The Earth’s crust and upper part of the mantle are broken into large pieces called tectonic plates. These are constantly moving at a few centimetres each year. Although this doesn’t sound like very much, over millions of years the movement allows whole continents to shift thousands of kilometres apart. This process is called continental drift.
There are three different types of plate boundary, depending on how the plates are moving relative to one another.
A boundary where the plates are moving towards each other. Often, the denser of the two plates is pushed under the other plate where it moves into the mantle and melts. This creates molten rock (magma) which can then be pushed up through the plates, causing earthquakes and volcanoes. The molten rock then cools to form new igneous rock.
A boundary where the plates are moving away from each other. Magma from beneath the plates is released from the gap and rises up, cools and forms new igneous rock. If this happens under pressure, it is known as a volcanic eruption. An example of this boundary type is the Mid-Atlantic ridge.
A boundary where the plates slide past one another without moving towards or away from each other. If the plates move a significant distance very suddenly, this causes an earthquake. There are no volcanoes at this type of boundary as melting of the rock does not occur. An example of this type of boundary is the San Andreas fault, California.