Volcanic eruptions big and small can be difficult to see coming. Like meteorologists forecasting weather, volcanologists today combine information from as many methodologies as possible to predict eruptions more accurately. Using thermal, gas emissions, and seismic data, among other resources, scientists study preeruption clues to understand explosive volcanic behavior better.
Társilo Girona, a volcanologist at the Geophysical Institute of the University of Alaska Fairbanks, and his colleagues have, for example, spent the past 3.5 years analyzing ground radiant temperatures around various volcanoes before eruptions. “Of the eruptions we studied, 90% were not forecasted,” said Girona. Now in a study in Nature Geoscience, Girona and his team report that ground temperatures around the volcanoes increased notably in the years leading up to eruptions, a finding that could inform efforts to monitor volcanoes and forecast eruptions.
Satellite Data Reveal Increased Ground Temperatures
In 2016, Girona learned of research showing that the temperatures of gas emissions collected at the ground surface near multiple volcanoes were nearly the same as the ambient air temperature, whereas gases contained in deep magma are thought to be several tens of to a thousand times hotter. “That means that during its trip up to the surface, the gas is losing heat,” Girona said. Girona wanted to know where the extra heat was going—hypothesizing that it was absorbed by the surrounding subsurface rock.
To test his idea, Girona’s team investigated five volcanoes that erupted within the past 20 years: Redoubt, Alaska (2009); Calbuco, Chile (2015); Pico do Fogo, Cape Verde (2014); Ontake, Japan (2007 and 2014); and Ruapehu, New Zealand (2006 and 2007).
“Each location experienced a different type of eruptive behavior,” said Girona, who selected the eruptions to represent a broad range of volcanic behavior so that information from this analysis could be applied to many other volcanoes. Some of the eruptions were magmatic (e.g., Redoubt, Calbuco, and Pico do Fogo), whereas others were phreatic (e.g., Ontake and Ruapehu). Also included were events of varying eruptive magnitudes and volcanoes with different formation histories and at different latitudes.
Gathering ground temperature data from active volcanoes is simply too dangerous, “so we went to space,” Girona said. He and his colleagues accessed nearly 2 decades’ worth of thermal radiance data collected by the Moderate Resolution Imaging Spectroradiometers on NASA’s Terra and Aqua satellites and created algorithms to convert these data into temperatures for each location and time frame for the eruptions. They then compared differences between ground radiant temperatures at the top and upper flanks of the volcano to temperatures in the area around the volcano, which yielded long-term radiant temperature anomalies leading up to each eruption.
“We saw a well-defined increase of the median anomaly” in each case, Girona said. He and his colleagues found that the radiant temperature at all five of the volcanoes increased by up to about 1°C with respect to their surroundings between 2 and 7 years before an eruption.
Subtle Changes, Major Impacts
In the case of Calbuco’s destructive eruption in 2015, Girona and his team found that the median radiant temperature increased by about 0.3°C in comparison to the surrounding ground nearly 7 years before the event. It’s a subtle change in temperature, but in volcanoes, subtleties can have large impacts. A 1°C increase in temperature can cause pressure changes in the shallow ground surrounding a volcano of roughly 1 megapascal, Girona said. “These are critical pressure changes for a volcano.”
To understand how the temperature of land surrounding a volcano may be related to eruptive events, we must look underground. Before any eruption, the shallowest magma chamber beneath a volcano, usually about 10 kilometers below the surface, begins to warm up. The molten rock, crystals, and gas in these sorts of reservoirs are typically between 700°C and 1,200°C.
Warming magma releases huge amounts of water vapor, carbon dioxide, and sulfur dioxide gas that gradually rise through the subsurface, transferring heat upward as well, which further vaporizes overlying groundwater. As much of this water vapor then condenses again as it cools near the ground surface, it releases latent heat that raises the temperature of the ground near a volcano. This process, according to Girona and his colleagues, may explain the diffuse heating they’ve observed before eruptions.
A Tool for Volcanic Monitoring
“To help get people out of harm’s way, we need to have an earlier heads-up of when volcanoes become more active,” said Florian Schwandner, deputy chief of the NASA Ames Earth Science Division. Volcanologists have been working for years to uncover clues of preeruptive behavior. “Girona’s research has shown that satellite data can be used to detect a volcano heating up years before an eruption—we didn’t know that before,” Schwandner said.
The current research is limited to volcanoes with infrequent, large eruptions, but Girona plans to apply what his team has learned to more types of volcanoes, including those with shorter and more frequent eruptions.
The most direct application of this research, according to Girona, is in the potential development of a new method to detect volcanic unrest. He’s hopeful that monitoring ground temperatures around volcanoes can be added to the current lineup of predictive tools. “To truly understand volcanoes,” Girona said, “we have to have as many tools as possible.”
—Ashleigh Papp (@heysmartash), Science Writer