Most of the 500 active volcanoes in the world are strung like beads along, or near, the margins of the continents, and more than half of those encircle the Pacific Ocean as a "Ring of Fire."
Volcano monitoring is the keeping of a detailed "diary" of the changes--visible and invisible--in a volcano and its surroundings. Between eruptions, visible changes of importance to the scientists would include marked increase or decrease of steaming from existing vents; emergence of new steaming areas; development of new ground cracks or widening of old ones; unusual or inexplicable withering of plant life; changes in the color of mineral deposits encrusting fumaroles; and any other directly observable, and often measurable, feature that might reflect a change in the state of the volcano.
But the "diary" keeping during eruptive activity presents additional tasks. Wherever and whenever they can do so safely, scientists document, in words and on film, the course of the eruption in detail; make temperature measurements of lava and gas; collecting the eruptive products and gases for subsequent laboratory analysis; measuring the heights of lava fountains or ash plumes; gauging the flow rate of ash ejection or lava flows; and carrying out other necessary observations and measurements to fully document and characterize the eruption. For each eruption, such documentation and data collection and analysis provides another building block in constructing a model of the characteristic behavior of a given volcano or type of eruption.
Volcano monitoring also involves the recording and analysis of volcanic phenomena not visible to the human eye, but measurable by precise and sophisticated instruments. These phenomena include ground movements, earthquakes (particularly those too small to be felt by people), variations in gas compositions, and deviations in local electrical and magnetic fields that respond to pressure and stresses caused by the subterranean magma movements.
The USGS Cascades Volcano Observatory (CVO), located in Vancouver, Washington, not only serves a national, or even global interest, in monitoring local volcanic activity, but also endeavors to help people live knowledgeably and safely with volcanoes and other related natural hazards including earthquakes, landslides, and debris flows in the western United States. CVO assesses hazards before they occur by identifying and studying past hazardous events. CVO also provides warnings during volcanic crises by intensively monitoring restless volcanoes and interpreting the results in the context of current hazards assessments. Additionally, CVO investigates and reports on hazardous events after they occur to improve their assessment and prediction skills, and to help develop new concepts of how volcanoes work.