Before a volcano erupts, there are usually signs of what’s to come; small earthquakes, heat emission, and ground swelling, to name a few, have all been observed to precede eruptions. Volcanologists also monitor the changing composition of gas emissions at volcanoes in the months leading up to an eruption. However, precursory variations in gas have not yet been verified to occur in volcanoes hosting active lava lakes.
To find out if volcanic gas emissions might also be used to predict eruptions at lava lake–hosting, carbon-poor volcanoes, Aiuppa et al. looked at Chile’s Villarrica volcano. Villarrica towers 2847 meters over the town of the same name; magma bubbles and spatters in an open-vent lava lake at its peak.
In this study, the researchers installed a multicomponent gas analyzer system (Multi-GAS) to bring the volcano into the Deep Carbon Observatory’s Deep Earth Carbon Degassing network. The instrument was placed on the outer rim of the lava lake in November 2014 to record the ratios of carbon dioxide and sulfur dioxide gas emitted from the magma. Magma, which contains dissolved gas, rises toward the surface of the magma chamber. As the pressure decreases on its ascent, the gas bubbles burst at the surface, and the degassed magma sinks back down into the chamber.
At this stage on Villarrica, the median carbon dioxide level was approximately 476 parts per million by volume (ppmv), and sulfur dioxide was 5.4 ppmv. To put this in perspective, Earth’s atmosphere has about 400 ppmv of carbon dioxide, and the threshold limit for human exposure to sulfur dioxide is about 5 ppmv.
A few months later, between February and March, the Multi-GAS recorded carbon dioxide levels increased to 488 ppmv, and sulfur dioxide increased to 9.2 ppmv. On 3 March 2015, the volcano erupted, spouting a 1500-meter-tall lava fountain into the air and covering the Multi-GAS tool with lava spatters.
The data gathered in the weeks leading up to the eruption revealed that the magma degassing occurred in three phases. Phase 1 was labeled the background degassing phase, where the gas ratio between carbon dioxide and sulfur dioxide was relatively low. The second phase, starting on 26 January, included wildly fluctuating gas concentrations. Phase 3 exhibited elevated degassing from 6 February until the eruption. Over the three phases, the gas composition coming from the lake became richer in carbon dioxide.
After analyzing the gas observations, the authors propose that overpressured, carbon dioxide–rich gas bubbles from very deep in the volcano, at 20–35 megapascals of pressure or more, are what escalated the volcanic eruption.
The results show that volcanic gas emissions could be a useful indicator to predict imminent eruptions on a range of volcanic types. (Geochemistry, Geophysics, Geosystems, https://doi.org/10.1002/2017GC006892, 2017)
—Alexandra Branscombe, Freelance Writer