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The volume of rocks from which heat can be extracted is called the geothermal reservoir, which contains hot fluids, a summary term describing hot water, vapour and gases. A geothermal reservoir is usually surrounded by colder rocks that are hydraulically connected with the reservoir. Hence water may move from colder rocks outside the reservoir (recharge) towards the reservoir, where hot fluids move under the influence of buoyancy forces towards a discharge area.

Before it erupts, a volcano produces many thousands of earthquakes. Some of these happen when rock inside the volcano cracks or slips along a fault. Others are caused by magma (molten rock) and gases trying to move under pressure within channels and cracks.
Select from the links below to find out about the four primary types of volcano-related earthquakes and to see the seismic signals that they produce. (The movement of a signal from one peak to the next is equal to one cycle.)