The Earth's Structure And The Theory Of Plate Tectonics - BBC Bitesize

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  1. The Earth's structure and plate tectonics
  2. Global distribution of earthquakes and volcanoes
  3. Different types of plate boundaries

The Earth's structure and the theory of plate tectonics

The Earth is made up of different layers:

The crust is the outer layer of the Earth. Under this is the mantle, then the outer core and the inner core in the middle.
Figure caption, Cross section of the Earth's structure
  1. The inner core is in the centre and is the hottest part of the Earth. It is solid and made up of iron and nickel with temperatures of up to 5,500°C.
  2. The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel.
  3. The mantle is the thickest section of the Earth at approximately 2,900 kilometres. The upper part of the mantle is called the asthenosphere. It is made up of semi-molten rock called magma.
  4. The crust is the surface of the Earth. It is a rock layer forming the upper part of the lithosphere. The lithosphere is split into tectonic plates.

Demonstrating the movement of the Earth's crust and how tectonic plates interact at plate boundaries

The theory of plate tectonics

The crust is the outer layer of the Earth. It is a thin layer between 0 - 60 km thick. The crust is the solid rock layer upon which we live. It is either continental or oceanic.

The Earth's crust is broken into tectonic plates. It was once believed that convection currents in the mantle caused the plates to move. However, it is now recognised to be more complicated than this. Mechanisms called slab pull and ridge push are believed to move the tectonic plates. Ridge push is where the new crust formed at divergent plate margins is less dense than the surrounding crust and so it rises to form oceanic ridges. The older seafloor either side of the ridge slides away and this moves the seafloor apart – moving the tectonic plates.

Therefore, instead of tectonic plates moving because of the convection currents, evidence suggests it is the plates that drive the convection. Slab pull occurs where older, denser tectonic plates sink into the mantle at subduction zones. As these older sections of plates sink, newer and less dense sections of plate are pulled along behind. Sinking in one place leads to plates moving apart in other places.

The movement of the plates and the activity inside the Earth, is called the theory of plate tectonics.

Circular convection currents in the mantle move the plates on top - either diverging (plates move away from each other) or converging (plates move towards each other).
Figure caption, Plates and convection currents
Next pageGlobal distribution of earthquakes and volcanoes

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