PLATE TECTONICS

TECTONICS

  • Ridges, Trenches, seamounts, etc are the various physical features found on the oceanic surface. Ridges and trenches tell us about natural boundaries between various lithospheric plates.
  • Tectonics is the scientific study of convection currents in the mantle and collisions of the lithospheric plates, folding, faulting, volcanism that control the structure of the Earth’s crust and its evolution through time. The large-scale deformation of the lithospheric crust and upper mantle above asthenosphere and the forces that produce such deformation.
  • It deals with the folding and faulting associated with mountain building; the large-scale, gradual upward and downward movements of the crust.

THEORIES PUT FORWARD TO EXPLAIN TECTONICS

 

 

CONTINENTAL DRIFT THEORY (ALFRED WEGENER, 1922)

 

  • Alfred Wegener suggested continental Drift Theory in the 1920’s.
  • He proposed that their existed one big landmass called as Pangaea which was covered by one big ocean called Panthalassa.
  • A sea called Tethys divided the Pangaea into two huge landmasses: Laurasia to the north and Gondwanaland to the south of Tethys.

 

 

  • Drift started around 200 million years ago and the continents began to break up and drift away from one another.

A) Forces behind the drifting of continents.

  • According to Wegener, the drift was in two directions:
  • Equator wards 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), and
  • 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 of the moon and of 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). This increase in centrifugal force has led to pole fleeing, as per Wegener.
  • Tidal force is due to the attraction of the moon and the sun that develops tides in oceanic waters.
  • According to Wegener, these forces would become effective when applied over many million years, and the drift is continuing.

B) Various Evidence in support of Continental Drift

 

 

  1. Apparent Affinity of Physical Features

 

  • The bulge of Brazil fit into the Gulf of Guinea in Africa. Similarly, Greenland seems to fit in well with Ellesmere and Baffin islands of Canada.
  • The west coast of India, Madagascar and Africa seem to have been together.
  • North and South America on one side and Africa and Europe on the other fit along the mid-Atlantic ridge.
  • The Caledonian and Hercynian mountains of Europe and the Appalachians of USA seem to be one continuous series.

This point was criticised by following ways –

  • Coastlines are a temporary feature and are liable to change.
  • Several other combinations of fitting in of unrelated landforms can be attempted.
  • Continental Drift Theory shifts India’s position too much to the south, distorting its relationship with the Mediterranean Sea and the Alps.
  • The gravity of the earth, the buoyancy of the seas and the tidal currents were given as the main factors causing the drift seems irrational because these small factors cannot cause a drift of such a magnitude.
  1. Botanical Evidence

 

 

  • Presence of glossopteris vegetation in Carboniferous rocks of India, Australia, South Africa, Falkland Islands (Overseas territory of UK), Antarctica and can be explained from the fact that parts were linked in the past but it may be an assumption as similar vegetation is found in unrelated parts of the worldlike Afghanistan, Iran and Siberia.
  1. Distribution of Fossils
  • The observations that Lemurs occur in India, Madagascar and Africa led some to consider a contiguous landmass “Lemuria” linking these three landmasses.
  • Mesosaurus was a small reptile adapted to shallow brackish water. The skeletons of these are found only in South Africa and Brazil. The two localities presently are 4,800 km apart with an ocean in between them.
  1. Rocks of Same Age Across the Oceans
  • The belt of ancient rocks of 2,000 million years from Brazil coast matches with those from western Africa but the Rocks of the same age and similar characteristics are found in other parts of the world too.
  1. Tillite deposits
  • Tillite deposits are sedimentary rocks formed out of deposits of glaciers.
  • The Gondwana system of sediments is found in India, Africa, Falkland Island, Madagascar, Antarctica and Australia (all were previously part of Gondwana).
  • Overall resemblance demonstrates that these landmasses had remarkably similar histories.

 

 

  1. Placer Deposits
  • Rich placer deposits of gold are found on the Ghana coast (West Africa) but the source (gold-bearing veins) are in Brazil, and it is obvious that the gold deposits of Ghana are derived from the Brazil plateau when the two continents lay side by side.

 

 

 

C) Drawbacks of 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 general assumptions
  • 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.

 

SEAFLOOR SPREADING

To understand the concept of Seafloor Spreading first discussion of below mentioned two theories is important: –

A) Convection Current Theory

  • Convection Current Theory is the soul of Seafloor Spreading Theory.
  • Arthur Holmes in 1930s discussed the possibility of convection currents in the mantle.
  • These are generated due to radioactive elements causing thermal differences in the mantle.

 

 

  • The intense heat generated by radioactive substances in the mantle (2600 km below the earth surface) needs escape giving rise to the formation of convection currents in the mantle.
  • Wherever rising limbs of these currents meet, oceanic ridges are formed on the seafloor due to the divergence of the lithospheric tectonic plates, and wherever the failing limbs meet, trenches are formed due to the convergence of the lithospheric tectonic plates.
  • The movement of the lithospheric plates is caused by the movement of the magma in the mantle.

B) Paleomagnetism

 

 

  • Paleomagnetism is the study of the record of earth’s magnetic field with the help of magnetic fields recorded in rocks, sediment, or archaeological materials.
    • Rocks formed from underwater volcanic activity are mainly basaltic (low silica, iron-rich) that makes up most of the ocean floor.
    • Basalt contains magnetic minerals, and as the rock is solidifies, these minerals align themselves in the direction of the magnetic field.
    • Paleomagnetic studies of rocks have demonstrated that the orientation of the earth’s magnetic field has frequently alternated (geomagnetic reversal) over geologic time.
  • The polarity of the Earth’s magnetic field and magnetic field reversals are thus detectable by studying the rocks of different ages.
  • The regions that hold the unique record of earth’s magnetic field lie along the mid-ocean ridges where the sea floor is spreading.
    • The fissure or vent (in between the ridge) between the plates allowed the magma to rise and harden into a long narrowband of rock on either side of the vent.
    • Rising magma assumes the polarity of Earth’s geomagnetic field at the time before it solidifies on the oceanic crust.
    • As the conventional currents pull the oceanic plates apart, the solidified band of rock moves away from the vent (or ridge), and a new band of rock takes its place a few million years later when the magnetic field was reversed. Thisresults in this magnetic striping where the adjacent rock bands have opposite polarities.
    • On studying the paleomagnetic rocks on either side of the oceanic ridges, it is found that alternate magnetic rock stripes were flipped so that one stripe would be of normal polarity and the next,
    • Hence, paleomagnetic rocks on either side of the mid-ocean or submarine ridges provide the most important evidence to the concept of Sea Floor Spreading.
    • This process repeats over and over giving rise to a series of narrow parallel rock bands on either side of the ridge and alternating pattern of magnetic striping on the seafloor.

 

 

C) The concept of Sea Floor Spreading by Harry Hess

      • When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere.
      • Basaltic magma rises from the fractures and cools on the ocean floor to form new seafloor.
      • The newly formed seafloor (oceanic crust) then gradually moves away from the ridge, and its place is taken by an even newer seafloor and the cycle repeats.
      • With time, older rocks are spread farther away from the spreading zone while younger rocks will be found nearer to the spreading zone.

 

 

D) Evidence for Seafloor Spreading

  1. Nature of oceanic rocks around mid-ocean ridges
  • Rocks on either side of the crest of oceanic ridges having equidistant locations from the crest were found to have similarities both in terms of their constituents, their age and magnetic orientation.
  • Rocks closer to the mid-oceanic ridges have normal polarity and are the youngest and the age of the rocks increases as one moves away from the crest (ridge).
  • The rocks of the oceanic crust near the oceanic ridges are much younger than the rocks of the continental crust.
  1. Distribution of Earthquakes and Volcanoes along the mid-ocean ridges

 

 

  • The normal temperature gradient on the sea floor is 9.4° C/300 m, but near the ridges it becomes higher, indicating an upwelling of magmatic material from the mantle.
  • Dots in the central parts of the Atlantic Ocean and other oceans are almost parallel to the coastlines. This indicates that the seafloor has widened with time.
  • In general, the foci of the earthquake in the areas of mid-oceanic ridges are at shallow depths whereas, along the Alpine-Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-seated ones.

 

 

 

 

PLATE TECTONICS

  • McKenzie and Parker suggested the theory of plate tectonics in
    • According to this theory, the earth’s lithosphere is broken into distinct plates which are floating on a ductile layer called asthenosphere (upper part of the mantle). And these plates move horizontally over the asthenosphere as rigid units.
    • The lithosphere includes the crust and top mantle with its thickness range varying between 5-100 km in oceanic parts and about 200 km in the continental areas.
    • The oceanic plates contain mainly the Simatic crust and are relatively thinner, while the continental plates contain Sialic material and are relatively thicker.
    • Lithospheric plates vary from minor plates to major plates, continental plates (Arabian plate) to oceanic plates (Pacific plate), sometimes a combination of both continental and oceanic plates (Indo-Australian plate).
    • The movement of these crustal plates (due to convection currents in the mantle) causes the formation of various landforms on the surface of the earth.

A) Major tectonic plates

  1. Antarctica and the surrounding oceanic plate
  2. North American plate
  3. South American plate
  4. Pacific plate
  5. India-Australia-New Zealand plate
  6. Africa with the eastern Atlantic floor plate
  7. Eurasia and the adjacent oceanic plate

 

 

B) Minor tectonic plates.

    1. Cocos plate: Between Central America and Pacific plate
    2. Nazca plate: Between South America and Pacific plate
    3. Arabian plate: Mostly the Saudi Arabian landmass
    4. Philippine plate: Between the Asiatic and Pacific plate
    5. Caroline plate: Between the Philippine and Indian plate (North of New Guinea)
    6. Fuji plate: North-east of Australia
    7. Turkish plate
    8. Aegean plate (Mediterranean region)
    9. Caribbean plate
  • Juan de Fuca plate (between Pacific and North American plates)
  • Iranian plate
  • Most of these minor plates were formed due to stress created by converging major plates.
  • Example: the Mediterranean Sea is divided into numerous minor plates due to the compressive force exerted by Eurasian and African plates.

C) Interaction of Plates

    • Major geomorphological features such as fold and block mountains, mid-oceanic ridges, trenches, volcanism, earthquakes etc. are a direct consequence of the interaction between various lithospheric plates.
    • There are three ways in which the plates interact with each other.

 

 

  1. DIVERGENT Boundary / Constructive Edge
    • When plates diverge from each other Mid-ocean ridges (e.g. Mid-Atlantic Ridge) are formed making way for the basaltic magma to erupt and move apart (seafloor spreading).
    • On continents, East African Rift Valley is the most important geomorphologic feature formed due to the divergence of African and Somali plates.
    • Divergent edges are sites of earth crust formation (hence the name constructive edge), and volcanic earth forms are common along such edges.
    • Earthquakes (shallow focus) are common along divergent edges.

 

 

  1. Convergent Boundary or Destructive Edge
    • In this kind of interaction, two lithospheric plates collide against each other.
    • The zone of collision may undergo crumpling and folding, and folded mountains may emerge (orogenic collision). Himalayan Boundary Fault is a great example.
    • When one of the plates is an oceanic plate, it subsides and gets embedded in the softer asthenosphere below the continental plate, and as a result, trenches are formed at the zone of subduction.
    • Near the convergent edge a part of the crust is destroyed, hence the name Destructive
    • The subducted material gets heated up and is thrown out forming volcanic island arc and continental arc systems.

 

 

  1. Transform boundaries
    • The two plates slip against each other, and there is no creation or destruction of landform but only deformation of the existing landform.
    • San Andreas Fault (Silicon Valley lies dangerously close to the Faultline) along the western coast of USA is the best example for a transcurrent edge on continents.

 

 

D) Evidence in Support of Plate Tectonics

  1. Paleomagnetism: Paleomagnetic rocks are the most important evidence.
  2. Older rocks form the continents while younger rocks are present on the ocean floor .On continents, rocks of up to 3.5 billion years old can be found while the oldest rock found on the ocean floor is not more than 75 million years old.
  3. Gravitational anomalies
    • In trenches, where subduction has taken place (convergent edge), the value of gravitational constant ‘g’ is less. This indicates a loss of material.
    • For instance, gravity measurements around the Indonesian islands have indicated that large gravity anomalies are associated with the oceanic trench bordering Indonesia.
  1. Earthquakes and Volcanoes
  • Plate boundary regions are areas of earthquake and volcanic disturbances goes to prove the theory of plate tectonics.

E) The significance of Plate Tectonics

  • Almost all major landforms formed are due to plate tectonics.
  • New minerals are thrown up from the core with the magmatic eruptions.
  • Economically valuable minerals like copper and uranium are found near the plate boundaries.
  • It will help in extrapolating the shape of landmasses in future.
  • For instance, if the present trends continue, North and South America will separate. A piece of land will separate from the east coast of Africa. Australia will move closer to Asia.

 

THE MOVEMENT OF INDIAN PLATE

  • The Indian plate includes Peninsular India and the Australian continental portions.

A) Indian Plate Boundaries

  • The subduction zone along the Himalayas forms the northern plate boundary in the form of continent-continent
  • In the east, it extends through Arakan Yoma of Myanmar towards the island arc along the Java Trench.
  • The eastern margin is a spreading site lying to the east of Australia in the form of an oceanic ridge in SW Pacific.
  • The Western margin follows Kirthar Mountain of Pakistan. It further extends along the Makrana coast (Pakistan and Iranian coasts) and joins the spreading site from the Red Sea rift (Red Sea rift is formed due to the divergence of Somali plate and Arabian plate)south-eastward along the Chagos Archipelago (Formed due to hotspot volcanism).
  • The boundary between India and the Antarctic plate is also marked by an oceanic ridge (divergent boundary) running in roughly W-E direction and merging into the spreading site, a little south of New Zealand.

B) The Movement explained
 

 

  • India was a large island situated off the Australian coast. The Tethys Sea separated it from the Asian continent till about 225 million years ago.
  • India is supposed to have started her northward journey about 200 million years (Pangaea broke).
  • About 140 million years ago, the subcontinent was located as south as 50S latitude.
  • The Tethys Sea separated the Indian plate and the Eurasian plate.
  • The Tibetan block was a part of the Asiatic landmass.
  • India collided with Asia about 40-50 million years ago causing rapid uplift of the Himalayas.
  • It’s thought that India’s coastline was denser and more firmly attached to the seabed, which is why Asia’s softer soil was pushed up rather than the other way around.
  • The process is continuing, and the height of the Himalayas is rising even to this date.
  • The northward movement of the Indian tectonic plate pushing slowly against the Asiatic plate is evident by the frequent earthquakes in the region.
  • During the movement of the Indian plate towards the Asiatic plate, a major event that occurred was the outpouring of lava and formation of the Deccan Traps (which are shield volcano).
  • This Shield volcanism started somewhere around 60 million years ago and continued for a long period.

 

error: Content is protected !!
  • Sign up
Lost your password? Please enter your username or email address. You will receive a link to create a new password via email.
Cart Item Removed. Undo
  • No products in the cart.
%d bloggers like this: