Activity is monitored using the technique of Volcanic Surveillance.
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Magma moving into the volcano to generate volcanic earthquakes and change its shape.
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Volcanic Surveillance is based on the assumption that movement of molten rock or magma beneath a volcano will occur before any eruption can start and this movement of magma is detectable using various methods. Volcanologists use many techniques to monitor an active volcano; some of the main techniques are:
In this image you can see the magma (orange) which moves into the volcano to generate the volcanic earthquakes and change the shape of the volcano.