Most volcanoes provide warnings before an eruption. Magmatic eruptions involve the rise of magma toward the surface, which normally generates detectable earthquakes. It can also deform the ground surface and cause anomalous heat flow or changes in the temperature and chemistry of the groundwater and spring waters. Steam-blast eruptions, however, can occur with little or no warning as superheated water flashes to steam.
Notable precursors to an eruption might include:
- An increase in the frequency and intensity of felt earthquakes
- Noticeable steaming or fumarolic activity and new or enlarged areas of hot ground
- Subtle swelling of the ground surface
- Small changes in heat flow
- Changes in the composition or relative abundances of fumarolic gases
These precursors do not indicate the type or scale of an expected eruption (that information is best obtained by mapping previous eruptions). Precursors can continue for weeks, months, or even years before eruptive activity begins, or they can subside at any time and not be followed by an eruption. Italy’s Campi Flegrei volcano has been showing signs of unrest for over 60 years.