The Earth’s crust is relatively thin, and is formed of large, flat pieces, called ‘plates’. Each crustal plate may be thousands of kilometres across. These plates are moved very slowly by movements of the magma underneath. Where two plates push against each other, the crust crumples to form mountains. Where they move apart, magma can escape to form new rocks. Originally the Earth’s land was close together, but over time the crustal plates have moved the land apart.
Changing times
200 million years ago the main land masses that we know today were all grouped close together (1). Over many millions of years, crustal plates carrying the continents have moved away from one another (2). 65 million years ago the continents had moved even further apart (3). Today the continents are still moving a few centimeters each year (4). The coastlines are also being slowly eroded by the sea. How will the continents look in 100 million years time?
When crustal plates press against each other, they do not slide smoothly. Instead, they press and press until suddenly they break at a weak point. As they break, there is a sudden movement of the crust, which we feel as an earthquake.
Occasionally, some of the mantle can push through weak points in the crust, and form a volcano. The crust is usually weaker along or near the lines where plates meet.
Earthquakes can cause a great deal of destruction.
The Sierra Nevada Mountains, USA, were formed by two crustal plates pushing against each other.