth’s interior

 

Direct evidence

  • samples drilled from deep inside Earth help in making inferences.
  • Deepest drill at Kola (in Arctic Ocean) has so far reached a depth of 12 km
  • Volcanic eruptions

Indirect evidence

  • Seismic waves produced by earthquakes.
  • Meteors are likely to have a similar internal structure as of Earth (Both are solar objects and born from the same nebular cloud).
  • Gravity anomalies due to uneven distribution of mass of material within the earth (Provide information about the materials in the earth’s interior).
  • Magnetic field study gives information about magnetic material distribution in crustal portion.

 

SEISMIC WAVES

 

  • Seismic wavesare the waves of energy caused by the sudden breaking of rock within the earth or an explosion. These are mechanical waves and requires medium to propagate.
  • These are the energy that travels through the earth and is recorded on seismographs.
  • Seismology= Study of earthquakes and seismic waves that move through and around the earth.
  • Seismologist= Scientist who studies earthquakes and seismic waves.

Types of Seismic Waves

  • Two main types —
    • Body Waves
    • Surface Waves
  • Body waves can travel through the earth’s inner layers, but surface waves can only move along the surface of the planet like ripples on water.
  • Earthquakes radiate seismic energy as both body and surface waves.

Body waves

  • Generated due to the release of energy at the focus. It moves in all directions travelling through the body of the earth.
  • 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
    • P-Wave
    • S-Wave

 

P wave or Primary wave

  • Fastest kind of seismic wave.
  • First to ‘arrive’ at a seismic station.
  • Can move through solid rock and fluids.
  • Velocity = Solids > Liquids > Gases
  • It pushes and pulls the rock as it moves through, just like sound waves push and pull the air.
  • Hence, it is also known as compressional waves because of the pushing and pulling they do.
  • Particles move in the same direction of wave propagation.

 

The windows during the thunder rattle because the sound waves push and pull on the window glass much like P waves push and pull on rock.

 

S wave or Secondary wave

  • Second wave felt in an earthquake.
  • The S wave is slower than a P wave.
  • It can only move through solid rock (not through any liquid medium).
  • Property helped seismologists to conclude that the Earth’s outer core is a liquid.
  • S waves move rock particles up and down (or side-to-side), perpendicular to the direction of wave propagation.

 

Surface wave

  • The body waves interact with the surface rocks and generate new set of waves called surface waves.
  • It travels only through the crust and have lower frequency than body waves.
  • Arrive after body waves.
  • Also known as long period waves.
  • Almost entirely responsible for the damage and destruction associated with earthquakes.
  • This damage and the strength of the surface waves are reduced in deeper earthquakes.
  • There are two types of surface waves
    1. Love Waves = Zig-Zag movement
    2. Rayleigh Waves = Elliptical movement

How the properties of ‘P’ and ‘S’ waves help in determining the earth’s interior?

  • Change of density in different layers of the Earth greatly varies the wave velocity.
  • The density of the earth as a whole can be estimated by observing the changes in velocity of waves.
  • By the observing the changes in direction of the P and S waves (emergence of shadow zones), different layers can be estimated.

Emergence of Shadow Zone

  • Shadow zone is the area of Earth’s surface where seismographs cannot detect an earthquake after the waves have passed through the earth.
  • P-waves are refracted by the liquid outer core and are not detected between 104° and 140°.
  • S-waves cannot pass through the liquid outer core and are not detected beyond 104°.
  • This information led scientists to deduce a liquid outer core.

 

CONCENTRIC LAYERS OF EARTH’S INTERIOR

 

  • Layers distinct in terms of temperature, composition, and density
  • Heat radiation à Conduction mainly and convection in the magma chamber
  • Temperature increases with depth = 1 °C/32 meters (at 48 km @ 1200-1300 °C)
  • This rate is mainly due to presence of radioactive materials in the crust (up to 100kms)

Temperature

  • @ 48 Km = 1100 °C
  • @ oceanic crust – (upper part- 0 °C; lower part- 1200 °C)
  • @ 400-700 Km = 1500-1900 °C
  • @ Junction of the mantle and outer core = 2900-3700 °C
  • @ Outer and inner Core = 4300 °C

 

Density

  • Outer thinner part is sedimentary rocks (0.8 to 1.6 km).
  • Below this there are crystalline rocks up to 3 km.
  • Average density of the earth is 5.5 g/cm3.
    • Outer layer (continents) = SIAL or granitic rocks- 2.7 g/cm3
    • Mantle and Oceanic crust = SIMA or Basaltic- 4.3 g/cm3
    • Core = NiFe- 11.0 g/cm3

Crust

  • Layer of solid rock that forms Earth’s outer “skin”
  • Above mohorovic discontinuity
  • Less than 1% of volume
  • It includes both dry land (Continental) and ocean floor (Oceanic)
  • Oceanic crust consists mostly of basalt
  • Continental crust consists mainly of granite

Mantle

  • Layer of solid, hot rock 40 kilometers beneath the surface- Largest layer
  • Between Mohovoric and Gutenberg discontinuity
  • Density = 3.3 g/cm3 at mohovoric and 5.7 g/cm3 at Gutenberg
  • Divided into layers
    1. Lithosphere – Uppermost part of mantle and the crust for a ridge layer about 100 kilometers thick
    2. Asthenosphere – Softer part of mantle below the lithosphere which is hotter and under increased pressure
    3. Lower Mantle – Solid material extending all the way to Earth’s core

Core

  • Made mostly of the metals Iron and Nickel (NiFe)
  • Consists of two parts
    • Outer core – Layer of molten metal that surrounds inner core  (P waves slow down, while S waves stop)
    • Inner core – Dense ball of solid metal
  • Movement of liquid outer core creates Earth’s magnetic field

Earth’s chemical composition

 

MINERALS
  • A mineral is a naturally occurring organic and inorganic substance, having an orderly atomic structure and a definite chemical composition and physical properties.
  • A mineral is generally composed of two or more elements but sometimes single element minerals like sulphur, copper, silver, gold, graphite etc. are also found.
  • These are usually solid and inorganic, and have a crystal structure.
  • Exceptions– Minerals such as coal, petroleum, and natural gas are organic substances found in solid, liquid, and gaseous forms respectively.
  • The basic source of all minerals is the hot magma in the interior of the earth.
  • Feldspar and quartz are the most common minerals found in rocks.

Some major minerals and their characteristics

 

Feldspar

  • Silicon and oxygen are common elements.
  • Half of the earth’s crust is composed of feldspar.
  • It is used in ceramics and glass making.

 

Quartz

  • It is one of the most important components of sand and granite.
  • It consists of silica.
  • It is a hard mineral virtually insoluble in water.
  • It is white or colourless and used in radio and radar.

 

Pyroxene

  • Pyroxene forms 10 per cent of the earth’s crust.
  • Commonly found in meteorites.
  • It is in green or black colour.

 

Amphibole

  • Used in asbestos industry.
  • Hornblende is another form of amphiboles.

 

Mica

  • Found in igneous and metamorphic rocks. It is used in electrical instruments.

 

Olivine

  • Used in jewellery.
  • Found in basaltic rocks.

 

Bauxite

  • It is the ore of aluminium (hydrous oxide of aluminium).
  • It is non-crystalline.

 

ROCKS
  • A rock is an aggregate of one or more minerals.
    • Do not have definite composition of mineral constituents.
  • Petrology is the scientific study of rocks.
  • Denudation: An erosive process of breaking and removing the rocks from the surface of the earth.
    • It is the wearing away of the terrestrial land by weathering, erosion, moving water, ice waves.
  • Lithification: Process of compaction that turns denuded sediments into the sedimentary rock.
    • g. sandstone, limestone, shale, chert

 

Types of rocks

  1. Igneous Rocks — Solidified from magma and lava
  2. Sedimentary Rocks — Result of deposition of fragments of rocks
  3. Metamorphic Rocks — Formed out of existing rocks undergoing recrystallization

Most Abundant = Igneous Rocks > Metamorphic Rocks > Sedimentary Rocks

 

Igneous Rocks

  • Formed out of magma and lava from the interior of the earth.
  • They are known as primary rocks.
  • Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the examples of igneous rocks.
  • Based on the presence of acid forming radical (silicon), igneous rocks are divided into Acid Rocks and Basic Rocks.

2 types

  1. Intrusive rocks (Granite)
  • Plutonic rocks
  • Slow cooling (at great depths) of molten material allows big-sized crystal. It results in large mineral grains.
  • Less dense and are lighter in color than basic rocks.

 

  1. Extrusive rocks (Basalt- Deccan Traps)
  • Volcanic rocks
  • Rapid cooling (at the surface) molten material prevents crystallization. It results in small and smooth grains.
  • Denser and darker in color.
  • Their origin under conditions of high temperatures makes them Unfossiliferous.

 

Sedimentary or Detrital Rocks

  • Formed as a result of lithification of denuded rocks (Exogenic agents lead to weathering and erosion of rocks exposed to it).
    • Deposits through compaction turn into rocks.
    • They are layered or stratified of varying thickness.
  • Sometimes, the deposits layers retain their characteristics even after lithification.
  • It possesses 75% of the earth’s crust but occupy only 5% on volume basis.
  • Till/Tillite à Ice deposited sedimentary rocks
  • Loess à Wind deposited sediments

Depending upon the mode of formation, they are classified into

  • Mechanically formed – sandstone, conglomerate, limestone, shale, loess etc.
  • Formed by mechanical agents like running water, wind, ocean currents, ice, etc.
  • Arenaceous rocks: More sand and big sized particles. They are porous and hard. E.g. sandstone.
  • Argillaceous rocks: More clay and are fine-grained. They are softer, impermeable and non-porous. They are easily weathered and eroded. E.g. shale.
  • Organically formed — geyserite, chalk, limestone, coal etc.
    • The remains of plants and animals buried under sediments undergo a transformation due to the heat and pressure from overlying layers.
    • Marine animals like corals and algae extract CaCO3 form sea water and deposit it in their skeletons. When they die, their skeletons together form limestone.
  • Chemically formed — chert, limestone, halite, potash etc.
  • Such rocks are precipitated chemically from solutions of one kind or another.
  • Example: Gypsum
  • Water containing minerals in caves evaporate and give rise to Stalactites and stalagmites.

 

Chief Characteristics of Sedimentary Rocks

  • These rocks consist of a number of layers or strata.
  • Marks of left behind water currents and waves etc.
  • Have fossils of plants and animals.
  • Generally porous (allow water to percolate through them).

Metamorphic Rocks

  • Metamorphic means ‘change of form’.
  • Metamorphism is a process by which already consolidated materials within the original rocks undergo recrystallization and reorganization.
  • These rocks form under the action of pressure, volume and temperature (PVT) changes.
  • Metamorphism occurs mainly by two processes
  • Dynamic Metamorphism
    • Rocks are forced down to lower levels by tectonic processes.
    • The minerals of sedimentary and igneous rocks re-crystallize under the influence of pressure without any appreciable chemical changes.
    • Examples
      • Granite à Gneiss
      • Clay, Shale à Schist

 

Thermal Metamorphism

  • Minerals of sedimentary and igneous rocks re-crystallize under the influence of high temperatures.
  • Examples
  • Sandstone à Quartzite
  • Clay, Shale à Slate à Phyllit
  • Coal à Anthracite, Graphite
  • Limestone à Marble
  • types of thermal metamorphism
    • Contact metamorphism
      • Rocks come in contact with hot intruding magma and lava and the rock materials recrystallise under high temperatures.
      • Quite often new materials form out of magma or lava are added to the rocks.
    • Regional metamorphism
      • Rocks undergo recrystallisation due to deformation caused by tectonic shearing together with high temperature or pressure or both.

 

Foliation or Lineation

  • Arrangement of rocks grains during the process of metamorphism.

Banding

  • Structure in metamorphic rocks in which minerals or materials of different groups are arranged into alternating thin to thick layers appearing in light and dark shades.

Rock cycle

  • A continuous process through which old rocks are transformed into new ones.
  • Igneous rocks are primary rocks and other rocks form from these rocks.

 

Oceans And Continents Distribution

To explain the present distribution of oceans and continents, various theories have been proposed.

Continental Drift Theory

  • By Alfred Wegener.
  • According to the continental drift theory, the world was made up of a single continent called Pangaea surrounded by ocean called Panthalassa through most of geologic time.
  • A sea called Tethys divided the Pangaea into two huge landmasses
    • Laurentia (Laurasia) to the north
    • Gondwanaland to the south
  • Drift started around 200 million years ago (Mesozoic Era). The continents began to break up and drift away from one another.
  • The continent eventually separated and drifted apart and the seven continents emerged that exist today

 

Force for Continental Drift

  • Pole-fleeing force (Rotation of the earth causes centrifugal effect)
  • Buoyant force (Object floats in the fluid due to this property)
  • Tidal force (Due to the attraction of the moon and the sun that develops tides in oceanic waters)
  • Gravitational force

Wegener believed that these forces would become effective when applied over many million years and the drift is still continuing.

 

Bouyancy = Buoyancy is an upward force exerted by a fluid on an immersed object in a gravity field.

Archimedes’ principle = Buoyant force on an object is equal to the weight of the fluid displaced by the object.

 

The drift was in two directions

  • Equator wards due to the interaction of forces of
    • Gravitational force
    • Pole-fleeing force
    • Buoyant force
  • Westwards due to tidal currents caused by earth’s rotation form west to east (Tidal currents act from east to west)

Evidence in support of Continental Drift

  1. Geomorphologic and geological similarities along the coasts of South America-Africa and Europe-North America.
      1. The Jig-Saw-Fit of Continents.
      2. Placer Deposits of gold in the Ghana coast without any source rock in nearby region.
  2. Tillite
      1. It is the sedimentary rock formed by glacier deposits.
      2. The Gondawana systems of sediments from India have counter parts in six different landmasses of the Southern Hemisphere.
  3. Same Age of rocks across the Oceans. For example
      1. Ranges in Canada match Norway and Sweden.
      2. Appalachian Mountains match UK Mountains.
  4. Fossil evidence for ancient climates
      1. Plant and animal fossils on very different continents
            • Mesosaurus, which was a freshwater reptile.
            • Glossopteris, which was a tropical fern.
      2. Same plants and animals on such different land masses indicates oneness of continents.

 

Drawbacks of Continental Drift Theory

  • Too general with silly and sometimes illogical evidences.
    • Buoyancy and Gravity act in opposite directions.
    • The Earth would have stopped rotation if the tidal force exerted by sun and moon is strong enough to rift the continents.
  • It doesn’t explain why drift initiated only in Mesozoic era and not before.

The continental drift theory was unable to provide the strong reason for continent movement. This issue was eliminated by the later studies.

 

Gyanbazi (Something Extra)

Density – Measure of how much mass there is in a volume of a substance.

3 types of heat transfer

1. Radiation – The transfer of energy through empty space. There is no direct contact between heat source and an object.

·   Sunlight warming Earth’s surface

2.  Conduction – Conduction takes place when two bodies of unequal temperature are in contact with one another, there is a flow of energy from the warmer to cooler body. The transfer of heat continues until both the bodies attain the same temperature or the contact is broken. Hence, it is a process of heat transfer by direct contact of particles of matter.

·    Metal spoon heating up in a pot of hot soup.

3.  Convection – Transfer of heat by the movement of a heated fluid (includes liquids and gases). Heat transfer by convection is caused by differences in temperature and density within a fluid

 

 

CONVECTIONAL CURRENT THEORY

 

  • Given by Arthur Holme.

Heat generated due to the decay of radioactive elements creates thermal differences in the mantle portion.

  • To vent out, the thermal differences builds a convection current cycle in the mantle.
    • Rising (ascending) limbs of these currents = Oceanic ridges are formed (Due to the divergence of the lithospheric plate)
    • Falling (descending) limbs of these currents = Trenches are formed (Due to the convergence of the lithospheric plate)
  • The movement of the lithospheric plate is driven by movement of magma in the mantle (Driven by convective process).

This system of currents exists in the entire mantle portion.

 

Mapping of the Ocean Floor

  • During the World War II, mankind reached to the bottom of ocean floor through the usage of submarines.
  • Detailed studies of bottom of ocean revealed that the floor is full of relief with mountain ranges, deep trenches etc.

Ocean Floor

  • Continental shelf: Angle is 1º, depth is 120-150 meter, and it extends generally 70 km into the sea. But this varies a lot
    • The continental shelf is virtually absent in west coast of South America.
    • It is 120 km wide in east coast of North America. In Bay of Bengal, it is very wide as well.
  • Continental slope: At the end of continental shelf slope steepens abruptly. Its end marks the end of continental blocks.
  • Continental rise: At the end of continental slope, slope becomes gentle again to 0.5º to 1º. Its end marks the end of continental margin.
  • Abyssal plains (Deep Sea Plain): Undulating plain lies 2-3 miles below sea level and cover 2/3rd of ocean floor.  Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plainscover more than 50% of the Earth’s surface.
  • Oceanic Ridges: The oceanic ridge system is a continuous underwater mountain range. It is created when magma rising between diverging plates of the lithosphere cools and forms a new layer of crust.
  • Abyssal hills: Sea hills on abyssal plains rising less than 1000 meters from the floor are called Abyssal hills.
  • Sea mounts: Sea hills on abyssal plains rising above 1000 meters from the floor are called sea mounts.
  • Guyots: Guyots are seamounts which have flat tops. All of them are generally of volcanic origin.
  • Submarine trenches/deeps: Long narrow and steep depression on abyssal plain is called a trench. The deeper trenches (> 5500 meters) are called deeps.
  • Canyons: Canyons are deep concave gorges on continental shelf, slope or rise.
  • Strait, sound / channel: Both straits and channels are narrow pieces of water connecting two larger bodies of water. Straits are narrower than a channel or sound.

 

PALEOMAGNETISM 
  • The word paleomagnetism literally means ancient magnetism.
  • Studies of different-aged rock give information about past strength and orientation of the Earth’s magnetic properties.
  • Minerals of rocks assumed the direction of magnetic field polarity when they were formed and preserved their property after the solidification.
  • Normal Polarity = North-seeking end of the compass needle points toward the present north magnetic pole.
  • Reversed Polarity = North-seeking end of the compass needle points toward the present south magnetic pole

Features of ocean floor

  • The mapping of the ocean floor and Paleomagnetic studies of rocks from oceanic regions reveals:
    • Volcanic eruptions are common all along the mid-oceanic ridges and they bring huge amounts of lava to the surface.
    • Remarkable similarities of rocks at equidistant locations on either side of the crest in terms of constituents and age.
    • Rocks closer to the mid-oceanic ridges are normal polarity and are the youngest. The age of the rocks increases as one move away from the crest.
    • Rocks on the oceanic crust are much younger than those on the continents.
    • The deep trenches have deep-seated earthquake occurrences while in the mid-oceanic ridge areas, the quake foci have shallow depths.

 

SEA FLOOR SPREADING

The features of oceanic relief provided many insights but following questions remained unanswered

  • Why there is no oceanic crust before the mid Mesozoic era?
  • Why the age of crust increases while going away from mid oceanic ridge?
  • Why the continuous creation of crust is not causing the increase of oceanic floor?

To answer the above questions, Harry Hess utilised the study of convection current and paleomganetism and proposed the theory of Sea Floor Spreading.

 

  • The younger age of the oceanic crust and the fact that the spreading of one ocean does not cause the shrinking of the other indicates the consumption of the oceanic crust.
  • Hot magma from Earth’s mantle rises up through the mid-oceanic ridges and constantly produces new oceanic crust.
  • Crust cools and flows sideways forming new seafloor. It is recycled millions of years later when it returns to the mantle by descending into the deep ocean trenches.
  • This indicates that the crust near the oceanic ridge would be youngest (due to its recent creation by outpour of basaltic lava from the interior of the earth) and near the trenches would be oldest.
  • Rate of sea floor spreading is decided by age and distance between two equal magnetic strips.
  • This indicates the formation and consumption of the oceanic crust is a cyclical process driven by convection currents in the mantle.
  • Seafloor spreading theory helps in providing explanation of continental drift in the theory of plate tectonics.

 

PLATE TECTONIC THEORY

The sea floor spreading theory explained the oceanic crust movement effectively but it did not explain the reason of continental plate movement. To remove this drawback, plate tectonic theory is given by McKenzie and Parker.

  • Tectonic plates are pieces of Earth’s crust and uppermost mantle, together referred to as the
  • The earth’s lithosphere is broken into distinct plates. These plates float on a ductile layer of asthenosphere (upper mantle) as a rigid unit.
  • Lithospheric plates (or crustal plates or tectonic plates) can be differentiated as
    • Minor plates or major plates
    • Continental plates (Arabian plate) or oceanic plates (Pacific plate)
      • Sometime a combination of both (Indo-Australian plate)
      • A plate may be referred to as the continental plate or oceanic plate depending on which of the two occupy a larger portion of the plate.
      • Oceanic plates = Simatic and relatively thinner
      • Continental plates  = Sialic and relatively thicker
  • Plate density (Denser plate goes for the subduction)
    • Oceanic Plate > Continental Plate
    • Bigger Plate > Smaller Plate

Types of Tectonics Plates

The earth’s lithosphere is divided into seven major and some minor plates.

 

Major tectonic plates

  • Antarctica and the surrounding oceanic plate
  • North American plate
  • South American plate
  • Pacific plate
  • India-Australia-New Zealand plate
  • Africa with the eastern Atlantic floor plate
  • Eurasia and the adjacent oceanic plate

Minor tectonic plates

  • Cocos plate: Between Central America and Pacific plate
  • Nazca plate: Between South America and Pacific plate
  • Arabian plate: Mostly the Saudi Arabian landmass
  • Philippine plate: Between the Asiatic and Pacific plate
  • Caroline plate: Between the Philippine and Indian plate (North of New Guinea)
  • Fuji plate: North-east of Australia.
  • Turkish plate
  • Aegean plate (Mediterranean region)
  • Caribbean plate
  • Juan de Fuca plate (between Pacific and North American plates)
  • Iranian plate

There are lots of other minor plates than the above mentioned one. Most of these minor plates were formed due to stress generated by converging major plates. For example, the Mediterranean Sea is divided into numerous minor plates because of the compressive force exerted by Eurasian and African plates.

 

Force for the Plate Movement

  • Convection current cycle

Types of Plate boundaries interaction

  • Divergence or Divergent Edge or the Constructive Edge
  • Convergence or Convergent Edge or Destructive Edge
  • Transcurrent Edge or Conservative Edge or Transform Fault.

 

Divergence forming or Divergent Edge or the Constructive Edge

  • Such edges are sites of earth crust formation (hence constructive) and volcanic earth forms are common along such edges.
  • Earthquakes (shallow focus) are common along divergent edges.
  • The sites where the plates move away from each other are called spreading sites.
  • The best-known example of divergent boundaries is the Mid-Atlantic Ridge.

At Ocean

  • Formation of Mid-oceanic ridges through which Basaltic magma erupts and moves apart (sea floor spreading).

At Continents

  • Formation of Rift Valley (East African Rift Valley on African and Somali plates).
  • Rifts are the initial stage of a continental break-up.
  • The continuous diverging force below the rifts can lead to the formation of a new ocean basin.

 

New Ocean Formation from Rift Valley

A huge rift forming in the Ethiopian Afar desert is expected to become the world’s newest ocean. When this happens, the Afar Rift will turn into a new ocean that will split the African continent and release the Horn of Africa from its land mass.

Convergence or Convergent Edge or Destructive Edge

There are mainly 3 ways in which convergence can occur

Oceanic-Oceanic Boundaries

  • When two oceanic plates collide, the denser plate sinks below the lighter plate.
    • Normally the older plate will subduct because of its higher density.
  • It forms trench along the boundary.
  • The rocks in the subduction zone become
    • Low density and high pressure magma forms that rises upward due to the buoyant force.
  • A continuous upward movement of magma creates volcanic islands on the ocean floor.
  • Earthquake and Volcanism is common.
  • The deepest oceanic trench, the Mariana Trench, is the result of the Pacific Plate moving beneath the Mariana Plate.
  • Example = Indonesian Archipelago, Philippine Island Arc

Oceanic-Continental Boundaries

  • When oceanic and continental plates collide, the oceanic plate undergoes subduction.
    • Oceanic plates are denser than continental plates, which mean they have a higher subduction potential.
    • They are constantly being pulled into the mantle, where they are melted and recycled into new magma.
    • It forms trench along the boundary. The trenches formed here are less deep than formed in ocean-ocean convergence.
  • The rocks in the subduction zone become metamorphosed.
    • Low density and high pressure magma forms that rises upward due to the buoyant force.
  • Earthquake and Volcanism is common.
  • A continuous upward movement of magma creates constant volcanic eruptions at the surface of the continental plate along the margin.
  • The Cascade Mountains of western North America and the Andes of western South America feature such active volcanoes.

Continental-Continental Boundaries

  • Continental-continental convergent boundaries pit large slabs of crust against each other.
  • There is very little subduction as most of the rock is too light to be carried very far down into the dense mantle.
    • The zone of collision may undergo crumpling and folding and folded mountains may emerge.
  • Magma cannot penetrate this thick crust; instead, it cools intrusively and forms granite. Highly metamorphosed rock, like gneiss, is also common.
    • Earthquake is frequent but no volcanic activity occurs.
  • The ocean basin or a sedimentary basin (geosynclinal sediments found along the continental margins) is squeezed between the two converging plates.
    • Himalayan mountains have come out of a great geosynclinecalled the Tethys Sea.
  • This is an orogenic collision. Himalayan Boundary Fault is one such example.

Geosyncline theory of mountain building

  • Geosyncline: A large-scale depression in the earth’s crust containing very thick deposits.

 

Phases of formation

  • Continent à Oceanic Crust ß Continent
    • Creation of depression between continental plates
    • Sediments deposit in this depression
    • Depression also contain marine origin sediments due to the presence of erstwhile ocean

 

Ocean-Ocean Convergence (Island-Arc Convergence) Ocean-Continent Convergence (Cordilleran Convergence) Continent-Continent Convergence (Himalayan Convergence)
Denser plate subduction and trench formation Oceanic plate subduction and trench formation Neither plate subduct or shallow subduction leading  crumpling and folding of plate
Earthquake and Volcanism Earthquake and Volcanism Earthquake
Low density high pressure Magma (matamorphosed rocks) formation that rises upward leading volcanic eruptions Magma (matamorphosed rocks) rises upward leading volcanic eruptions all along the boundaries Fragments of oceanic crust get plastered leading welding of two plates (known as Suture Zone)
Chain of volcanic islands formation known as Island Arc Extensive volcanic mountain formation known as Continental Arc Fold mountain formation
Examples = Indonesian Archipelago, Philippine Island Arc Examples = Rockies (North America) and Andes (South America) Mountains Examples = Himalayas, Alps, Urals, Appalachians

 

  • There are evidences of marine origin sediments have been found in Karakoram range of Trans-Himalayas. This shows the presence of ocean in between of Indian and Eurasian plate before the collision that had been ultimately transformed into the geosyncline due to the compressive forces of two plates.

 

ranscurrent Edge or Conservative Edge or Transform Fault

  • Formed when two plates passes each other and grind against each other.
  • There is only deformation of the existing landform (no creation or destruction).
    • Neither creation of crust (no basaltic lava eruption) nor destruction of crust (no subduction)
  • Seismic activity occurs during grinding of passing plates.
  • Example = San Andreas Fault @ western coast of USA

 

Ring of Fire

  • The Ring of Fire is also known as the Circum-Pacific Belt.
  • It is a path along the Pacific Ocean characterized by active volcanoes and frequent earthquakes.
  • Along much of the Ring of Fire, plates overlap at convergent boundaries (falling limbs creating trenches) called subduction zones.
    • Plate that is underneath is pushed down (subducted) by the plate above, melts and becomes magma.
  • The abundance of magma so near to Earth’s surface gives rise to conditions ripe for volcanic activity.

Rates of Plate Movement and its significance

  • The strips of normal and reverse magnetic field that parallel the mid-oceanic ridges help scientists determine the rates of plate movement.
  • These rates of movement for these plates vary considerably.
  • The Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and the East Pacific Rise near Easter Island, in the South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr).

Importance of plate tectonic

  • It helps in understanding of large-scale geological phenomena, such as earthquakes, volcanoes, and the existence of ocean basins and continents.
  • It aids in the interpretation of landforms.
  • The concept of plate tectonics explains mineralogy. New minerals pour up from the mantle and deposits on lithosphere. These rocks are the source of many minerals.
    • The famous Pacific Ring of fire known for its violent volcanic activity is also a ring of mineral deposits.

 

Formation of Himalayas
  • 225 million years ago (mya) India was a large island situated off the Australian coast and separated from Asia by the Tethys Ocean.
  • The supercontinent Pangea began to break up 200 Ma and India started a northward drift towards Asia.
  • 80 Ma India was 6,400 km south of the Asian continent but moving towards it at a rate of between 9 and 16 cm per year.
    • At this time Tethys Ocean floor would have been subducting northwards beneath Asia and the plate margin would have been a Convergent oceanic-continental one just like the Andes today.
  • From about 50-40 Ma the rate of northward drift of the Indian continental plate slowed to around 4-6 cm per year.
    • This slowdown is interpreted to mark the beginning of the collision between the Eurasian and Indian continental plates, the closing of the former Tethys Ocean, and the initiation of Himalayan uplift.
  • The Eurasian plate was partly crumpled and buckled up above the Indian plate but due to their low density/high buoyancy neither continental plate could be subducted.
  • This caused the continental crust to thicken due to folding and faulting by compressional forces pushing up the Himalaya and the Tibetan Plateau.
  • The Himalayas are still rising by more than 1 cm per year as India continues to move northwards into Asia, which explains the occurrence of shallow focus earthquakes in the region today.

 

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