Plate tectonics is the theory that the lithosphere (crust and upper mantle) consists of large, rigid sections, called plates .

These plates move slowly with respect to each other and if we map out the divisions between these plates, it would correlate significantly with the distribution of volcanoes around the world. These plates are either continental, e.g. The North American Plate, or oceanic, e.g. The Nazca Plate.

Tectonic plates are powered by convection currents, which is the circular movement of magma within the mantle.

These currents are powered by the core, which heats the magma, causing it to rise, cool and fall back down.

This circular motion causes the plates, which float on the mantle, to move.

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However, tectonic plates can only move in three directions. They converge, diverge or transform.

This movement can result in activities such as folding, earthquakes or volcanic activity.

Proof of plate movement is evident in the theory of plate tectonics and continental drift, put forward by Alfred Wegener.

He suggested that the world’s continents are drifting across the globe, powered by convection currents.

He believed that the countries of the world were once one big super continent called Pangaea and a single ocean called Panthalassa.

Approximately 200 million years ago, Pangaea began to break apart and the continents began to drift as a result of convection currents underneath the earth’s crust.

This was based on the evidence of continental fit. For example, the Mountains of Caledonia, which stretch from the North-west of Ireland, Scotland, and Scandinavia.

Also, Ireland’s most common rock, limestone, requires warm, tropical waters to form, therefore proving Ireland’s past position of 30? South of the equator.

The Theory of Seafloor Spreading, put forward by another scientist Henry Hess, also proves that the continents are moving as a result of plate movement.  

He observed volcanic eruptions taking place beneath the surface of the earth whilst in a submarine. The theory suggests that ocean floors are widening as new rock is formed where the plates separate, e.g. the Eurasian and North American plates separating, creating the Mid-Atlantic ridge.

As plates separate, driven by convection currents, molten magma rises up from within the mantle to fill the opening. This magma cools and solidifies to form a new rock, e.g. Basalt. This goes on to form a volcano, such as that at Iceland (Hekla, Krafla)

Proof of seafloor spreading and thereby plate movement is evident in the existence of mid-ocean ridges e.g. the Mid Atlantic ridge. Also, the formation of new land such as Iceland and the varying ages of the sea floor. (i.e. youngest where the new rock is formed along mid-oceanic ridges and oldest along continental edges), further proves that the tectonic plates are moving.

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In conclusion, within the body of this answer I have explained how the study of plate tectonics helps us to understand global plate movement.