YSM Issue 86.1

Predicting Volcanic Eruptions
Modeling Magma Wagging to Anticipate Volcanic Behavior
BY THERESA OEI
On Mount Vesuvius, broad sheets
of fire and leaping flames blazed
at several points, their bright glare
emphasized by the darkness of night.” –
Pliny the Younger’s account of Mount Vesuvius
and the destruction of Pompeii, 79 AD.
For centuries, volcanoes have been an
incomprehensible natural phenomenon. To
further our understanding of unexplained
eruptions, Professor David Bercovici of
Yale’s Geology and Geophysics Department
studies volcano behavior and recently proposed
a new predictive model that describes
a mechanism called magma wagging that
occurs just before a volcano erupts.
Understanding Volcanoes
There are three major types of volcanoes:
scoria cone, shield volcano, and stratovolcano,
which have eruption types of Strombolian,
Hawaiian, and Plinian, respectively.
The most dangerous of these classified
volcanoes is the Plinian, the type of volcano
that famously buried Pompeii. These volcanoes
have an explosive column of lava that
typically reaches 45 km.
For each of these volcano types, plate tectonics
are an integral part of why volcanoes
erupt. The majority of volcanoes occur in
subduction zones, an area where the sea floor
sinks into the Earth’s mantle. The slabs of
sea floor drag down wet crust and sediment
into the hot mantle, which cooks out their
water into the surrounding mantle rock. The
wetted mantle melts and the resulting magma
rises into the over-lying crust. As it passes
through the crust, it melts other rocks and
creates a viscous paste. This paste gets stuck
in the column and causes a buildup of pressure.
When the pressure reaches a certain
threshold, the volcano erupts and breaks
the rock surrounding the column. Ash and
sulfate are blasted from the volcano and enter
Photograph of the Santiaguita Volcano
Magma Ring. Courtesy of Professor
Bercovici.
the surrounding air, creating an atmosphere
that may endanger the health of nearby
plants and animals.
Volcanic Tremors and Magma Wagging
Volcanic tremors occur before the eruption
of a volcano and may last for a few
minutes or several days. The tremors are
defined by a low frequency shaking, about
0.5 to 5 hertz, that is approximately the same
despite the size or location of the volcano.
The universality of this tremor frequency was
puzzling to geologists, who sought to create
mathematical models that could predict
volcanic eruptions.
The model created by Bercovici explains
low frequency tremors by studying magma
wagging in the magma-conduit system. This
system encompasses a column of magma and
a surrounding area of gas. Magma consists
of a mixture of viscous rock melt, crystals
and bubbles. As it rises within the walls of
the conduit, the edges of the column develop
into a thin, permeable annulus that surrounds
the central magma column. The permeability
of the annulus allows it to transport gas into
the surrounding space.
Magma wagging occurs when magma is
20 Yale Scientific Magazine | January 2013 www.yalescientific.org