A plate is made part of lithosphere which comprises of the complete solid crust and the top solid mantle. The whole lithospheric earth surface is divided into seven major and many minor plates. Plate means the top solid fractured part of the lithosphere covering the whole surface of the earth as a solid rocky layer. Plates move over the asthenosphere horizontally as rigid units. Tectonic means the movement and associate building of plate.
Plates can be major plate and minor plate based on size. For example, India-Australia-New Zealand plate is a major plate, whereas the Arabian plate is a minor plate. And based on nature, it is the continental plate or oceanic plate, decided by whichever occupy a large part of the plate. For example, the Eurasian plate may be called as a continental plate, whereas Antarctica and the surrounding oceanic plate is mostly an oceanic plate.
The plate tectonics theory proposes that the earth's lithosphere is divided into seven major and various minor plates. The plate movement results in the building up of stresses within the plates and the continental rocks above, which leads to folding, faulting and volcanic activity. The major plates are surrounded by young fold mountains, trenches, ridges and faults.
Major plates of the world:
- Antarctica and the surrounding oceanic plate
- North American (including western Atlantic floor separated from the South American plate alongside the Caribbean islands) plate
- South American (including western Atlantic floor separated from the North American plate alongside the Caribbean islands) plate
- Pacific plate
- India-Australia-New Zealand plate
- Africa with the eastern Atlantic floor plate
- Eurasia and the adjacent oceanic plate.
Important minor plates of the world:
- Cocos plate: Between Central America & Pacific plate
- Nazca plate: Between South America & Pacific plate
- Arabian plate: Mostly the Saudi Arabian landmass
- Philippine plate: Between the Asiatic & Pacific plate
- Caroline plate: Between the Philippine & Indian plate (North of New Guinea)
- Fuji plate: North-east of Australia.
Type of convectional movements:
- Rising convection (Divergent movement)
- Sinking convection (Convergent movement)
- Rising & sinking but not align convection (Slide past or Transform movement)
Types of plate margin interaction:
- Oceanic-Oceanic plates
- Oceanic-continental plates
- Continental-continental plates
Plate types, movements and related outcomes:
- Divergent movement: Where the new crust is formed as the plates pull away from each other. Spreading sites is the place where the plates move away from each other.
- Continental-Continental: When divergent convection flows below two continental plates due to tensional force, plates crackdown and create faults and move away, forming block mountains and rift valleys. E.g. the East African Great Rift Valley. Block mountains are also volcanic peaks. Some depression turned into lakes.
- Oceanic-Oceanic: The magma comes out of asthenosphere as it rises and due to its denser and hot nature, it starts spreading in the horizontal direction which takes time to cool down. Magma is basaltic and forms elongated sea between rifts and later forming the oceanic floor — for example, Red sea.
- Due to rising convection, magma comes out of the earth, and it starts to form mid-oceanic ridges. E.g. the Mid-Atlantic Ridge (between the South America and Africa continents separated by Atlantic ocean) and the East Pacific Rise. Formation of volcanism on Mid Oceanic Ridge with silent eruption is also present in this divergence. The mid-oceanic ridges formed along the divergent oceanic-oceanic boundary are longest mountains stretching nearly 17000 km.
- Continental-Oceanic: Basaltic magma will always form oceanic floor only, and it is not possible that magma comes out and forms continent on one side and ocean on the other side and hence continental-oceanic divergence is not possible.
The divergent boundary will form oceanic plates only, and it does not form a continental plate. Magma forms an oceanic crust on both sides. Even if it starts with the oceanic-continental divergence it turns into oceanic-oceanic divergence as it cannot remain continuously as continental-oceanic.
Outcomes of these movements lead to the various forms such as HORST (peak) and the GRABEN (valley) leading to block mountains and rifts, respectively. For example, Black Forest, Vosges are Horst and Rhine river is Graben (Germany and France) & Vindhyachal range, Satpura rang are Horst and Narmada river is Graben (India).
- Convergent movement: Where one plate dived under another, and the crust is destroyed. A subduction zone is a location where the sinking of a plate occurs.
- Oceanic-Oceanic: When two oceanic plates converge due to sinking convection, and compressional force; hence oceanic crust starts to subduct Whichever crust is denser and heavier or relatively older goes down and start to subduct first. After partial subduction, melting starts. Magma created by melting of the lithospheric plate, which is lighter, less dense and less hot compared to magma below the asthenosphere, try to move upward and fracture the plate and erupt through the plate which is not subducted and cool down faster to form more vertical formation like seamounts and Island Arc. Examples of such arcs are Japan, the Philippines, the Tonga Islands, the Aleutian Islands, and the West Indies Islands etc.
- Oceanic-Continental: When oceanic-continental plate converges the lighter continental plate folds at its margin, forming what is called as Anticline (upper part) and Syncline (lower part). The anticline is called a marginal fold mountain formed on the margin of the continent. The folds can be more than one creating parallel fold mountains, also called as cordilleras. Such fold mountains are volcanic peaks also. Compression gives fold mountains, and subduction gives volcanic peaks- for example, Andes Mountains off the west coast of the U.S. and Rockies (Mt. St. Helens) and Chimborazo.
- Continental-Continental: When both the continental plates convergence, there is no subduction as the continents never subduct, and there is the only compression between continents which leads to its folding, and it forms Fold Mountains (not marginal) which are on the interior of continents along with compressed marine deposits of the un-subducted oceanic plate. E.g. Himalaya mountains, the sea that was compressed to form marine deposits was Tethys sea.
The Indian plate Movement:
The Indian plate consists of Peninsular India and the Australian continental parts. The continent-continent convergence boundary is in the northern plate with the subduction zone alongside the Himalayas. It extends in the east through Rakinyoma Mountains of Myanmar in the direction of the island arc along the Java Trench. To the east of Australia, the eastern margin is a spreading site lying inside the shape of an oceanic ridge in South-West Pacific. The Western margin follows Pakistan's Kirthar Mountain. It, in addition, extends alongside the Makrana coast and joins the spreading site from the Red Sea rift south-eastward alongside the Chagos Archipelago. The boundary between the Antarctic plate and India is also marked by way of an oceanic ridge (divergent boundary) running in West-East route and merging into the spreading site, a little south of New Zealand.
India was a large island located off the Australian coast, in a boundless ocean. The Tethys Sea separated India from the Asian continent until approximately 225 million years ago. It is supposed that India had started her northward journey approximately two hundred million years ago at the time when Pangaea broke. India collided with Asia approximately 40-50 million years in the past resulted in fast uplift of the Himalayas. Nearly 140 million years before the present, the subcontinent was positioned at the south as 50 degrees South Latitude. The Tethys Sea has separated the two primary plates, and the Tibetan block was near to the Asiatic landmass.
During the Indian plate movement toward the Eurasian plate, a major event that happened was the eruption of lava and formation of the Deccan Traps. This began somewhere around 60 million years in the past and continued for a long period of time. Note that the subcontinent was still near the equator. From 40 million years in the past and thereafter, the occasion of the formation of the Himalayas took place. Scientists agree that the process is still continuing, and the height of the Himalayas is growing even to this date.
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