PREDICTING FIRE WITH BRIMSTONE
Improving the forecasting of volcanic eruptions.
PERCHED ON THE EDGE OF a crater up high on the slopes of Costa Rica’s Turrialba volcano, an instrument is sniffing the volcano’s bad breath. If the gaseous mix changes, it’s likely to be the signal the volcano is set to erupt.
Maarten de Moor, from Costa Rica’s Volcanic and Seismic Observatory, is monitoring the signals closely. In recent months there have been troubling minor eruptions. In January a state of emergency was declared for a few days, postponing international flights.
Volcano watchers like de Moor know all too well that their job can mean the difference between life and death. Nearby Colombia is a tragic reminder. On one November day in 1985, just after 3pm, Nevado del Ruiz erupted without warning. Within minutes, four deadly rivers of clay, ice and molten rock raced down its flanks, destroying towns and villages. More than 23,000 people died. While there had been mini-eruptions and earthquakes prior to the blast, scientists were unable to convince the community of the risk in time.
The decades since the Colombian tragedy have seen major improvements to the science of eruption forecasting. Early warning systems enabled 75,000 people to evacuate prior to the massive explosion of the Mount Pinatubo on the Philippine island of Luzon in 1991. More than 70,000 people were moved out of harm’s way before Indonesia’s Mount Merapi erupted in in 2010. But forecasting is not infallible. In 2014, Mount Ontake in Japan erupted unexpectedly, killing 57 people.
OUR PLANET HOSTS an estimated 1,550 active volcanoes. Most signal their vitality with just an occasional rumble, approximately 20 are non-stop fumers that don’t erupt, and about 50 explode each year. A handful of these are big enough to cause problems.
To forecast big blasts, scientists measure fumes emanating from within the volcano. When magma starts to move upwards, carbon dioxide, being less soluble, bubbles out first. It’s followed by a belch of sulfur dioxide as the magma nears the surface. So an increase in the ratio of carbon dioxide to sulfur dioxide can provide an early warning that the magma is rising.
For decades, the only way volcano researchers
could measure these gases involved walking up the slopes towards the crater, or swooping past in an aircraft – both risky activities, especially once an eruption was underway and researchers wanted to see how gas composition changed during the event.
Being a volcanologist has become somewhat less risky since 2005. Two international groups – the Network for Observation of Volcanic and Atmospheric Change (NOVAC) and the Deep Carbon Observatory – have been developing instruments to monitor the target gases remotely and continuously. Both use portable low-cost spectrometers that analyse gas concentrations based on how sunlight is absorbed as it passes through the volcanic plume. Another type of meter measures changing levels of sulfur dioxide and can be installed kilometres downwind of active vents or on aircraft and satellites, allowing continuous monitoring. At present, some 35 volcanoes around the world are watched this way.
EARLY IN 2014, de Moor installed gas sensors on the lips of Turrialba’s three craters. Since then he and his colleagues have observed sharp increases in the carbon-sulfur ratio in the months prior to small eruptions. “It is a really promising result,” he says, “and a huge step forward for eruption forecasting.”
Nevertheless de Moor is worried by the similarities he is seeing to the last large eruption on Turrialba, in 1864: “The ash deposits suggest that it started with small eruptions, like those we are seeing now.” Those little disturbances, he says, gave way to an enormous “Strombolian” outburst (a reference to the hyperactive volcano on the island of Stromboli off the coast of Sicily).
Such an eruption from Turrialba, just 50 kilometres from the outskirts of Costa Rica’s capital San Jose, would devastate the surrounding terrain, potentially killing thousands and crippling the nation’s economy. In March de Moor installed more sensitive instruments on the volcano, to ensure even the smallest murmurings are detected.
Turrialba’s lessons, however, cannot necessarily be directly applied to other volcanoes. Take the rising ratio of carbon dioxide to sulfur dioxide. Turrialba’s menacing neighbour, Poás, produced the opposite signal prior to small eruptions.
The reason for this lies with the acidic lake that fills the Poás crater. Its waters normally absorb sulfur dioxide while allowing carbon dioxide to bubble through; that creates a permanently high carbon-to-sulfur ratio in the gas cloud plume. But in the days prior to an eruption, the lake’s ability to absorb sulfur dioxide reached saturation point. Unable to be absorbed by the lake, the excess gas bubbled out. As a result, the carbon/sulfur ratio fell prior to an eruption.
Deciphering the signal from Poás was a milestone, de Moor says, since many of the world’s most unpredictable and explosive volcanoes – including Nevado del Ruiz and Mount Ontake – have crater lakes. The findings from the Costa Rican volcanoes, he says, underscore “there is no one size fits all” eruption signal. THE KEY TO SUCCESSFUL prediction is to combine different techniques. Besides the established methods of gas and seismic monitoring, new satellite imaging techniques can reveal whether a volcano is actually swelling with magma. Volcanologist James Hickey at the University of Exeter in Britain is taking this approach to generate a computer model of what is happening underneath Sakurajima, an active volcano on the Japanese island of Kyushu.
Sakurajima’s last major eruption took place in 1914, killing 58 people and causing a massive flood in the nearby seaside city of Kagoshima. Its magma chambers have been refilling since, causing minor eruptions virtually every day.
Hickey’s model incorporates the area’s topography and underlying rock types, along with very precise GPS measurements of surface movement, to gauge just how fast the magma is replenishing. The results, published in Scientific Reports in September 2016, indicate the tank needs roughly 130 years to fill.
“In other words,” Hickey says, “enough magma