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What Equipment Is Used to Detect a Volcanic Eruption Before It Happens?

The United States has 169 active volcanoes, 54 of them officially classed as a threat to public safety. Volcanic eruptions can impact roads, towns, plane flights and human survival. The U.S. Geological Survey uses several technologies and methods to monitor volcanoes. Volcano monitoring was deemed important enough that during the 2013 partial government shutdown, volcano monitoring operations were maintained.

Temperature's Rising
- An increase in temperature is one of the warning signs that something is happening underground. Volcano watchers spot problems with thermal-imaging cameras that photograph heat signatures: new lava flows moving toward eruption photograph hotter than old flows, for instance. Satellite-mounted cameras provide regular, real-time monitoring. For closer, high-resolution images, the USGS and other organizations use aircraft-based cameras or land-based scanners. The more detailed close-up images provide information about the size and movement of high-temperature patches.
The Earth Moves
- Ground movement is another sign that a volcano is becoming active. Earthquakes around a volcano often indicate a coming eruption, so vulcanologists use seismometers -- earthquake detectors -- to track small tremors. Hawaii's largest island has more than 60 seismic monitoring stations. Tiltmeters detect very slight changes in the volcano's slope, often invisible to the naked eye. The movement can be a sign, for instance, of gas and molten rock building up inside the volcano, making the walls swell.
Something's in the Air
- Another clue to an impending eruption is when volcanic gases, such as hydrogen sulfide, vent into the surrounding air. Collecting samples for analysis is risky, so vulcanologists use a spectrometer, which identifies elements by their effect on light. Volcano watchers set up a spectrometer so that it can analyze light passing through a volcanic plume. In 1991, such gas monitoring helped predict the eruption of Mt. Pinatubo in the Philippines, saving the lives of potential victims.
Watching for Lahars
- A lahar is a magma flow from a snow-covered volcano. When it bursts out, it melts snow and ice and creates a sliding mass of rock, mud and water that can travel far beyond the eruption. But visual sightings of a lahar often detect danger too late to be useful. The USGS now uses acoustic-flow monitors, seismometers set to detect high frequencies. Lahar vibrations have a higher frequency than earthquakes, so the monitors can tell one from the other.

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