• thick continental crust which makes up the continents
• much thinner oceanic crust which makes up the ocean floors

The plates of the Earth's crust area always moving - gradually pushing together, pulling apart or sliding past each other. These plate movements are caused by currents of magma (molten rock) beneath the crust. Heat from the Earth's core warms the magma, making it rise and push the plates along before it cools and sinks back down again.

There are different types of plate boundaries:

• At constructive plate boundaries, new rock is made as two plates are pulled apart and hot magma oozes up to fill the gap. The new rock may form a ridge along the plate boundary. For example, the Mid-Atlantic Ridge which runs beneath the Atlantic Ocean has formed in this way.

• At destructive plate boundaries where two plates of similar density collide, they push upwards and produce fold mountains. The Himalayas and Alps are chains of fold mountains. Some mountains, such as Mount Everest, are still growing in this way.

• At destructive plate boundaries where two plates of different densities are pushed together, one plate slides over the other and the lower one disappears into the magma, where it eventually melts. The groove where the plates meet is called a trench. These boundaries are called destructive plate boundaries. An example is the Andes Mountains and the Peru-Chile trench.

• At conservative plate boundaries, plates rub past each other. Crust is neither created nor destroyed but often the plates will get stuck and earthquakes happen when the stress that builds up is released. At the San Andreas Fault in California, the Pacific and North American plates are sliding past each other and there are regular earthquakes.

The map below shows the positions of the world's plate boundaries. Most of the world's earthquakes and volcanoes occur near plate boundaries because the crust is weaker there.