Volcanoes change shape before and during eruptions

The surface of a volcano often changes shape when magma moves beneath it or rises into its cone. Hundreds of shallow cracks or deep faults tens to hundreds of meters long may develop in hours or days. The ground can change shape by rising up, subsiding, tilting, or forming bulges that are clearly visible to people familiar with the volcano.

We use a variety of methods to monitor a volcano's changing shape or deformation. Some methods are as simple as using a steel tape to measure a widening ground crack. Most volcano deformation, however, can only be detected and measured with precise surveying techniques, sensitive instruments placed on the ground or in deep holes, and satellite-based technology. Whatever the method, our goal is the same: determine the changes occurring at a volcano that help us provide eruption warnings and to understand how volcanoes work.

One of the most dramatic examples of ground deformation on a volcano occurred before the eruption of Mount St. Helens in 1980. The volcano's north flank grew outward more than 100 m between late March and 18 May 1980 to form an enormous bulge. The way that scientists monitored the bulge is well illustrated and described in a day by day account of this pre-eruption period.