In the United States, nearly a quarter of the country’s 169 active volcanoes could pose a threat to public safety as more communities settle and grow in areas adjacent to them. Monitoring volcanoes in real time allows scientists to understand potential future volcanic activity, which helps public officials and emergency managers to make decisions and minimize losses during an eruption. Volcanoes can erupt with little to no warning, so finding ways to continually monitor small changes in activity could provide important information before potential future eruptions.
Part of the challenge of forecasting eruptions is that scientists can’t look directly inside a volcano to gauge what’s happening. Currently, scientists track seismic activity, gas emissions, and surface deformation for clues. Researchers can also look at changes in gravity, which reflect small variations in activity beneath the surface. Since anything with mass has a gravity field, Earth’s gravitational pull is stronger in areas with more mass and weaker in areas with less mass. Therefore, a change toward stronger gravity in a certain area—from, for instance, more magma—can potentially be indicative of future volcanic activity.
In a new study, Poland and Carbone continually monitored gravity changes at Kīlauea Volcano in Hawaii from 2011 to 2015 to understand how gravity varied with volcanic activity. Using data from a gravimeter located within the volcano’s caldera, they noted that gravity change correlated strongly with both deformation and the depth of a lava lake inside the summit’s eruptive vent. This allowed the scientists to assess the density of the lava lake over time.
Using this 5-year set of observations, the researchers found that the density was relatively low overall—slightly more than that of water—which reflects the large amount of gas in the lava lake. A few spikes in density over time were also recorded, possibly indicating accumulation of a small amount of magma beneath the surface that was not accompanied by surface deformation—a process that would not have been recognized by any other monitoring method. Some of these spikes were associated with changes in seismicity that had been interpreted as possible magmatic intrusions, emphasizing the importance of gravity in mapping transient and potentially hazardous volcanic activity.
Continually tracking changes in gravity near highly active and accessible volcanoes, the authors say, could have great potential for sensing previously overlooked or underappreciated volcanic signals. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2016JB013057, 2016)
—Wudan Yan, Freelance Writer