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| March
2, 2001 |
| Rocks
Reveal Amazing Dino Lights |
By BBC
News Online science editor Dr David Whitehouse
BBC News
 | | At
high latitudes the night sky could have looked spectacular |
When
the dinosaurs walked the Earth, the planet's magnetic field was three times stronger
than it is today. It means the creatures would probably have seen amazing coloured
lights in the night sky.
The discovery about the strength of the field
has been made possible by a new research technique. It could help scientists better
understand the ancient Earth and how its molten core behaved millions of years
ago.
The field interacts with particles streaming from the Sun - it is
this that gives us the Northern and Southern Lights.
The field also protects
us from much of the Sun's harmful radiation. So, understanding how it works, and
if it might temporarily vanish, could help scientists predict potentially harmful
fluctuations.
North-South flips
It is known, for example, that
the Earth's magnetic poles have flipped many times in the past. A compass used
100,000 years ago would have pointed south instead of north.
The evidence
for these polarity changes is captured in tiny magnetic particles in new rock.
As lava cools around the particles, they align themselves with the ambient magnetic
field like a compass.
Great bands of rock displaying North-South flips
are laid across the ocean floors, showing how new rock produced at volcanic ridges
spreads out over time to make the sea bed. The ages of the rock bands date the
great changes in the Earth's magnetic field.
"We know a lot about the
directions of the Earth's magnetic field," said John Tarduno, professor of geophysics
at the University of Rochester, US. "It is how we unravel plate tectonics and
learn something about the Earth's core.
"But to understand the way the
field works, you also need to know the field's strength in the past, and we don't
know nearly enough about that."
New technique
The traditional
way to measure the ancient Earth's magnetic field strength (called paleointensity)
was developed more than four decades ago. It requires a piece of igneous rock
to be heated and cooled in a chamber that is shielded from any outside magnetic
field.
In this way, the magnetism in the rock's particles can be "drained".
Scientists can then increase the magnetic field and measure how much magnetism
the particles in the rock can hold. However, because of contamination, scientists
do not regard the technique as being particularly accurate.
To develop
an improved method, scientists at the University of Rochester used a superconducting
quantum interference device (Squid). This equipment is normally employed in computer
chip design, and is extremely sensitive to the tiniest magnetic fields.
"With
the Squid, we realised that we could start measuring the magnetic fields in single
crystals instead of whole rocks," said Dr Tarduno. "That let us use samples we
knew had no contamination."
Early tests have worked well on feldspar,
the most common mineral on the Earth's surface. The mineral has a microscopic
shell around its slivers of magnetic crystal that protect it from contamination.
Inside the core
"We can now measure paleointensity in places we
could never measure anything before," said Dr Tarduno. "And the results are more
reliable than ever before."
Measurements have been made on 100-million-year-old
rock samples from India. Scientists are particularly interested in them because
they come from a period when the Earth's magnetic field was not reversing.
It
was found that during this time the Earth's magnetic field was three times stronger
than the old method suggested.
Researchers say that besides possibly giving
T. rex a better display of the Northern and Southern Lights, the field strength
indicates what was happening inside the Earth's hot, molten core - the geodynamo
that drives the magnetic field.
"Our findings suggest that there is a
relationship between magnetic reversals and paleointensity," Dr Tarduno said.
"Such a relationship fits very well with supercomputer models. It's an exciting
time. We're really starting to understand how the heart of our planet works."
The research is published in the journal Science. |
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