Thursday, February 19, 1998
Mantle magma answers some quake questions
Molten rock pools may help scientists ascertain causes of
seismic disturbances
By Matt Grace
Daily Bruin Contributor
On the other side of the globe, earthquakes greater than 6.0 on
the Richter scale pound the tiny island of Tonga in the Southwest
Pacific.
At the same time they provide scientists on South Campus with
unique insights into the solid-liquid interfaces governing the
unpredictable activity of earthquakes.
Using equipment originally designed to spy on the nuclear
activities of Russia, John Vidale of UCLA and Michael Hedlin of UC
San Diego have provided the strongest evidence to date for the
existence of molten magma reservoirs, suspended thousands of miles
beneath the earth’s surface.
"This is an incremental and important step in the
characterization of the core-mantle boundary," said Jim Whitcomb,
the program director for geophysics at the National Science
Foundation.
The military-subsidized network of 130 seismometers dotted over
60 square miles in southern Norway, detect faint seismic
vibrations, created by anything from earthquakes to atomic
bombs.
The discovery of molten lakes of rock – roughly the size of
California – provide strong evidence against the earth’s mantle
consisting entirely of solid rock.
The scientists based their analysis on seismic waves which
passed through the mantle from 25 earthquakes hitting Tonga in the
last two decades.
They found obstacles within the mantle which changed the
direction of the seismic waves. If the mantle was composed
completely of solid rock, then the seismic waves would travel in a
straight line.
The existence of a liquid-solid interface offers the only
explanation for the discrepancies in seismic wave activity.
"The melt scatters (the seismic waves) off in all directions,"
said John Vidale, associate professor of seismology in the
department of earth and space science. "It affects the direction
they’re going and their amplitude."
The stiffer, colder mantle conducts the waves at a higher
velocity compared to the pools of mushier, molten rock near the
core-mantle boundary, Vidale said.
The characterization of this region offers new insight into the
unconventional behavior of the liquid rock as it migrates
throughout the earth’s inner layers.
"Convection of mantle is the driving force behind earthquakes,"
said Whitcomb.
As scientists learn more about the dynamic relationship between
shifting temperatures and densities in the core-mantle boundary,
the more they understand the behavior of earthquakes, Whitcomb
said.
In the meantime, scientists diversify their research efforts so
they can attack different angles of seismology.
Michael Hedlin is currently studying how the heterogenities in
the mantle vary across the earth by comparing different sets of
data to the global average.
This, he realizes, will be one more step to understanding
earthquakes.
"Science moves in little jumps, and this might get us a little
closer," concluded Hedlin.
RACHEL FACTOR
John Vidale is a member of the research team that is
investigating the Earth’s core.