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magazine / apr09
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April 2009 issue |
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What lies beneath
Shedding light on the unseen forces of global magnetism
By Steven Fick and Dan Rubinstein
It resembles a psychedelic poster one might see on the wall
of a teenager’s unkempt bedroom, but beyond looking “far
out,” the first Earth Magnetic Anomaly Grid (EMAG) is a
window into the planet’s crust. The product of an international
scientific joint effort, the map (above) depicts what are known as
magnetic “peculiarities” near the Earth’s surface. These anomalies
are caused by differences in the magnetic strength and alignment
of rocks in the crust.
On land, the crust’s age is the dominant factor. Young crust is
typically thinner and has fewer magnetic minerals. These areas,
such as western Europe, appear in blue. The opposite is true
for old crust, such as the iron-rich Kursk anomaly in western
Russia, which features more contrasting colours. Under the
ocean, stripes abound — further evidence of both continental
drift and the regular reversal of the Earth’s magnetic field. (No
data are available for the grey areas.)
Think of the planet as a giant magnet. As tectonic plates move
apart, new crust is created at volcanic midocean ridges. When
the material in this crust spreads and cools, it is magnetized in
the direction of the Earth’s magnetic field at the time. But the
planet’s poles have “flipped” approximately once every 200,000
years over the past billion years, a gradual, complex process which
scientists believe is spurred by disturbances in the churning liquid
that surrounds the Earth’s solid inner core — the “geodynamo”
thought to generate the planet’s magnetic field. When the poles
reverse, the material on either side of the midocean ridges is,
in effect, frozen in time, with undersea stripes on the EMAG
reflecting this change in polarity. According to the geological
record, the poles haven’t flipped for nearly 800,000 years. The
next reversal is overdue, geologically speaking.
Stefan Maus of the Cooperative Institute for Research in
Environmental Sciences at the University of Colorado at
Boulder spent five years compiling and collating data from
about 85 institutions to help create the EMAG, which should
prove useful to scientists as well as groups involved in mineral
and oil exploration. “From the public perspective, the greatest
value is educational,” says Maus. “If I’m a teacher trying to
explain continental drift and can show students these stripes,
that’s visual evidence. The map shows us some very impressive
things about the Earth that one doesn’t otherwise see.”
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