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<DIV><FONT size=4><!--StartFragment --><FONT size=6><STRONG>Earthquakes
rearranging Earth's map</STRONG></FONT>
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<FONT size=2>Monday, January 10, 2005 Posted: 10:28 AM EST (1528 GMT)
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<P><B style="FONT-SIZE: 14px">(AP) -- Sumatra's catastrophic December 26
earthquake flattened cities and shifted landscapes in seconds with the force of
a million atom bombs. Within hours, ferocious waves had turned the Indian Ocean
into a cauldron of death and debris.</B></P>
<P>But the geologic circumstances that set up the worst natural disaster in a
century were much longer in the making. How long?</P>
<P>Try 300 million years. Maybe twice that long.</P>
<P>Once, scientists believe, all the earth's continents were combined in a
single gigantic land mass they call Pangea (Greek for "all lands"). But
geological forces caused it to break apart; and ever since then, the pieces,
which scientists call plates, have been drifting across the planet at an average
rate of a few inches a century.</P>
<P>As the plates move, grinding collisions between them trigger earthquakes and
even build mountains. The Indian subcontinent, for example, has been moving
inexorably northward for millions of years, colliding with Asia like a
slow-motion car wreck, the land at the edge of the collision buckling to form
the Himalayas. Mount Everest and other mountains in the chain are still growing
at a rate of about a half-inch per year.</P>
<P>Geologists say it was a lurching collision between the Indian plate and the
Burma plate, which grind together along a 750-mile (1,200-kilometer) long,
north-south fault in the Indian Ocean, that triggered the recent earthquake off
Sumatra, and the resulting tsunami.</P>
<P>The quake is believed to have shifted north Sumatra and smaller nearby
islands by as much as 60 feet (18.2 meters).</P>
<P>In human time, earthquakes that powerful are rare, but in the vastness of
geologic time, they are commonplace. "An earthquake of this magnitude, in this
part of the world, has probably occurred about a million times since the breakup
of Pangea," said Chris Scotese, a geophysicist at the University of
Texas-Arlington. "No exaggeration."</P>
<P>Geologists believe they understand, at least generally, what causes the
plates to move and collide.</P>
<P>The earth, they explain, is made up of pressurized layers. At the center is a
hot metal core about 2,160 miles (3,500 kilometers) thick, the center of it
solid and the outer layer molten. Then comes the hot, rocky mantle, about 1,800
miles (2,900 kilometers) thick. On top of that is the part we live on, a thin,
cooler crust, perhaps 30 miles (48 kilometers) thick.</P>
<P>The crust is not solid and unbroken like the coating on a gumball. Rather, it
is fractured into more than a dozen overlapping, rigid plates of rocky armor.
The plates move relative to one another as they slide atop the hotter layers
below.</P>
<P>The overlapping points between plates are called subduction zones, and that
is where the biggest earthquakes strike, changing Earth's map slightly each
time. Volcanoes most often erupt in these boundaries between the plates.</P>
<P>Understanding these forces more precisely in enough detail to predict
earthquakes has proven elusive. For one thing, many of the boundaries between
plates are covered by deep oceans, making them inaccessible to study. Even when
plates come together on land, as they do in California, the real action occurs
out of sight miles (kilometers) below the surface.</P>
<P>Only now, in central California, have scientists started to drill about two
miles (three kilometers) into a fault zone to learn more about these forces. But
even that will offer only a blurry snapshot; the December 26 earthquake occurred
on a larger fault nearly six miles (10 kilometers) down.</P>
<P>Really big earthquakes, those like the Sumatra quake that noticeably
rearrange the landscape are so rare that there are few opportunities to study
them. Until the Sumatra quake, it had been 40 years since a magnitude 9 temblor
occurred in Alaska. Like fishermen waiting for the big one, geologists can spend
their entire careers waiting for the chance to study the huge temblor that never
comes.</P>
<P>"A lot of what we see that is catastrophic occurs in the snap of a finger,"
says David Wann, a geophysicist for the U.S. Geological Survey. "And then
nothing happens for hundreds of years."</P>
<P>Obviously, there were no human beings around when Pangea broke apart. So how
do scientists know it ever existed?</P>
<P>"Look at the fit of the matching coasts of Africa and South America," Scotese
said, the continents fitting together like jigsaw puzzle pieces.</P>
<P>More detailed evidence can be found in both the geologic and fossil
record.</P>
<P>For example, researchers studying one of Antarctica's rare rocky exposures in
2003 reported finding metals with the same chemistry as those in China,
suggesting the two were once joined together.</P>
<P>Meanwhile, fossils of extinct tropical plants have been found in Antarctica
and Australia, indicating those continents were once located near the
equator.</P>
<P>Fossils of Lystrosaurus, a reptile that lived during the Triassic Period 200
million years ago, have been found in Africa, India and Antarctica -- strong
evidence that those continents were still connected then. Much later, during the
Cretaceous Period 65 million years ago, the continents had separated into an
arrangement roughly like today's map, making it nearly impossible for
Tyrannosaurus rex to range beyond his isolated home in North America.</P>
<P>Scotese and many other geologists believe that Pangea was not the first
supercontinent on the planet, nor do they believe it will be the last. Rather,
they say, supercontinents have formed and broken apart several times during the
earth's 4.6 billion-year history.</P>
<P>Computer models of the earth's moving plates show that the continents are
again drifting together. For the last 40 million years, Africa has been melding
into Europe, its northern drift pushing up the Alps and the Pyrenees and causing
earthquakes in Italy, Greece and Turkey. The Mediterranean is slowly shrinking,
a small remnant now of what was once a large ancient ocean.</P>
<P>Australia, meanwhile, is on a collision course with southeast Asia.</P>
<P>Beyond that, the computer models become increasingly speculative, but they
suggest North America is inching toward the building Euro-African-Asian
monolith.</P>
<P>In another 250 million years, Scotese believes, Earth will be one
supercontinent again Pangea Ultima.</P></DIV></BODY></HTML>