| While new ocean crust is constantly
being created at mid-ocean ridges, old crust must
either be destroyed or reduced at the same rate
(or else the planet would be continually expanding
and increasing in volume). The plates, therefore,
emerging along mid-ocean ridges, sliding over the
athenosphere, and grinding past other plates along
transform faults, are almost all headed on a collision
course. When two continents carried on converging
plates ram into each other, they crumple and fold
under the enormous pressure, creating great mountain
The highest mountain range in the world, the snow-capped
Himalayas, is an example of a continent-to-continent
collision. This immense mountain range began to
form when two large landmasses, India and Eurasia,
driven by tectonic plate movement, collided. Because
both landmasses have about the same rock density,
one plate could not be subducted under the other.
The pressure of the colliding plates could only
be relieved by thrusting skyward. The folding, bending,
and twisting of the the collision zone formed the
jagged Himalayan peaks. This string of towering
peaks is still being thrust up as India, embedded
in the Indo-Australian Plate, continues to crunch
relentlessly into Tibet, on the southern edge of
the Eurasian Plate.
a more detailed chronological explanation.
About 220 million years ago, India was an island
situated off the Australian coast, and separated
from the Asian continent by a vast ocean called
the Tethys Sea. When Pangaea broke apart about 200
million years ago, India began to move northward.
Scientists have been able to reconstruct India's
northward journey. When India rammed into Asia about
50 million years ago, its northward advance slowed.
The collision and decrease in the rate of plate
movement mark the beginning of the Himalayan uplift.
Sea Shells near Himalayan Peaks?
When archaeologists found the fossilized remains
of ancient sea-creatures near the peaks of the Himalayas
they were, understandably, puzzled. Intriguing questions
were raised. Was there once an ocean or other large
body of water at the top of this enormous mountain
Had the entire planet, Himalayas and all, at some
point in Earths long history, been submerged
underwater? Possibly - but highly improbable. No
theory could fully explain this apparent paradox.
Until the theory of plate tectonics was put forth.
Briefly, it goes like this: As the Indo-Australian
Plate, with India firmly embedded, approached the
Eurasian continent 20 million years ago, its leading
edge, comprised of oceanic crust, was first to be
crumpled and uplifted. Slowly, the Himalayas rose
and the leading oceanic crust of the Indian sub-continent,
carrying the fossilized remains of its ancient ocean
inhabitants, was thrust up by the crumpling crust
in its wake. Thus, plate tectonics explains how
the majestic peaks of one of the worlds great
mountain ranges were once the deep sea-floors of
an ancient drifting plate.
The European Alps have been formed in similar
fashion, starting some 80 million years ago when
the outlying continental fragments of the African
Plate collided with the Eurasian Plate. Unyielding
pressure between the two plates continues even
today, resulting in the gradual closing up of
the Mediterranean Sea.
As an underlying oceanic plate tips down, its
ocean-floor sediment is scraped off along the
front edge of the overriding continental plate.
The result is an increase in the width and thickness
of the overriding plate. This could be why the
Andes, a long mountain range bordering the west
coast of South America, appears to be growing
higher. Perhaps sediment from the Nazca Plate,
which is diving under South America in the Peru-Chile
Trench, is scraping off on the roots of the Andes.
This scraping adds thickness and buoyancy to the
mountains so that they float upward more rapidly
than their peaks can be eroded by wind and rain.
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