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Earth Structure and Plate Tectonics

For an introduction to the Earth's structure, click HERE!

👉lithosphere 

The lithosphere is the rigid outer part of the earth consisting of the crust and the upper mantle.

👉asthenosphere

The asthenosphere is the upper layer of the earths mantle, below the lithosphere

👉gutenburg discontinuity

This is the distinct boundary between the core and the mantle

👉mohorovic discontinuity

The boundary surface between the earth's crust and the mantle

👉crustal plates

Crustal plates are large blocks or rigid layers of the earth's crust that are said to drift slowly. There are two types of plates. Oceanic and Continental. 

👉 oceanic plate

Oceanic plate or oceanic crust is also called SIMA as it is made up of basalt where silica (Si) and magnesium (Ma) are dominant. Oceanic crust is continuous around the earth surface and is 6-10km thick. 

👉continental plate

Continental plate or continental crust is also called SIAL as it consists of silica (Si) and aluminum (Al) which are very common on the surface. When these are combined with oxygen, they make up the most common rock which is granite. Continental crust can be up to 70km thick. Continents occur where continental crusts rests on oceanic crust. 

The Theory of Continental Drift 

Alfred Wegener in 1912 said that the continents are slowly drifting apart from one super continent that was called PANGAEA which existed 200 million years ago. The evidence for this include 

- the fit of the continents if you were to put them back together

- similar plant and animal fossils found in neighbouring continents now separated by water.

- rocks of similar type and age found at the edges of continents that could have once fitted together. 

The theory of Plate Tectonics 

J. Wilson in 1965 linked the idea of continental drift and sea-floor spreading (put forward by Harry Hess) and introduced the idea of moving belts and rigid plates which formed the basis of the plate tectonics theory. He stated that plates move because of what happens in the mantle below. The intense heat coming from the earth's core causes the magma in the mantle to move very slowly in giant convection currents. These movements occur 

-upwards towards the crust (In this case, plates are forced apart)

-sideways or horizontal to the crust (In this case, plates slide along each other)

- downwards towards the core (In this case, plates are brought together)

Plate movement is usually continuous and moves approximately 2cm per year.