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Time to worry?
live in Costa Rica the time to worry is nigh - or not, depending
on how optimistic you feel. US geologists have pinned down the
timing of recent catastrophic eruptions in the central valley of
Costa Rica that suggest the next major volcanic event will occur
within the next few hundred thousand years.
Speaking at the annual meeting of the Geological Society of
America in April, in Puerto Vallarta, Mexico Phillip Gans, of
the University of California, Santa Barbara, told his audience
that although the Costa Ricans were not around for the last big
one, the next is inevitable. "Another pyroclastic flow like the
last one big one in Costa Rica will make the Mount St. Helens
eruption look like nothing!" he exclaimed. Like most geological
events major volcanic eruptions are so infrequent that
prediction on the human timescale is irrelevant; the eruption
might happen in five or five hundred thousand years, it is
impossible to say for certain.
not that we don't think there will be any warning," he later
told Spotlight, "it is that we don't know what the warning signs
will be like and whether we will be able to tell whether these
signs are indicative of a large or just another small eruption."
Several of the volcanoes around the central valley are currently
active, but their activity for the past 300 thousand years has
been of mainly small eruptions.
volcanoes in Costa Rica are formed by subduction. An oceanic
tectonic plate sliding under the country initiates melting
deeper in the Earth. The molten magma rises to the surface and
erupts as volcanoes, producing pyroclastic flows, lava flows,
and ash fall deposits. Pyroclastic flows are high-speed
avalanches of hot ash, rock fragments, and gas that race down
the sides of volcanoes during explosive eruptions or when the
steep edge of a dome breaks apart and collapses. These
pyroclastic flows, move at over 100 km per hour, knocking down
and burning everything in their path with scorching matter at
more than 500 Celsius.
volcanoes are unpredictable beasts, says Gans, Mount St. Helens
provided a four-month warning, revealing its intent through
small earthquakes within the volcano, monitored with sensitive
seismic equipment. 25 people died in the eruption, but the
majority were evacuated in plenty of time.
don't know if we will get a similar warning for a very large
eruption like the ones that have occurred prehistorically in the
Central Valley of Costa Rica," explains Gans. The Central
Plateau of Costa Rica is home to more than half of Costa Rica's
population and is flanked by several large volcanoes, some of
which are still active.
has spent many years dating with great precision hundreds of
rock samples from Costa Rica, using the radioactive decay of
potassium. The data he has accrued has allowed him to piece
together a detailed history of volcanic activity in the region
as well as to create a geological map of the country. His latest
prediction was the result of research carried out with G.
Alvarado-Induni and W. Perez of the University of Costa Rica in
San Jose, Costa Rica, Master's student Ian MacMillan of the
University of California, Santa Barbara; and Andrew Calvert of
the US Geological Survey in Menlo Park, California.
Subduction-related volcanism has been taking place in Costa Rica
for at least 24 million years, according to Gans' calculations.
Major pyroclastic eruptions have been commonplace during the
last million years in the vicinity of the Central Valley of
Costa Rica; the most recent was about 324,000 years ago. Indeed,
the cities and towns here, including the capital San Jose are
built on the pyroclastic flow deposits produced by that
eruption. A pyroclastic flow today of that magnitude would bury
them all in tens of metres of hot rock and ash. Just a few days
warning could save thousands of lives.
To predict an eruption a dense
array of seismographs will be needed to spot characteristic
tremors, detailed surveys and levelling data must be recorded to
indicate whether the ground surface is moving upward on the
flanks or at the summit of the volcano. Increased hot springs
activity around and at the summit of the volcano provides a
third indicator of a likely eruption. "The Costa Ricans are
definitely doing some of this type of monitoring," Gans told us,
"but not to the same degree that was done at Mount St. Helens."