[tt] geomagnetic field to reverse polarity +1.5 kYrs

[Eugen Leitl]

http://www.sciencedaily.com/releases/2008/09/080926105021.htm

Earth's Magnetic Field Reversals Illuminated By Lava Flows Study

ScienceDaily (Sep. 26, 2008) — Earth's north magnetic pole is shifting and
weakening. Ancient lava flows are guiding a better understanding of what
generates and controls the Earth's magnetic field – and what may drive it to
occasionally reverse direction.

The main magnetic field, generated by turbulent currents within the deep mass
of molten iron of the Earth's outer core, periodically flips its direction,
such that a compass needle would point south rather than north. Such polarity
reversals have occurred hundreds of times at irregular intervals throughout
the planet's history – most recently about 780,000 years ago – but scientists
are still trying to understand how and why.

A new study of ancient volcanic rocks, reported in the Sept. 26 issue of the
journal Science, shows that a second magnetic field source may help determine
how and whether the main field reverses direction. This second field, which
may originate in the shallow core just below the rocky mantle layer of the
Earth, becomes important when the main north-south field weakens, as it does
prior to reversing, says Brad Singer, a geology professor at the University
of Wisconsin-Madison.

Singer teamed up with paleomagnetist Kenneth Hoffman, who has been
researching field reversals for over 30 years, to analyze ancient lava flows
from Tahiti and western Germany in order to study past patterns of the
Earth's magnetic field. The magnetism of iron-rich minerals in molten lava
orients along the prevailing field, then becomes locked into place as the
lava cools and hardens.

"When the lava flows erupt and cool in the Earth's magnetic field, they
acquire a memory of the magnetic field at that time," says Singer. "It's very
difficult to destroy that in a lava flow once it's formed. You then have a
recording of what the paleofield direction was like on Earth."

Hoffman, of both California Polytechnic State University at San Luis Obispo
and UW-Madison, and Singer are focusing on rocks that contain evidence of
times that the main north-south field has weakened, which is one sign that
the polarity may flip direction. By carefully determining the ages of these
lava flows, they have mapped out the shallow core field during multiple
"reversal attempts" when the main field has weakened during the past million
years.

During those periods of time, weakening of the main field reveals "virtual
poles," regions of strong magnetism within the shallow core field. For
example, Singer says, "If you were on Tahiti when those eruptions were taking
place, your compass needle would point to not the North Pole, not the South
Pole, but Australia."

The scientists believe the shallow core field may play a role in determining
whether the main field polarity flips while weakened or whether it recovers
its strength without reversing. "Mapping this field during transitional
states may hold the key to understanding what happens in Earth's core when
the field weakens to a point where it can actually reverse," Hoffman says.

Current evidence suggests we are now approaching one of these transitional
states because the main magnetic field is relatively weak and rapidly
decreasing, he says. While the last polarity reversal occurred several
hundred thousand years ago, the next might come within only a few thousand
years.

"Right now, historic records show that the strength of the magnetic field is
declining very rapidly. From a quick back-of-the-envelope prediction, in
1,500 years the field will be as weak as it's ever been and we could go into
a state of polarity reversal," says Singer. "One broad goal of our research
is to provide some predictive capability for what could happen and what could
be the signs of the next reversal."

Hoffman and Singer's research has been supported largely by grants from the
National Science Foundation.