Geologically, volcanic eruptions are predictable. The main methods of predictions are seismographic monitoring, tiltmeters, surveillance by satellite. Generalizations based on broad trends are thus of limited use, because little preventive action could be taken about them. If, also, a volcano behaves in a consistent way, like many active cinder cones, for example, then it is often not very dangerous and its behavior can be readily assessed. After all, the expected scarcely needs a forecast. In such cases, the people keep their distance-and usually know what distance to keep.
It is, however, the most violent and dangerous volcanoes that display the most irregular behavior, and have erupted most lethally, and often without apparent warning in the past. Forecasting the unexpected is thus the real problem. Also, researchers take into account gas and stream emissions and create hazard assessment maps which help to monitor and analyze geological activity in a particular region. This requires more sophisticated analysis, which has been developed with increasing success, so that volcanologists are becoming more ambitious about predicting eruptions. The study is still, however, in its infancy and problems remain. Prediction has to be set in a broad perspective.
The terms “active”, “repose”, “dormant” and “extinct”, that are so often applied to volcanoes, still cannot be clearly defined. An active volcano need not necessarily be in eruption, just as someone with a cough does not cough all the time. Eruptions are separated by periods of repose, which eventually last for so long that the volcano is considered dormant and eventually extinct. The main precursors of eruptions are earth tremors (frequency and intensity) and gas and stream emissions (mudpots and salfataras), ground deformations and displacements.
The time scale varies from several hours till several decades. Different volcanoes, however, experience dormant periods that can last for decades or centuries. Vesuvius, for instance, presents challenging problems, because it has been the most violent volcano in Europe in historic time, because it lies in a densely populated area, and because its present state of activity is uncertain. Geologically determined prediction is based on the assumption that the past is the key to the future. Shallow volcanic earthquakes are perhaps the most frequent and reliable sign that a volcano is about to burst back into life. The tremors often indicate that magma is approaching the surface when their frequency and intensity increase more than a hundred-fold.