Plate tectonic theory
Theories that explains various Earth's movement -
Plate tectonic theory - The term plate was coined by J.T. Wilson in 1965. Plates are the broad rigid segments of lithosphere which includes the rigid upper part mantle and the oceanic and continental crust. It moves on underlying Asthenosphere.
The theory of Plate tectonic was proposed by Morgan and Mc Kenzie & Parker separately in 1967.
Plate tectonic is the study of deformation within plate and of interaction of plates around their margins.
The plates are 100 KM thick and have high rigidity and are unable to deform except in response to a very strong and prolonged forces.
There are 7 major plates and many minor plates.
The 7 major plates are -
- Pacific plate - which is largest and contains only oceanic crust
- North American plate
- South American plate
- Eurasian plate
- African plate
- Indo-Australian plate
- Antarctica plate
Some important minor plates are -
- Nazca plate
- Cocos plate
- Arabian plate
- Philippines plate
Movement of plates - The plates are constantly in motion but with different speed and direction. This causes 3 types of plate boundary, i.e., of contact between two plates.
- Divergent/Constructive plate boundary
- Convergent/Destructive plate boundary
- Conservative/Parallel/transform plate boundary
Divergent plate boundary - When both plates moves away from each other.
Types of divergent plate boundary -
- Ocean-Ocean divergence
- Continent-Continent divergence
Ocean-Ocean Divergence - These are the zones of tension where the lithosphere splits, separates and moves apart into two opposite direction.
Hot magma comes up through the cracks, solidifies leading to the formation of new oceanic crust and gradually a mid oceanic ridge is formed.
The tectonic activities, i.e., earthquake and volcanism are limited to the surface, i.e., shallow earthquake and slow and continuous eruption of lava from mid oceanic ridges.
.jpg "ocean ocean divergence")
Continent-Continent Divergence - A divergent plate boundary along continents involves 3 different stages.
- Intra continental rifting, i.e., fragmentation of the continents by an upward movement of convection current in the form of rising magma from below. It causes the creation of numerous cracks along the continental crust and the formation of rift valley. Rising magma starts to come out of this rift. Example - East African Rift Valley
- Intra plate thinning - It involves partial melting of lithosphere and gradual thinning of continental plate. The rift valley starts to widen and may gradually get filled with ocean water. Example - Red sea (between Asia and Africa)
- Formation of oceanic ridge - The continuous spreading of plates and creation of new plate material along the rift valley pushes the two continental mass sufficiently apart. At this stage there will be a new ocean basin along both the sides of the ridge. Example - Mid Atlantic ridge (between Africa and South America)
Continent-Oceanic divergence - It is a very rare chance but in case if this happens, then after a while magma accumulates forming oceanic crust and this making it Ocean-Ocean divergence.
Convergent plate boundary - When plates are moving towards each other.
Types of Convergent plate boundary -
- Ocean-Ocean convergence
- Ocean-Continent convergence
- Continent-Continent convergence
Ocean-Ocean convergence - When two oceanic plates collide, then the oceanic plate with higher density descends down into the Asthenosphere. This process is called 'Subduction' and the region of subduction is called 'Subduction zone'.
It leads to the formation of 'Trenches' which are the deepest part of the oceanic surface. For example, the collision of Pacific plate with its surrounding plate has led to the formation of Philippines trench, Mariana trench (formed by the subduction of Pacific plate beneath Philippines plate), Japan trench, Kurile trench, etc.
The subducting plate starts to melt as it reaches the deeper portion and some of its molten material finds its way upward in the form of volcanic activity. This molten material piles continuously on the adjacent oceanic floor forming volcanic islands.
Volcanic islands are not made up of only oceanic crust, i.e., Basaltic in nature. Since, there is a thin deposition of continental sediments over oceanic floor. Thus, the molten material is not purely Basaltic rather it is Andesitic in nature.
The continuous volcanic activity results in gradual increase in size and elevation of volcanic islands.
Island Arc - These islands when are arranged parallel to the tranches in an arc shape are known as island arc.
Archipelago - An archipelago is a group of island scattered in the ocean. It is formed along ocean ocean convergent plate boundary with intense volcanic activity or shifting of subduction zone. Example - Indonesia, Philippines, etc.
Subduction zones are the sites of most widespread and intense earthquakes. Most earthquakes will have forces at Benioff zone which is the zone of friction along the subducting plate (Earthquake - Shallow, intermediate & deep).
Ocean-Continent convergence - It takes place when an oceanic crust of one plate collides with the continental crust of another. Continental crust being more buoyant (i.e., lighter) forces the higher density oceanic crust into the mantle.
Example - Nazca plate and Cocos plate are subducting below South American plate.
During subduction the thicker sequence of rocks along the continental margin is compressed and deformed to form a chain of fold mountains.
Example - Andes and Rockies mountains
As the oceanic plate subducts, it creates tranches. Earthquake are generated at Benioff zone. As the oceanic plate plunges deeper it gets melte3d and starts to rise. Thus, rising magma will be emplaced in the overlying continental crust which may eventually migrate to the surface leading to the formation of volcanoes.
For example - Nazca plate subducting below the South American plate has resulted in a series of volcanoes along Andes such as Cotopaxi, Chimborazo, etc.
Continent-Continent convergence - It was always preceded by Oceanic Continental collision. It is when the two plates with continental margin collides.
Example - Collision of Indian and Eurasian plate
Prior to the continental collision, the land masses were separated by oceanic crust. As the two plate converges, the intervening sea encloses and the sea floor subducts beneath one of the plate. The continues convergence results in complete subduction of ocean crust and the two continental masses get stick together at a zone called 'Suture zone'.
When the continental margin continued to collide, the lower density of continental crust does not permit its complete subduction (only partial subduction).
The oceanic material completely breaks from the continental block and gets completely assimilated in the mantle. At this point of time volcanic activity ceases.
The continued convergence forces the continental crust partially under another one creating an unusually thick layer of intense folding. This results in the formation of very high fold mountains.
Example - Himalayas
This type of convergence results in shallow and intermediate earthquake.
Conservative plate boundary - At the conservative plate boundary, the plates slides past each other without any formation of new crust. This boundary is also called parallel or transform plate boundary. It roughly moves parallel to the direction of plate movement.
It results in the formation of faults.Example - San Andreas fault of California
Most intense earthquakes happen here.
It provides continuity to the oceanic ridges.
Transform faults are also found on the ocean crust where they offset the oceanic ridges through out their length.
.jpg "conservative plate boundary")
Note - The Pacific ocean is surrounded by trenches on all the sides which are the zone of intense volcanic activities and earthquake. That's why it is also known as 'Ring of Fire'.
Cause of plate movement -
- Convection current - These currents are of two types - one involving the whole of mantle and another involving only Asthenosphere. These currents ascends from below. They diverges and spread laterally. As the magma moves laterally, the current carries the slab of lithosphere along with them. This is the sight of oceanic ridge. When these currents encounters a similar current from opposite direction, they descends into the deeper part of the mantle and drag the lithosphere down into the mantle at tranches.
- Mantle plumes - This is a type of mantle convection which includes jet like plumes of low density material from the core- mantle boundary. As the plumes reaches the lithosphere, it spreads out laterally, doming the surface zones of the Earth and moving them along the direction of mantle plumes. Mantle plumes results into the formation of volcanic hotspots. Example - Hawai'i island, Reunion islands, etc
- Ridge push - Magma rising along the oceanic ridges form wedges of new lithosphere on either side of the trailing edge of the plate. It causes the plates to be pushed apart.
- Gravity sliding - The spreading centres stand high on the ocean floor. This results in gravitational sliding of lithospheric slab away from the oceanic ridge.
- Slab pull - It is along the subduction zone where the subducting plate pulls the rest of the slab along.
Limitation of Plate Tectonic Theory - The plate tectonic theory although was widely accepted, it did not explain the following -
- Both African and Antarctician plate are surrounded by ridges and have no subduction zone to accommodate the new lithosphere being generated.
- Plate tectonic, in the past, i.e., formation of earlier mountains, movements of different plates and how they are different from the present one can't explain.
Difference between different plate boundaries -
Ocean Ocean Divergence
- Landform formed - Mid-Oceanic ridge & Some transform faults
- Earthquakes - Shallow
- Volcanic Activity - Yes, Slow and Continuous
- Example - Mid-Atlantic ridge
Continent Continent Divergence
- Landform formed - Rift valley
- Earthquake - Shallow
- Volcanic Activity - Yes, Slow and Continuous
- Example - East African Rift Valley
Ocean Ocean Convergence
- Landform formed - Tranches, Subduction zone, Benioff zone, Island Arc, Archipelago, etc
- Earthquake - Shallow, Intermediate & Deep
- Volcanic Activity - Yes, Explosive and Discontinuous
- Example - Japan, Philippines, Indonesia
- Landform formed - Tranches, Subduction zone, Benioff zone, Fold Mountains
- Earthquake - Shallow, Intermediate & Deep
- Volcanic Activity - Yes, Explosive and Discontinuous
- Example - Andes and Rockies
- Landform formed - Very high Fold Mountains
- Earthquake - Shallow and Intermediate
- Volcanic Activity - No
- Example - Himalayas
- Landform formed - Faults and Cracks
- Earthquake - Shallow
- Volcanic Activity - No
- Example - San Andreas Fault
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Note - This is my Vision IAS Notes (Vision IAS Class Notes) and Ashutosh Pandey Sir's Public Administration Class notes. I've also added some of the information on my own.
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