Nearly 3 million
people live in close proximity to Mount Vesuvius, which infamously erupted in
A.D. 79, burying the prosperous towns of Pompeii and Herculaneum. Researchers
agree that it is not a matter of if Vesuvius will erupt again, but when and
how violently. The Italian government is even offering 30,000 Euros to anyone
living in one of the 18 towns in the immediate area who is willing to move out
of the area. Meanwhile, volcanologists are reconstructing the volcano’s
past to better predict just what might happen when it blows its top again.
Close to 3 million people live near Mount Vesuvius, which has erupted explosively
eight times over the last 17,000 years, including the A.D. 79 eruption that
destroyed Pompeii and other towns. Researchers are studying the volcano to figure
out what might happen when Vesuvius erupts again. Photo by Dan Dzurisin, 1983,
U.S. Geological Survey.
Mount Vesuvius has a varied history, sometimes going thousands of years between
eruptions, and sometimes going just a few years. Sometimes, like in A.D. 79,
the eruptions have been explosive; while at other times, the volcano peacefully
effused lava. Furthermore, the pumice composition in each of Vesuvius’
major eruptions has been slightly, but significantly, different, says Dan Morgan,
a volcanologist at the University of Durham in the United Kingdom. Thus, he
says, determining what is “typical” and likely future behavior at
Vesuvius is “exceedingly difficult.” It is those questions that a
new model, developed by a team led by Andrea Borgia, a volcanologist at the
European Development and Research Agency in Rome, tries to answer.
According to Borgia et al.’s model, published in the Feb. 2 Geophysical
Research Letters, a strong Strombolian eruption — a voluminous lava
fountain with occasional explosions — is the most likely type of eruption
to occur at Vesuvius in the short-term. This type of eruption is not as risky
to people living in the area, Borgia says, because there is usually ample time
to get out of the way of the lava flows.
An eruption that presents a larger risk but is less likely to occur is the Plinian
type — named for Pliny the Younger, who witnessed the A.D. 79 eruption
of Vesuvius and provided the first-ever written account of a volcanic eruption.
This violent, explosive eruption sends plumes of hot gas, ash and rock tens
of kilometers into the atmosphere and cascading down the mountainside in the
form of pyroclastic flows.
The evacuation plans and hazard assessments at Vesuvius are based on Plinian
or sub-Plinian eruptions, which pose a different hazard than large Strombolian
eruptions, Borgia says. “The problem is our model shows that it is very
probable that the next eruption will be Strombolian, with large, destructive
lava flows,” Borgia says. Based on geologic and structural data and synthetic
aperture radar images, the model indicates that eruptive activity at Vesuvius
can be characterized by three phases.
From 25,000 years ago to 18,000 years ago, the volcano was “building its
edifice up” with large lava flows (mostly Strombolian eruptions). The second
phase, from 18,000 years ago through the A.D. 79 eruption, involved the accumulation
of gas-rich magma beneath the volcano that led to explosive Plinian eruptions.
During the third phase, beginning after the A.D. 79 eruption and continuing
through the next 7,000 years, Borgia says, the volcano “spreads” —
creating subsidence on the summit area, new ridges around the base (Pompeii
is actually built on one of these ridges) and occasionally fractures and large
lava flows, but no more Plinian eruptions.
Since the A.D. 79 eruption, the explosivity of eruptions — the most recent
being in 1944 — has decreased, Borgia says, and will continue to remain
low. However, “the trend of diminishing volcanic explosivity is perhaps
a little more complex to interpret than the authors claim,” says Morgan,
who is involved in the ERUPT (European Research on Understanding Processes and
Timescales in magma systems) program that studies Vesuvius and other European
volcanoes.
During what Borgia calls the spreading phase (over the past 2,000 years), two
explosive eruptions have occurred, contradicting the model, Morgan says. Furthermore,
the type of eruption also depends on the chemistry of the magma and the tectonics
of the volcano.
Vesuvius “is a very complicated volcano,” Morgan says. But large lava
flows are certainly a possibility there, he says, as most of the area between
the volcano and the sea has been buried under lava at some point in the past
400 years.
If the past can predict the future, then generally, the longer the quiescent
period at Vesuvius, the more violent the renewal of activity, say Morgan and
others. At the International Geological Congress meeting last summer in Florence,
Franco Barberi of Rome University called Vesuvius the world’s most dangerous
volcano, with a high probability of an explosive Plinian eruption by the end
of this century on the order of the one that buried Pompeii.
Megan Sever
Link:
ERUPT
project
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