People living near volcanoes may detect premonitory events before an eruption. Both the frequency of occurrence and intensity of felt earthquakes commonly increase before eruptions begin. Eruptions may also be preceded by noticeable steaming or fumarolic activity and perhaps by new or enlarged areas of hot ground. However, most precursory changes are subtle and the most effective means of monitoring are instrumental and include a variety of geophysical, geodetic, and geochemical techniques. Seismometers are used to detect and locate earthquakes associated with the rise of magma. Swelling of the ground surface can be detected by using precision instruments and techniques that measure minute changes in slope, distance, or elevation at the ground surface. Other techniques involve measurement of changes in heat flow at a volcano by repeated infrared surveys or by direct measurements of hot spring or fumarole temperatures. Changes in the composition or relative abundances of fumarolic gases may also precede eruptions and can be detected by frequent or continuous analysis of gases.

These and other types of monitoring may be useful in detecting warning signs of impending eruption. However, the overall success of a monitoring system depends on detection an interpretation of precursory events in time to warn and evacuate people from threatened areas and to initiate other measures to mitigate the effects of the eruption. Although monitoring systems may be useful in indicating an increase in the probability of volcanic activity and its possible location, they typically do not indicate the kind or scale of an expected eruption, particularly the first magmatic event, or the surrounding areas that might be affected. Precursors to volcanic activity may 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. Thus, monitoring of volcanic precursors may provide a general warning that volcanic activity in a specific area is becoming more likely, but it often does not pinpoint the nature or timing of an eruption or even its certainty.