Poás volcano in Costa Rica is one of the most active volcanoes in Central America as well as a popular tourist destination. Inside its main crater sits a warm, hyperacidic lake saturated with sulfur. An underground hydrothermal system of water-filled conduits feeds the pond and also periodically releases impressive geyser-like jets of water. In mid-April, 2017, these occasional “phreatic” eruptions of water, steam, and other gases lead to “phreatomagmatic” eruptions that also included chunks of glowing rocks that damaged nearby structures. Historically, eruptions at “wet” volcanoes have been difficult to predict, but research at Poás is demonstrating that clear warning signals often exist in the gas emissions.
In a new paper  in Geophysical Research Letters, DCO researchers report that the gases emitted from Poás can give clues to interactions between water and melted rock, called magma, in the subsurface, and may signal an upcoming eruption. Reservoirs and Fluxes Community Members Maarten de Moor (Universidad Nacional, Costa Rica, and University of New Mexico, USA), Andrés Diaz (Universidad de Costa Rica), John Stix (McGill University, Canada), Alessandro Aiuppa, (Università di Palermo, Italy), Tobias Fischer (University of New Mexico, USA), and colleagues, tracked and analyzed the volcano’s emissions over five years. During quieter times, they used fixed MultiGAS stations that periodically detect different gases and transmit the data in real-time though radio waves. When an eruption destroyed the MultiGAS instrument, they conducted measurements by drone. The researchers propose that employing this type of monitoring on wet volcanoes worldwide would provide warnings for imminent eruptions and better estimates of the global release of carbon dioxide through volcanoes.
In magmatic eruptions, where volcanoes release mostly melted rock, scientists have identified clear warning signals, such as earthquakes, elevation changes as the region inflates with magma, and changes in gas emissions. But for wet volcanoes, these warning signs are not yet well defined. “Lately, our work has focused on phreatic eruptions and making the case that gas monitoring is a very important aspect to include when monitoring these types of volcanoes,” said de Moor.
In collaboration with DCO’s Deep Earth Carbon Degassing (DECADE) initiative to improve estimates of carbon dioxide emissions from subaerial volcanoes, de Moor has tracked gas emissions at Poás and other volcanoes in Central America for the past five years. He and his colleagues monitored the concentrations of sulfur dioxide (SO2), carbon dioxide (CO2), and hydrogen sulfide (H2S) in the volcano’s plume using the MultiGAS instrument installed on the side of the crater. “That works very well until the volcano erupts and destroys the instrument,” said de Moor.
After the April 2017 eruption took out the MultiGAS station, the researchers resorted to flying drones from a tourist platform overlooking the crater to continue monitoring during the eruptive period. They believe this is the first time that scientists have assessed a volcanic crisis by collecting a time series of gas measurements via drone. Once the volcano quieted down, they installed a new MultiGAS instrument to continue monitoring remotely.
In the year before Poás’ April 2017 eruption, the team saw the ratio of sulfur dioxide to carbon dioxide escaping from the crater drop very low, then skyrocket just before the eruption. Both of these gases come from magma. Typically, carbon dioxide passes through the hydrothermal system and escapes, but sulfur dioxide can react to create sulfur minerals, clogging up the channels and effectively creating a “hydrothermal seal,” blocking the release of gases from below.
“We think that this hydrothermal sealing resulted in pressure buildup beneath the seal,” said de Moor. “Eventually that seal had to fail, and when it did, it resulted in an explosive eruption that had a top-down triggering effect that expelled the seal and disrupted the underlying magma system, resulting in the first magmatic eruption of Poás since 1953.”
The researchers propose that by monitoring gas fluxes and the ratio of sulfur dioxide to carbon dioxide, they can detect when the hydrothermal system is blocked, which may signal that a large eruption is imminent. Likewise, measuring the ratio of hydrogen sulfide to sulfur dioxide may also be useful, because these gases showed the opposite pattern. The ratio rose during the hydrothermal sealing and dropped sharply before Poás exploded.
Besides being useful for hazard monitoring, these measurements also inform our understanding of global degassing from volcanoes. The amount of carbon dioxide and other gases escaping from volcanoes can vary widely day to day, and global estimates are based on small numbers of measurements from a subset of Earth’s volcanoes. “We’re making a good start on understanding global volcano degassing, but we still have a long way to go,” said de Moor. “We just need a lot more instrumentation on more volcanoes to fully understand these temporal variations.”
In January, Poás begin emitting ash again, suggesting that the volcano may be entering a new active period. “It’s an incredibly dynamic system and every time we see a new phase of activity we’re learning more about its behaviors,” said de Moor.
Main image: Gases escaping from a vent in the Poás crater deposit a layer of yellow sulfur. A portable MultiGAS sits in the foreground for scale. Credit: Maarten de Moor