What are the principal signals of a volcanic unrest? How can we forecast volcanic eruptions?

Scientists have many tools to assess the likelihood of a future eruption. The most active volcanoes are closely monitored with different instruments to detect eruption precursors. The main indicators of a volcanic unrest are:

  • Earthquakes frequency: it increases before an eruption because of magma resupplying in the magma chamber. This process causes recognisable seismic patterns that can be monitored by volcanologists if seismometers are placed close enough to the volcano.
  • Gas emissions: the composition of gases released and the amount of gas emissions provide an insight on the volcano’s activity. Gases are collected near fumaroles and active vents.
  • Swelling of the volcano: the volcano swells before an eruption because of the magma accumulating beneath it. This swelling is not visible to the human eye, but GPS or optical instruments can measure it.
  • Other techniques are used to characterise a volcanic unrest, like hydrology (study of the water composition near a volcano) or thermal activity (monitoring of the temperatures inside the volcanic edifice) etc…

Sometimes, a volcano seems to wake up by emitting precursor signals, but no eruption occurs. Eruption forecasting is not an exact science and relies on the interpretation of numerous datasets.