The Himalayas: Two continents collide
Among the most dramatic and visible creations of plate-tectonic forces are
the lofty Himalayas, which stretch 2,900 km along the border between India
and Tibet. This immense mountain range began to form between 40 and 50 million
years ago, when two large landmasses, India and Eurasia, driven by plate
movement, collided. Because both these continental landmasses have about
the same rock density, one plate could not be subducted under the other.
The pressure of the impinging plates could only be relieved by thrusting
skyward, contorting the collision zone, and forming the jagged Himalayan
About 225 million years ago, India was a large island still situated off
the Australian coast, and a vast ocean (called Tethys Sea) separated India
from the Asian continent. When Pangaea broke apart about 200 million years
ago, India began to forge northward. By studying the history -- and ultimately
the closing-- of the Tethys, scientists have reconstructed India's northward
journey. About 80 million years ago, India was located roughly 6,400 km
south of the Asian continent, moving northward at a rate of about 9 m a
century. When India rammed into Asia about 40 to 50 million years ago, its
northward advance slowed by about half. The collision and associated decrease
in the rate of plate movement are interpreted to mark the beginning of the
rapid uplift of the Himalayas.
The 6,000-km-plus journey of the India landmass (Indian Plate)
before its collision with Asia (Eurasian Plate) about 40 to 50 million years
ago (see text). India was once situated well south of the Equator, near
the continent of Australia.
The Himalayas and the Tibetan Plateau to the north have risen very rapidly.
In just 50 million years, peaks such as Mt. Everest have risen to heights
of more than 9 km. The impinging of the two landmasses has yet to end. The
Himalayas continue to rise more than 1 cm a year -- a growth rate of 10
km in a million years! If that is so, why aren't the Himalayas even higher?
Scientists believe that the Eurasian Plate may now be stretching out rather
than thrusting up, and such stretching would result in some subsidence due
Sunset view of towering, snow-capped Mt. Everest, from the
village of Lobuche (Solu-khumbu), Nepal. (Photograph by Gimmy Park Li.)
Fifty kilometers north of Lhasa (the capital of Tibet), scientists found
layers of pink sandstone containing grains of magnetic minerals (magnetite)
that have recorded the pattern of the Earth's flip-flopping magnetic field.
These sandstones also contain plant and animal fossils that were deposited
when the Tethys Sea periodically flooded the region. The study of these
fossils has revealed not only their geologic age but also the type of environment
and climate in which they formed. For example, such studies indicate that
the fossils lived under a relatively mild, wet environment about 105 million
years ago, when Tibet was closer to the equator. Today, Tibet's climate
is much more arid, reflecting the region's uplift and northward shift of
nearly 2,000 km. Fossils found in the sandstone layers offer dramatic evidence
of the climate change in the Tibetan region due to plate movement over the
past 100 million years.
At present, the movement of India continues to put enormous pressure on
the Asian continent, and Tibet in turn presses on the landmass to the north
that is hemming it in. The net effect of plate-tectonics forces acting on
this geologically complicated region is to squeeze parts of Asia eastward
toward the Pacific Ocean. One serious consequence of these processes is
a deadly "domino" effect: tremendous stresses build up within
the Earth's crust, which are relieved periodically by earthquakes along
the numerous faults that scar the landscape. Some of the world's most destructive
earthquakes in history are related to continuing tectonic processes that
began some 50 million years ago when the Indian and Eurasian continents
Last updated: 05.05.99