The Plate Tectonic Theory is a major theory in global tectonic system that revolutionized our understanding of Earth's dynamic processes. This comprehensive theory explains the formation of mountains, occurrence of earthquakes and volcanoes, and the continuous movement of continents. It represents a significant advancement influenced by quantitative revolution, positivism, and system analysis in physical geography.
What is Plate Tectonic Theory?
The Plate Tectonic Theory (PTT) is a major theory in global tectonic system influenced by quantitative revolution, positivism, and system analysis in physical geography.
The term "plate" was given by Wilson, Morgan, and Mackenzie.
The plates are solid lithospheric slabs which slide on elastic/plastic asthenosphere.
Lithosphere = Crust + Upper Mantle
These plates experience continuous horizontal and vertical movement which causes deformation of rock on the Earth's crust or crustal rock.
Concept :-
Because of the hot convectional current in the mantle, the plates are continuously created or destroyed. The effects of the movement of the plates are mostly seen on their margin or boundaries.
The plate boundaries are divided into:
- Divergent Plate / Constructive Plate Boundary
- Transform Plate Boundary / Conservative Plate Boundary
- Convergent / Destructive Plate Boundary
1. DIVERGENT PLATE / CONSTRUCTIVE PLATE BOUNDARY
These are also known as Accreting Plate Margins
→ Due to ascending limbs of convectional current the crust breaks causing the plate to drift away.
→ Due to the drifting of the plate margin, there is upwelling of magma which cools to form mid-oceanic ridges.
The regions of mid-oceanic ridges are characterized by:
- Fissure eruption
- Low intensity of earthquake
- Upwelling of Peridotitic magma, forming Peridotite rocks
Similarly, the earthquakes which lie in the mid-oceanic ridges or the fracture zone are shallow focus earthquakes.
→ The expanding plate/divergent plate margin comes in contact with less dense continental rocks and therefore it is subducted along the continental margins forming oceanic trenches.
This zone of subduction is also known as Benioff Zones.
→ Sometimes the mid-oceanic ridges can project above the oceanic water.
Example: Iceland or Icelandic plateau
→ 15% of world's earthquakes and volcanoes are located along the divergent plate boundary.
2. TRANSFORM PLATE BOUNDARY / CONSERVATIVE PLATE MARGINS
→ Transform plate boundaries are also known as Shear Plate margins. Such margins are formed when two lithospheric plates pass horizontal to each other along a fault.
In this case, since the plate is neither created nor destroyed, it is also known as conservative plate boundaries.
→ The conservative plate boundaries are the regions of rift valleys, and such rift valleys may have volcanic activities.
The Great East African Rift Valley is the longest rift valley in the world.
→ The most powerful earthquakes in the world generally occur along the transform plate boundary. Such earthquakes are known as Earthquakes of fault zone.
San Andreas fault system is an example of transform plate boundary which occurs due to the movement of North American plate, Pacific plate, and Juan de Fuca plate.
→ San Francisco and San Diego are both located on the Pacific plate. The Pacific plate lies on the western side of the fault, while the North American plate lies on the eastern side of the fault.
The Pacific plate is being carried in the north-western direction, resulting in the formation of a fault which is 1050 km long. It is expected that in the next 10 million years this part of California will break from mainland U.S.A.
3. CONVERGENT / DESTRUCTIVE PLATE BOUNDARY
→ The convergent/destructive plate boundaries are known as consuming plate margins. It occurs when two lithospheric plates move towards each other and collide.
One of the plates bends and is subducted below the other plate and pulled into the mantle. The process of descent of a plate and its pulling in the mantle, melting, and it becoming magma/again converting into magma is technically known as subduction.
→ The point where the subduction occurs is marked by very deep ocean trenches, while this trenches arc are parallel to the fold mountains because the collision of the plates deforms or folds some part of the Earth's crust to give rise to depth highest mountains.
Peru-Chile trench is parallel to Andes mountain system.
Types of Convergent Boundaries
There are three different plate types of boundary :-
This explains the formation of mountains such as Rockies and Andes.
Both the converging plates are continents, and therefore such plates lead to very high mountains but no volcanoes due to absence of subduction.
There is huge subduction which leads to the formation of island arcs having significant volcanic activities.
The volcanoes of convergent plate boundary are rich in silica, and such magma is known as Andesitic or Granitic.
Such magma, being rich in silica, is viscous and therefore solidifies quickly. Hence, the subsequent explosions are of great intensity.
The volcanoes of Rockies and Andes are prominent examples.
→ The earthquakes along the subduction zone are caused when the downward movement of the lithospheric plate releases energy. Such earthquakes are generally deep focus earthquakes and are most destructive.
Pacific Ring of Fire
The earthquakes of the Circumpacific belt or the Pacific Ring of Fire are the best example because this zone has 60% of world's earthquakes and volcanoes.
This belt is formed because of subduction of the denser Pacific plate below the Asian, Indian, and minor Philippine plates.
Key Points to Remember
- 60% of earthquakes and volcanoes occur in the Pacific Ring of Fire
- San Andreas Fault is 1050 km long
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