YSM Issue 86.1

GEOLOGY
pushed up through the conduit construct
in the center of the volcano. A layer of gas
develops between the conduit and the rock
of the volcano, and magma comes out of
the conduit, causing the magma column to
shake within the gas layer. The shaking and
lateral movement of the column is countered
by the spongy, springy foam of the annulus,
which returns the column to its original
position, causing oscillation described as
“wagging.” The pressure changes within the
annulus are transmitted to the walls of the
magma-conduit system, which leads to an
observable tremor.
Bercovici developed a mathematical
equation to describe the oscillatory magma
wagging phenomenon. Bercovici’s equation
shows that the frequency of the magma
wagging is only weakly dependent on the
size of the volcano. This explains why all
volcanoes have frequencies close to 1 hertz
and little frequency variation is observed
across volcanoes.
Before a volcanic eruption, the frequency
of these seismic tremors increases. In
Sketch of the magma wagging model. Courtesy of David Bercovici.
Mathematical model for magma wagging.
Courtesy of David Bercovici.
general, the magma wagging frequency is
low and constant but closer to eruption
the frequency increases, as does the range
of the frequencies. This increase can be
explained by two different factors. First, the
gas layer surrounding the column or conduit
of magma becomes narrower. Second, the
walls of the volcano start to fall apart and
collapse inwards, increasing the pressure on
the column of magma. As these two events
occur, the frequency of magma wagging
increases.
Applying the Model
The model designed by Bercovici explains
the mechanism for volcanic tremors, but it is
difficult to apply this model to accurately predict
eruptions. Eruption depends on many
properties and environmental circumstances,
complicated by our inability to observe the
inside of volcanoes. However, it is possible to
observe the change in tremor frequencies as
volcanoes approach eruption, which allows
some forecasting of volcanic behavior.
To determine their predictive accuracy,
volcano models are tested using supercomputers
that simulate volcanic eruptions. Ultimately,
scientists hope that the magma wagging
model performs well in supercomputer
modeling trials. Bertovici notes, however,
that while theoretical modeling can be helpful,
“nothing can compare to empirical data.”
About the Author
Data from past volcanic eruptions may
serve as guidelines for judging the impact
of volcanoes in the future. For example, the
tremor frequency and amplitude may reveal
the amount of damage a volcano’s eruption
will cause. Although predicting volcano
behavior remains imperfect, models such as
Bercovici’s bring us closer to understanding
the incredibly powerful natural phenomenon
of volcanic eruptions.
Theresa Oei is a sophomore Molecular Biophysics and Biochemistry major in
Pierson college. She is on the board of Synapse, Yale Scientific Magazine’s Outreach
Program, and works in Professor Steitz’s lab studying target genes of the viral miRNAs
HSUR4 and HSUR5 for their role in tumorigenesis.
Acknowledgements
The author would like to thank Professor Bercovici for his time and consideration in
describing his research in geological development.
Further Reading
• Jellinek, Mark A., and Bercovici, David. “Seismic Tremors and Magma Wagging
During Explosive Volcanism,” Nature Journal, Vol. 470 (2011): 522-525.
www.yalescientific.org January 2013 | Yale Scientific Magazine 21