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Birth of the Himalaya
by Roger Bilham
Photography by Liesl Clark
The Continental Shuffle
Over two hundred fifty million years ago, India, Africa,
Australia, and South America were all one continent called
Pangea. Over the next several million years, this giant
southern continent proceeded to break up, forming the
continents we know today. Pangea essentially turned inside
out, the edges of the old continent becoming the collision
zones of new continents. Africa, South America, and Antarctica
began to fragment.
What ultimately formed Mt. Everest, about 60 million years
ago, was the rapid movement of India northward toward the
continent of EuroAsia;
Click here for a present-day map of the Indian
subcontinent. India charged across the equator at rates of up to 15
cm/year, in the process closing an ocean named Tethys that had
separated fragments of Pangea. This ocean is entirely gone
today, although the sedimentary rocks that settled on its
ocean floor and the volcanoes that fringed its edges remain to
tell the tale of its existence.
Click here to see a Shockwave animation sequence on the
formation of the Himalaya
(Get Shockwave).
Click here to see an
unanimated sequence of the
same thing.
Mechanics of Mountain Formation
To understand the fascinating mechanics of the collision of
India with Asia we must first look beneath the Earth's
surface. The continents are carried by the Earth's tectonic
plates like people on an escalator. There are currently 7
giant plates sliding across the Earth's surface, and a handful
of smaller ones. There may have been more or fewer plates in
the past. Currently they slide, collide, and recede from one
another at rates of 1-20 cm/year. They are driven by internal
heat deep in the earth that is able to escape efficiently only
by convection. Convection is the process that drives hot
currents of gas or liquid upwards because they are less dense,
and cold currents of liquid downwards because they are more
dense.
Continental Plates
In some ways, the continents are like giant accumulations of
rock debris lying atop the tectonic plates. Continents are the
"scum of the Earth," consisting mostly of light minerals like
quartz, which can't sink into the Earth's dense mantle.
For at least 80 million years the oceanic Indian Plate
continued its inexorable collision with southern Asia,
including Tibet. The heavy ocean floor north of India acted
like a giant anchor, plunging rapidly into the mantle, and
dragging the Indian continent along with it, northward,
towards Tibet.
As the plates collided, the sinking ocean floor generated
volcanoes in southern Tibet because the rock at the top of the
descending plate melted, from friction and the huge pressures
of collision. However, by 25 million years ago the fast moving
Indian continent had almost entirely closed over the
intervening ocean, squeezing the sediments on the ocean foor.
Since the sediments were lightweight, instead of sinking along
with the plate, they crumpled into mountain ranges—the
Himalayas.
By 10 million years ago the two continents were in direct
collision and the Indian continent, because of its enormous
quantity of light quartz-rich rocks, was unable to descend
along with the rest of the Indian plate. It was at about this
time that the anchor chain must have broken; the descending
Indian plate may have fallen off and foundered deep into the
mantle.
Although we don't fully understand the mechanism of what
happened next, it's clear that the Indian continent began to
be driven horizontally beneath Tibet like a giant wedge,
forcing Tibet upwards. Tibet, meanwhile, is behaving like a
giant roadblock that prevents the Himalaya from moving
northward. Under the peaks and under most of Tibet the Indian
plate is apparently gliding along almost frictionlessly.
Future of the Himalaya
Over periods of 5-10 million years, the plates will continue
to move at the same rate, which allows us to forecast fairly
reliably how the Himalaya will develop. In 10 million years
India will plow into Tibet a further 180km. This is about the
width of Nepal. Because Nepal's boundaries are marks on the
Himalayan peaks and on the plains of India whose convergence
we are measuring, Nepal will technically cease to exist. But
the mountain range we know as the Himalaya will not go away.
This is because the Himalaya will probably look much the same
in profile then as it does now. There will be tall mountains
in the north, smaller ones in the south, and the north/south
width of the Himalaya will be the same. What will happen is
that the Himalaya will have advanced across the Indian plate
and the Tibetan plateau will have grown by accretion. One of
the few clues about the rate of collision between India and
Tibet before the GPS measurements were made was the rate of
advance of Himalayan sediments across the Ganges plain. There
is an orderly progression of sediments in front of the
foothills. Larger boulders appear first, followed by pebbles,
and further south, sand-grains, silts, and finally very fine
muds. This is what you see when you drive from the last hills
of the Himalaya southward 100 km. The present is obvious, but
the historical record cannot be seen on the surface because
the sediments bury all former traces of earlier sediments.
However, in drill holes in the Ganges plain, the coarser rocks
are always on the top and the finer pebbles and muds are on
the bottom, showing that the Himalaya are relentlessly
advancing on India.
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