There are six major indicators of impending volcanic eruptions: gas leaks, bulges, tremors, dropping, bumps, and steam.
Gas leaks occur when magma rises within the
volcano and releases carbon dioxide gas. It can be measured at the volcano's fumaroles or where it seeps through the soil on the slopes. A small rise over a large area of a volcano's surface is an indication that the pressure of the magma
beneath is rising. This can be monitored using satelite-based radar interferometry. Small earthquakes or a constand shaking of the ground means that magma is rising under the volcano's surface or it is beginning to seep out of cracks under the surface. If these small quakes suddenly stop, though they don't always, it means that the magma has stopped just beneath the surface. A large bulge within a few hundred metres of a volcano's vent indicates an increase in pressure just below the surface. Explosions of steam occur when magma heats pockets of underground water, turning them into steam. All of these symptoms can occur for weeks or even months before a volcano erupts; however, they are a sure indication that an eruption will occur soon.