Monitoring and Predicting Eruptions: Reading a Volcano's Warning Signs
For most of history, volcanic eruptions struck without warning, and communities could only react once the danger was upon them. Today, that has changed. Using a growing array of instruments and techniques, scientists can detect the warning signs that often precede an eruption, giving communities precious time to prepare. Forecasting eruptions remains an inexact science, but modern monitoring has saved countless lives.
The signs of a stirring volcano
Before many eruptions, a volcano gives warning signs as magma rises beneath it. The ground may swell as magma accumulates, earthquakes may increase as rock fractures to make way for it, and gas emissions may rise. The volcano may heat up, its crater lake may change, and its surface may deform. Recognising and tracking these signs is the foundation of eruption forecasting.
Seismic monitoring
Earthquakes are among the most reliable precursors of an eruption. As magma forces its way upward, it cracks the surrounding rock, producing distinctive patterns of small earthquakes and tremor. Networks of seismometers around a volcano detect these signals, and changes in their number, depth, and character can warn that magma is on the move, sometimes days or weeks before an eruption.
Ground deformation
As magma accumulates beneath a volcano, it can push the ground upward, causing it to swell, tilt, or bulge. Scientists measure this deformation with GPS instruments, tiltmeters, and satellite radar, which can detect movements of mere centimetres. A swelling volcano is a strong signal of rising magma, and tracking deformation is a key tool in anticipating eruptions.
Gas monitoring
Changes in the gases a volcano releases can also signal an impending eruption. As magma rises, it releases gases such as sulphur dioxide and carbon dioxide, and increases in these emissions can warn of activity. Scientists measure volcanic gas using ground-based sensors, aircraft, and satellites, adding another vital strand to the forecasting effort.
Satellites and remote sensing
Satellites have transformed volcano monitoring, especially for remote and dangerous volcanoes. They can measure ground deformation, detect heat and gas emissions, and track ash clouds from space, monitoring volcanoes that would otherwise be impossible to watch closely. This global view is especially valuable for the many volcanoes without dedicated ground-based observatories.
The limits of prediction
Despite these tools, forecasting eruptions remains difficult. Some volcanoes give clear warnings; others, especially those driven by steam rather than magma, can erupt with little or no precursor, as the 2014 Ontake disaster showed. Scientists can often say that an eruption is likely and roughly when, but pinpointing the exact timing, size, and style of an eruption remains a profound challenge.
Saving lives through monitoring
When monitoring works, it saves lives. The successful evacuation before the 1991 Pinatubo eruption, guided by scientists tracking the volcano's escalating unrest, is a landmark example, sparing tens of thousands of people. The lesson of disasters like Armero, where warnings failed to translate into action, has driven continual improvement in both monitoring and communication.
Explore on the map
The world's volcanoes, from the closely watched giants near major cities to the remote peaks monitored by satellite, are the subjects of an ongoing global effort to read their warning signs. Explore them on the interactive map — filter by region to see the volcanoes that scientists watch to keep the communities around them safe.