Tectonic Plates

At one time, it was believed that the major features of the Earth’s surface were caused by the shrinking of the crust as the earth cooled down following its formation. 

Today, evidence suggests that the Earth’s lithosphere (crust and upper part of the mantle) is made up of a number of large pieces called tectonic plates.

The tectonic plates are constantly moving at relative speeds of a few centimetres a year.

Their movement is caused by convection currents within the earth’s mantle.

The convection currents are caused by heat generated by natural radioactive processes within the Earth.

If you look at the shapes of the different continents, you can see that they would fit together, even though they are now separated by thousands of kilometres of oceans.

The edges of these land masses have similar patterns of rocks and fossils.

This suggests that they were once part of a single land mass, which has split and been moved apart. This is the continental drift theory, put forward by Alfred Wegener in 1915. It was not generally accepted until more than 50 years later.

He named the original giant landmass Pangaea.

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Plate Boundaries

The diagram shows positions of the continents today and the position of the plate boundaries.

Earthquakes and/or volcanic eruptions occur at the boundaries between the tectonic plates.

The arrows show the direction in which the plates are moving.

They are sliding past each other along the Californian coast.

This is why earthquakes are common here.

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Subduction Zones

In some areas, for example along the western side of South America (the Andes), the plates are moving towards each other.

This causes a thinner, more dense part of the ocean floor to be forced downwards below a thicker, granite type of rock.

This is called subduction.

Friction may cause the rock to melt and the formation of metamorphic rocks.

Subduction causes the continental plate to be compressed, forming fold mountains and faults (breaks in the rock).

Earthquakes occur and magma may rise through the continental crust to form volcanoes.

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Sea Floor Spreading

Along oceanic ridges, such as the mid-Atlantic ridge at the bottom of the Atlantic ocean, the plates are moving apart.

This is called sea floor spreading.

This causes fractures which are filled by magma that solidifies to form new igneous rock, such as basalt.

The magma contains iron-rich minerals which record the direction of the Earth’s magnetic field at the time when the magma solidifies.

Magnetic reversal patterns in the oceanic crust occur in stripes parallel to the oceanic ridges.

These match the periodic reversals of the Earth’s magnetic field, so they support the idea that the sea floor is spreading.

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