Scientists closer to predicting phreatic volcanic eruptions
One type of a volcanic eruption, a phreatic eruption, which involves external water, is particularly energetic causing a disproportionate number of fatalities, Steve Carr reports.
Phreatic eruptions are extremely difficult to forecast, often occurring with little or no geophysical precursors.
Recently, researchers at the Deep Carbon Observatory (DCO), led by Maarten de Moor from the Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional, Heredia, Costa Rica, (and postdoc at UNM) along with University of New Mexico Professor Tobias Fischer, Department of Planetary Sciences and chair of the Deep Earth Carbon Degassing initiative, measured gas emissions from crater lake at Poás volcano in Costa Rica, in an attempt to determine some of the precursors to major volcanic eruptions.
"The initial goal of the study was to quantify gas fluxes (CO2, SO2, H2S) from Poas volcano and to monitor changes in gas compositions," said de Moor.
"The motivation behind the measurements was firstly to provide robust constraints on gas fluxes as a contribution to global volcanic gas emissions to the atmosphere, and secondly to monitor degassing in order to track volcanic activity for hazard mitigation purposes."
Excerpt from EPSL paper: "The Poás crater represents one of the most chemically extreme environments on Earth and Poás Volcano National Park was visited by more than 200,000 tourists in 2014.
About 60 seismically registered phreatic blasts occurred from the lake during the same year, ranging from minor "gas bursts" to highly explosive jets ejecting ballistics, sediments, vapor and lake water to more than 400 meters above the lake surface."
In a new article, published in Earth and Planetary Science Letters recently, the results from a DECADE (Deep Earth Carbon Degassing initiative) project to investigate gas emissions at Poás have delivered promising results.
"Before this study, phreatic eruptions were primarily thought to be generated by changes in hydrothermal systems, and usually occur with no appreciable precursors," said de Moor.
"Our study shows that there are clear short-term changes in gas compositions prior to phreatic eruptions at Poás, and are generated by short-period changes in high temperature volcanic gas input from the deep magmatic system."
The team measured gas emissions from the crater lake in situ using a fixed multiple gas analyzer station (Multi-GAS) during a two month period of phreatic activity in 2014. The lake was the site of intense phreatic eruptive behavior between 2006 and 2014.
Both accuracy and precision are important in the Multi-GAS measurements. The Multi-GAS instrument measures gas ratios, such as SO2/ CO2 and H2S / SO2. Precision, or the reproducibility, of the Multi-GAS measurements is important when comparing data points within the researchers' dataset.
"The accuracy, or proximity of the measured value to the true gas ratio, is most important for quantifying gas emission rates from the volcano and for comparing our measured gas compositions to those from other volcanoes or other studies at Poas," said de Moor.
"We did a series of laboratory tests using gas mixtures to estimate both accuracy and precision of the Multi-GAS measurements. These measurements give us confidence that the variations we see in the field data are real."
"Diagnostic tests prove that the occurrence of eruptions and high SO2/ CO2 are statistically correlated, and that the occurrence of quiescence (no eruptions) and low SO2/ CO2 are also correlated.
The results of these diagnostic tests from Poás show scientists that both true predicted values (successful "prediction" of eruption based on high SO2/CO2) and false predicted values (successful "prediction" of quiescence based on low SO2/CO2) are high, indicating strong evidential worth for the association between gas composition and eruptions." ■