Predicting when a volcano might erupt is very difficult. Some volcanoes erupt almost constantly, but others may have intervals of hundreds of thousands of years between eruptions.

But now we are much better at forecasting than we were 20 years ago, thanks to new or better developed techniques.

First of all, it is very important to know how each volcano has acted in the past, since each one behaves differently. Scientists, called volcanologists, find out by looking at materials that have erupted from volcanoes in the past.

If a volcano were to erupt gently, it would create lava flows, which are streams of molten rock. These cool and solidify to form sheets of hard rock. Other volcanoes create explosions when they erupt. These explosions produce rock fragments, crystals, and volcanic glass (molten rock that has cooled very quickly on the surface).

Studying these materials can help volcanologists understand the violence of eruptions and the frequency with which volcanoes explode.

Volcanologists can predict if a volcano is about to erupt using various techniques. The volcano’s surface can get hotter, as magma – the molten rock from underground that is poured out as lava when a volcano erupts – comes closer to the surface before an eruption. This can be controlled by detectors on satellites that measure heat.

looking from space

The volcano’s surface can also rise or fall due to the movement of magma below the surface. This can be detected on the ground, but it can also be measured from space using satellites using radar.

Radar detection of a volcano’s motion works by studying changes in the time it takes for radio waves to bounce from the satellite to the volcano and back. The time will be less if the volcano has risen.

Monitoring volcanoes from space is now very common. It is safer and less expensive than having volcanologists on the ground near the volcano being studied, especially if it is erupting or in a very remote area.

Another way to tell when a volcano is about to erupt is to measure the gases coming out of it. When the magma moves towards the surface, the gases escape and come out ahead of the magma. These gases can be measured from space or from the ground. If the mixture of gases coming out of the volcano changes, this can indicate that the magma below is moving.

moving magma

Volcanologists also often use two other methods to tell if a volcano is about to erupt. When magma moves, it shakes the earth, creating a type of earthquake called a harmonic tremor. These tremors can indicate how fast and where the magma is moving.

A second method that can detect the movement of magma is the measurement of gravity. Gravity is, of course, the force that keeps everything from being blown off Earth’s surface and into space. But the force decreases slightly if the surface under your feet becomes less dense.

This doesn’t mean you’re going to suddenly fly into orbit, as the changes are minuscule, but they can be measured with instruments called gravimeters. Molten rock has a lower density than when it is solid, so areas of lower gravity in a volcano, especially if they change over time, could indicate the presence of magma and the possibility of an eruption.

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By studying the history of a volcano and combining information from many different ground-based and space-based techniques, we can be reasonably sure that we will hit the mark and be able to issue early warnings to people living nearby, before the volcano erupts.

This article was originally published on The conversation. read the original.