The Earth's crust is made up of lots of separate pieces,
called plates, which fit together like an enormous jigsaw puzzle. There are two
types of crust:
continental crust which makes up the continents
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
- 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
- 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.