What happens before a volcano erupts?

Earthquakes and tremors almost always happen before an eruption. However, the U.S. Geological Survey notes that a volcano can display some if not all of these signs for a long time without actively erupting.

Predicting exactly when a volcano will erupt is an imperfect science.

Scientists look at a volcano's eruptive history, seismographic detections of earthquakes, and monitor ground deformation and changes in gas emissions to predict when an eruption is impending. According to the Smithsonian Global Volcanism Program, earthquakes and tremors almost always happen before an eruption. 

After thousands of earthquakes, officials in Iceland are warning that an eruption from the Reykjanes volcanic system is imminent, but exactly when it will happen is unknown. 

The Icelandic Met Office uses data from measurements taken at the dike in Grindavík to run models and determine when and where magma may emerge. 


However, the U.S. Geological Survey notes that a volcano can display some, if not all, of these signs for a long time without actively erupting.

"These precursors do not indicate the type or scale of an expected eruption (that information is best obtained by mapping previous eruptions)," according to the USGS. "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."

Campi Flegrei last erupted nearly 500 years ago. The volcano has been "restless" for more than 70 years, and tens of thousands of small earthquakes have occurred, along with ground lifting in Pozzuoli, Italy, stretching the volcano's crust. Yet, there has not been an active eruption with all the activity.

Geologists from the University of College London and Italy’s National Research Institute for Geophysics and Volcanology recently published using modeling of earthquake activity and ground uplift to show Campi Flegrei is "stretched nearly to a breaking point" and could rupture.

Still, geologists cannot predict precisely when a volcano will erupt. 


How long do volcano eruptions last?

Experts also say looking at a volcano's history alone is not the best way to forecast when an eruption begins or ends because volcanoes have been around for tens of thousands of years before we were actively monitoring volcanic behavior. 

Some volcanoes erupt for months, years or, in the case of Guatemala's Santa Maria volcano, a century. 

According to "Volcanoes of the World," 93% of volcano eruptions end within three years, 53% end within two months and 24% end within one week.