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Environment: The Real Story Behind the 'Roof of the World'

August 24, 2008

By Andrea Thompson, Senior Writer
Live Science
August 21, 2008

The southern face of Mount Everest, known locally as Sagarmatha,
soars above the monsoon clouds Saturday, Aug. 26, 2000 at the border
of Nepal and Tibet. AP Photo/John McConnico

It's called the "Roof of the World" with good reason -- the Tibetan
Plateau stands over 3 miles above sea level and is surrounded by
imposing mountain ranges that harbor the world's two highest summits,
Mount Everest and K2.

While the world's top mountaineers regularly attempt to summit the
forbidding peaks, the remote area is home to a rich variety of
cultures, from villages in Pakistan that practice the various sects
of Islam to the Buddhist communities of Tibet, now part of the
People's Republic of China. Perhaps the most well-known person of the
region is the Dalai Lama, Tibet's spiritual leader and an advocate
for a peaceful solution to Tibet's bid for independence.

Less well-known is the story of how the Tibetan Plateau and the
craggy peaks that surround it formed. The geologic tale, as it has
been known, is familiar to many schoolchildren: About 50 million
years ago, the Indian subcontinent began to collide with Eurasia, and
as it slammed into the bigger landmass, the plateau and the Karakoram
and Himalaya ranges were born.

But there's more to the story.

In a review of research on the evolution of the Tibetan Plateau,
published in the Aug. 22 issue of the journal Science and funded by
the National Science Foundation, a group of researchers put together
the geological puzzle pieces to develop a more intricate, and
somewhat controversial, picture of the development of the modern
Tibetan Plateau than was previously envisioned.

"It's a complicated place," said Leigh Royden of MIT, lead author of
the review.

Putting the pieces together could also help scientists determine the
cause of the earthquake that rocked China's Sichuan province in May,
killing tens of thousands.

Continents collide

Before India rammed into Eurasia, the Tethys Ocean, which separated
the two landmasses, was being subducted beneath Eurasia. In the Late
Cretaceous (about 100 to 65 million years ago) a volcanic mountain
range similar to the modern Andes developed along the southern edge
of the Eurasian plate. But these earlier mountains would have been
"nothing like what's there now," Royden told LiveScience.

These earlier tectonics would have begun to raise portions of the
Tibetan Plateau above sea level and thickened the continental crust
there, the researchers said, setting the stage for what would come later.

After the collision, more of the area now included in the plateau was
involved in the tectonic changes, with the southern and central
portions of Tibet reaching high elevations (the northern portions
remained low) as the crust "shortened," or smushed together. As the
crust was mashed, the towering peaks that make up the Himalayas and
the Karakoram were gradually pushed up to their dizzying heights.

As the collision progressed, material from the lithosphere (the solid
outer shell of the planet) below the surface crust was "shoved out"
toward the east, as Royden put it. These lozenges of lithosphere were
aided by the eastward movement of subduction trenches in the Pacific
Ocean to the east of what is now China.

Crustal movement

Eventually, around 20 millions years ago, the trenches halted in
their eastward march. As India and Eurasia continued to collide,
"stuff couldn't leave to the east," Royden explained.

While some geologists think crustal shortening continued to build up
the plateau, Royden says there is little evidence for this, and that
the pile-up of lithospheric material underneath the plateau continued
to thicken the crust and raise the plateau.

Whether or not the subsurface material is flowing faster or slower
now, geologists aren't sure, Royden said. By extension, they don't
know whether the plateau is getting higher or lower, though that may
depend on what part of the plateau you're talking about, Royden said,
with some parts possibly rising while others sink. Studies of the
rates at which rivers have cut down through the rock in these areas
may help geologists to suss out the vertical motions of the plateau.

Tibet and the Sichuan quake

The movement of the lithosphere under the plateau could also be
behind the Sichuan earthquake, Royden said.

The area where the quake occurred is traditionally considered one of
low seismic risk, Royden said. Some geologists have said the quake
was a result of traditional thrust faulting, where one piece of crust
is pushed up over another. But, "when you look at the whole geologic
context," Royden says, the movement of the lithosphere in the region
could be at fault.

As the material flows eastward it runs into an older, stronger piece
of crust at the Sichuan Basin and piles up at the basin's western
edge. The interpretation of Royden and her colleagues is that a fault
with vertical and eastward motion set up by this situation created the quake.

Though much about the Tibetan Plateau's geology, including the exact
cause of the Sichuan quake, remains a mystery, Royden is fairly
certain than in a decade or two, geologists will have a much clearer
picture of what is happening underneath the "Roof of the World."

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