GPS & Volcanoes

In early 1980, a bulge appeared on the north side of Mount St. Helens, a volcano in the state of Washington (Figure 1). By late March, the bulge was growing almost 2 meters per day. By May 17, the bulge had grown by over 130 meters (See also the USGS summary).

Volcanologists studying Mount St. Helens recognized that the volcano was building to an eruption. What they didn't anticipate was how the eruption would occur. They expected the explosions to be directed upward, toward the sky. Instead, on May 18, 1980, at 8:32 a.m., an earthquake trigged a landslide that in turn triggered an eruption—directed sideways out of the volcano (Figure 2).

Scientists now recognize that these lateral blasts are common, and have occurred at volcanoes around the world. By understanding volcano deformation, or movement of the surface of a volcano in response to what's going on in its plumbing system down below, we can better anticipate both how and when a volcano is likely to erupt.

Figure 1.Mount St. Helens before the eruption. (From USGS)

Mount St. Helens before the eruption. (From Close

Figure 2.Mount St. Helens after the eruption, blown out by the lateral blast. After the lateral blast, the eruption cloud went upward as the volcanologists expected. (From USGS)

Mount St. Helens after the eruption, blown out by the lateral blast. After the lateral bla...

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Changes in the magma system feeding a volcano can actually change the shape of a volcano. When magma moves into the system, pressurizing it, the ground above the magma moves upward and outward, "inflating" like a balloon. When magma or fluids move out of the system, the volcano contracts, or "deflates." (See Figure 3). Sometimes, these motions tell us what the magma is doing deep below a volcano. Sometimes, as in the case at Mount St. Helens, these motions can tell us what the magma is doing up high in the volcano, and what to expect when it reaches the surface.

GPS can precisely measure both horizontal and vertical motions. GPS and other instruments used to measure deformation may detect motion at a volcano before any earthquakes occur, and these changes in shape may accelerate immediately before an eruption, making GPS a valuable monitoring tool. GPS also helps us understand how magmatic systems work so that we may better plan for eruptions. Combining GPS with seismicity, gas emissions, and changes in water chemistry around the volcano paints an even better picture of what is going on below the surface.

Figure 3.Left and Middle Panels: The surface of a volcano "inflates" and "deflates" as the magma chamber below it pressurizes and depressurizes. Measuring this motion can help tell scientists whether a volcano is building to an eruption. During inflation, points (A, B, C, D) on the volcano move upward and outward (A^',B^',C^',D^'). During deflation, points move downward and inward. Right Panel: Geophysicists can produce simple models of how magma expands. These models are then used to predict how the Earth crust will move. The arrows show horizontal motions and the colors show vertical motion. Credit: USGS and Jessica Murray-Moraleda.

Left and Middle Panels: The surface of a volcano "inflates" and "deflates" as the magma ch...

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