Eruptions are almost always preceded and accompanied by volcanic unrest, indicated by variations in the geophysical and geochemical state of the volcanic system. Volcanic unrest can also be expressed by changes in temperature, composition, and level of crater lakes, and by anomalous melting or volume changes of glaciers and snow fields on volcanoes. By the year 2000, more than half a billion people will be at risk. Lava flows can cause great economic loss from property damage and decreased agricultural productivity, but they rarely cause deaths.
Volcanism is a natural process which has operated since the Earth was formed.
Although a few attempts have been made to divert lava flows, humans have had no influence whatsoever on the underlying causes of volcanism. Most active volcanic systems are located along or near divergent and convergent boundaries between the Earth's tectonic plates. Diagnostic monitoring sites commonly include active vents and fumaroles, crater lakes, and areas of ground cracking. Ideally the sites should be distributed over the entire volcanic system, to monitor both the summit and flank areas of eruptive centers. Optimum monitoring of volcanic unrest must be based on a combination of geophysical, geodetic and geochemical methods, rather than reliance on any single technique.