At any one time there are around 20 volcanoes actively erupting, and in the last 20 years approximately 2,000 people have been killed by volcanic eruptions.
Volcanologists use a range of techniques to try to predict volcanic eruptions. One thing which is measured and recorded is the increase in frequency and strength of earthquakes, measured by a seismograph, which may indicate an eruption is close. Tiltmeters and laser mapping are used to record swellings or deformations of the surface as this, along with high concentrations of gases such a sulphur dioxide, suggest that magma is rising.
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