The abnormal movements detected by a network of continuous GPS stations can alert scientists that a volcano is deforming and that magma might be intruding beneath the volcano

When a volcano is about to erupt, its surface deforms in ways that scientists can recognize from GPS data (http://volcanoes.usgs.gov/activity/methods/deformation/gps/). For instance, the rock near a volcano’s crater can rise by several centimeters as magma intrudes underneath. Such deformations can yield clues about what is happening deep below the surface and can help volcano observatories provide eruption warnings to local governments and residents. Recent efforts to monitor volcanoes in Colombia illustrate how eruption forecasting using GPS has the potential to save large numbers of lives. After a devastating and unexpected eruption of the Nevado del Ruiz in 1985 (http://news.bbc.co.uk/onthisday/hi/dates/stories/november/13/newsid_2539000/2539731.stm), the Colombian government searched for ways to ensure that the country would not be caught unawares again. Scientists have installed GPS receivers at strategic locations along Colombia’s volcanoes in recent years, and data collected are transforming how people cope with hazards from the slumbering volcanoes that dominate the horizon