Table of Contents
- In this post we will study about one of the most important concept of geomorphology called Plate Tectonics. This is the third post in ‘Tectonics’ after ‘Continental Drift Theory’ and ‘See Floor Spreading Theory’.
- In 1967, McKenzie and Parker suggested the theory of plate tectonics. The theory was later outlined by Morgan in 1968.
- By then, the ‘continental drift theory’ was completely discarded with the emergence of ‘convectional current theory’ and ‘see floor spreading theory’.
- Both ‘convectional current theory’ and ‘see floor spreading’ paved the way for the Theory of Plate Tectonics.
- According to the theory of plate tectonics, the earth’s lithosphere is broken into distinct plates which are floating on a ductile layer called asthenosphere (upper mantle). 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 (sometimes called crustal plates, tectonic plates) vary from minor plates to major plates, continental plates (Arabian plate) to oceanic plates (Pacific plate), sometime a combination of both continental and oceanic plates (Indo-Australian plate).
- The movement of these crustal plates causes the formation of various landforms and is the principal cause of all earth movements.
Rates of Plate Movement
- The Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and the East Pacific Rise in the South Pacific [about 3,400 km west of Chile], has the fastest rate (more than 15 cm/yr).
- Indian plate’s movement during its journey from south to equator was one of the fastest plate movements.
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 many more minor plates other than the above mentioned plates. Most of the 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.
- The figure below shows the changes in landform with time due to the interaction of various plates.
Force for the Plate Movement
- The slow movement of hot, softened mantle that lies below the rigid plates is the driving force behind the plate movement.
- The heated material rises to the surface, spreads and begins to cool, and then sinks back into deeper depths (convection currents – explained in the previous post – See Floor Spreading). This cycle is repeated over and over to generate what scientists call a convection cell or convective flow.
- Heat within the earth comes from two main sources: radioactive decay and residual heat. Arthur Holmes first considered this idea in the 1930s, which later influenced Harry Hess’ thinking about seafloor spreading.
Plate Tectonics – Interaction of Plates
- Major geomorphological features such as fold and block mountains, mid-oceanic ridges, trenches, volcanism, earthquakes etc. are a direct consequence of interaction between various lithospheric plates.
- There are three ways in which the plates interact with each other.
Divergence forming Divergent Edge or the Constructive Edge
- As the name itself suggests, in this kind of interaction, the plates diverge [move away from each other].
- Mid-oceanic ridges are formed due to this kind of interaction. Here, the basaltic magma erupts and moves apart (see floor spreading).
- On continents, East African Rift Valley is the most important geomorphological feature formed due to divergence of African and Somali plates.
- 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 the mid-oceanic ridge in Atlantic ocean, the American Plate(s) is/are separated from the Eurasian and African Plates.
Convergence forming Convergent Edge or Destructive Edge
- In this kind of interaction, two lithospheric plates collide against each other (in detail in the next post).
- The zone of collision may undergo crumpling and folding and folded mountains may emerge.
- This is an orogenic collision. Himalayan Boundary Fault is one such example.
- When one of the plates is an oceanic plate, it gets embedded in the softer asthenosphere of the continental plate and as a result, trenches are formed at the zone of subduction.
- The subducted material gets heated, up and is thrown out forming volcanic islands and dynamic equilibrium is achieved
- There are mainly three ways in which convergence can occur.
- between an oceanic and continental plate;
- between two oceanic plates; and
- between two continental plates.
Transcurrent Edge or Conservative Edge or Transform Fault
- Formed when two plates move past each other.
- In this kind of interaction, two plates grind against each other and there is no creation or destruction of landform but only deformation of the existing landform. [Crust is neither produced nor destroyed as the plates slide horizontally past each other].
- In oceans, transform faults are the planes of separation generally perpendicular to the midoceanic ridges.
- San Andreas Fault along the western coast of USA is the best example for a transcurrent edge on continents.
Evidence in Support of Plate Tectonics
- Evidences for both See Floor Spreading and Plate tectonics are complimentary (almost same evidences).
- Paleomagnetic rocks are the most important evidence. The orientation of iron grains on older rocks shows an orientation which points to the existence of the South Pole, once upon a time, somewhere between the present-day Africa and Antarctica (Paleomagnetism).
- Older rocks form the continents while younger rocks are present on the ocean floor. On continents, rocks of upto 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 (western part of Pacific floor). As we move, towards ridges, still younger rocks appear. This points to an effective spread of sea floor (See floor spreading is almost similar to plate tectonics except that it examines the interaction between oceanic plates only) along oceanic ridges which are also the plate margins.
- 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.
- 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.
- The fact that all plate boundary regions are areas of earthquake and volcanic disturbances goes to prove the theory of plate tectonics.
Significance of Plate Tectonics
- For the earth scientists, it is a fundamental principle for study. For physical geographers, this approach is an aid in interpretation of landforms.
- New minerals are thrown up from the core with the magmatic eruptions. Economically valuable minerals like copper and uranium are found more frequently near the plate boundaries.
- On the basis of present knowledge of crustal plate movement, the shape of landmasses in future can be guessed. 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.
Movement Of The Indian Plate
- The Indian plate includes Peninsular India and the Australian continental portions.
Indian Plate Boundaries
- The subduction zone along the Himalayas forms the northern plate boundary in the form of continent — continent convergence.
- In the east, it extends through Rakinyoma Mountains (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 divergence of Somali plate and Arabian plate) southeastward along the Chagos Archipelago (Formed due to hotspot volcanism).
- The boundary between India and the Antarctic plate is also marked by oceanic ridge (divergent boundary) running in roughly W-E direction and merging into the spreading site, a little south of New Zealand.
- India was a large island situated off the Australian coast, in a vast ocean.
- 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 ago at the time when Pangaea broke.
- India collided with Asia about 40-50 million years ago causing rapid uplift of the Himalayas.
- The positions of India since about 71 million years till the present are shown in the Figure. It also shows the position of the Indian subcontinent and the Eurasian plate.
- About 140 million years before the present, the subcontinent was located as south as 50◦ S. latitude. The two major plates were separated by the Tethys Sea and the Tibetan block was closer to the Asiatic landmass.
- 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. This started somewhere around 60 million years ago and continued for a long period of time.
- Note that the subcontinent was still close to the equator. From 40 million years ago and thereafter, the event of formation of the Himalayas took place.
- Scientists believe that the process is still continuing and the height of the Himalayas is rising even to this date.
- Around 220 million years ago, around the time that Pangea was breaking apart, India started to move northwards.
- It travelled some 6,000 kilometres before it finally collided with Asia around 40 to 50 million years ago.
- Then, part of the Indian landmass began to go beneath the Asian plate, moving the Asian landmass up, which resulted in the rise 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 mountain range grew very rapidly in comparison to most mountain ranges, and it’s actually still growing today.
- The continued growth in the Himalayas is likely due to the Indian tectonic plate still moving slowly but surely northward. We know the plate is still moving in part because of the frequent earthquakes in the region.
Comparison: Continental Drift – See Floor Spreading – Plate Tectonics
|Continental Drift||See Floor Spreading||Plate Tectonics|
|Explained by||Alfred Wegener in 1920s||Arthur Holmes explains Convectional Current Theory in 1930s.|
Based on convectional current theory, Harry Hess explains See Floor Spreading in 1940s
|In 1967, McKenzie and Parker suggested the theory of plate tectonics. The theory was later outlined by Morgan in 1968|
|Theory||Explains Movement of Continents only||Explains Movement of Oceanic Plates only||Explains Movement of Lithospheric plates that include both continents and oceans.|
|Forces for movement||Buoyancy, gravity, pole fleeing force, tidal currents, tides,||Convection currents in the mantle drag crustal plates||Convection currents in the mantle drag crustal plates|
|Evidences||Apparent affinity of physical features, botanical evidence, fossil evidence, Tillite deposits, placer deposits, rocks of same age across different continents etc.||Ocean bottom relief, Paleomagnetic rocks, distribution of earthquakes and volcanoes etc.||Ocean bottom relief, Paleomagnetic rocks, distribution of earthquakes and volcanoes, gravitational anomalies at trenches, etc.|
|Drawbacks||Too general with silly and sometimes illogical evidences.||Doesn’t explain the movement of continental plates||———————|
|Acceptance||Totally discarded||Not complete||Most widely accepted|
|Usefulness||Helped in the evolution of convectional current theory and see floor spreading theory||Helped in the evolution of plate tectonics theory||Helped understand various geographical features.|
a. Polar fleeing force relates to:
- Revolution of the Earth
- Rotation of the earth
b. Which one of the following is not a minor plate?
c. Which one of the following facts was not considered by those while discussing the concept of sea floor spreading?
- Volcanic activity along the mid-oceanic ridges.
- Stripes of normal and reverse magnetic field observed in rocks of ocean floor.
- Distribution of fossils in different continents.
- Age of rocks from the ocean floor.
d. Which one of the following is the type of plate boundary of the Indian plate along the Himalayan mountains?
- Ocean-continent convergence
- Divergent boundary
- Transform boundary
- Continent-continent convergence
Answer in about 30 words.
- What were the forces suggested by Wegener for the movement of the continents?
- How are the convectional currents in the mantle initiated and maintained?
- What is the major difference between the transform boundary and the convergent or divergent boundaries of plates?
- What was the location of the Indian landmass during the formation of the Deccan Traps?
Answer in about 150 words
- What are the evidences in support of the continental drift theory?
- Bring about the basic difference between the drift theory and Plate tectonics.
- What were the major post-drift discoveries that rejuvenated the interest of scientists in the study of distribution of oceans and continents?