As the volcanic island of El Hierro, the smallest of Spain's Canary Islands, rumbled and groaned over the course of seven months in 2011 and 2012, gases silently percolated up through the island's soil and groundwater.
Eventually, a spectacular plume appeared off the southern coast of the island, a sign that El Hierro volcano, an underwater volcano just offshore, had finally erupted.
During that time, researchers had been busy collecting and analyzing the helium gas content of more than 8,000 soil and water samples. Now, those data can be used to monitor El Hierro and forecast its next eruption, researchers say, and likely other volcanic eruptions around the globe as well.
"We believe that helium can anticipate the detection of magmatic movement even before those movements can be detected by seismic activity," said Eleazar Padrón, a geochemist at Spain's Technological Institute for the Renewable Energies, who led the work.
An almost ideal gas
Researchers have been using gas emissions to forecast volcanic eruptions for at least 30 years, but they usually focus on carbon dioxide, the second most abundant gas (after water vapor) in volcanic eruptions. Helium, a noble gas, is a better candidate for tracking and forecasting eruptions, Padrón explained, because it doesn't react with rocks or groundwater and microorganisms don't consume or produce helium.