Last month, researchers from the Na- tional Autonomous University of Mexico hoped to reach the top of Po- pocatépetl, a 5400-meter-tall volcano near Mexico City, to install monitoring equipment at its summit crater. But El
Popo, as locals call it, rebuffed them with
ash and belches of acrid gas—precisely what
the scientists wanted to measure. They settled for installing the sensor lower down the
mountain, and hope to move it higher next
year. The goal is to measure just what—and
how much—El Popo has been smoking, because the fumes may hold a promising way
to forecast eruptions.
A growing body of monitoring data sug-
The idea of sniffing out restlessness in
gests that a sharp jump in the ratio of
carbon to sulfur gases emanating from a
volcano can provide days to weeks of warn-
ing before an impending outburst. The
latest evidence comes from three recent
studies, focusing on volcanoes monitored
as part of the Volcano Deep Earth Carbon
Degassing (DECADE) initiative, managed
by the Carnegie Institution for Science in
Washington, D.C. They offer hope that geo-
chemical monitoring of gases could some-
day join the two geophysical mainstays
of forecasting: tracking the swelling of
Earth’s surface and the rise in earthquakes
that typically precede eruptions. “It’s sta-
tistically robust as a forecasting tool,” says
Tobias Fischer, a volcanologist at the Uni-
versity of New Mexico in Albuquerque, and
chair of the DECADE project.
volcanic fumes has been around for de-
cades. For instance, a sharp rise in sulfur
emissions helped scientists anticipate the
1991 eruption of Mount Pinatubo in the
Philippines. Scientists have also keyed in on
the carbon-to-sulfur (C-S) ratio in volcanic
gases as a particularly helpful metric. In
principle, it can signal when a fresh injection of magma is rising from deep in the
crust—a prelude to an eruption.
The ratio changes because carbon diox-
ide (CO2) dissolved in rising magma bubbles
out at depths of 10 kilometers or more, as
the pressure drops. Sulfur-rich gases, in con-
trast, stay in solution up to shallower depths.
A spike in the ratio can thus provide warning
that a new batch of magma has risen above
a deep threshold. A subsequent drop in the
C-S ratio could indicate that the magma has
climbed further, to depths where sulfur gases
are released, but Fischer says this hasn’t
been observed enough to be reliable.
Despite the simple mechanism, estab-
lishing a clear link between the ratios and
eruptions requires constant monitoring.
Historically, researchers just bottled a few
gas samples during a visit to a volcano or
used airplanes or remote-sensing tools to
watch a volcano for several days or weeks,
says Christoph Kern, a physicist with the
U.S. Geological Survey in Vancouver, Washington. Either way, Kern says, it was hard to
catch an eruption in the act.
But that changed in the early 2000s, when
scientists began to develop new devices that
could be left on volcanoes to make continuous measurements and transmit the data to
researchers. They were solar powered, hardy
enough to survive the elements, and cheap
enough to risk sacrificing in an eruption.
“They’re essentially expendable,” says Marie
Edmonds, a volcanologist at the University
of Cambridge in the United Kingdom.
Italian scientists were the first to deploy
these instruments at volcanoes like Etna
and Stromboli, and they began to notice
changes in the C-S ratio in the days and
hours prior to eruptions. Since then, U.S.
and Japanese geologists have installed in-
struments at a handful of volcanoes in those
countries, and the DECADE project has
added them at nine more around the world,
including El Popo. Overall, changes in C-S
gas ratios seem to be a powerful portent,
Fischer says. “Now, we’re seeing it at many
Perhaps the clearest illustration comes
from Turrialba in Costa Rica, a volcano
that poses a threat to the city of San José,
30 kilometers to the west. Maarten de Moor,
a researcher at the Volcanic and Seismic
Observatory of Costa Rica, helped install
gas sensors on Turrialba in early 2014, just
in time for the volcano to start erupting.
He led a study, published in the Journal of
Geophysical Research in August, reporting
sharp increases in the C-S ratio of gases a
few weeks before each outburst over two
eruption cycles (see chart, left). “What we’ve
seen is quite mind-blowing,” he says. “These
signals are eye-opening.”
But for monitoring gas ratios to become
a widely used forecasting tool, researchers
will need to understand many complicating
i o Eruptions
Popocatépetl, a volcano near Mexico City, may warn
of its eruptions in its gassy belches.
Gas changes signal eruptions
Shifting gas mix warns that magma is on the rise
By Julia Rosen
A whiff of future eruptions
Spikes in gas ratios occur just days before eruptions
at Turrialba, a volcano in Costa Rica.