Plate Tectonics: A whole new way of looking at your planet
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The Book  
Table of Contents
Introduction
In the Beginning
The Tectonic Plates
Mount St. Helen
How Plates Move
Plate Boundaries
A Changing Earth
Pangaea - All Lands
Mid-Ocean Ridges
An Ocean is Born
The Birth of an Island
Mountain Ranges
Subduction Zones
Island Arcs
The Ring of Fire
Faults
Earthquakes
Hot Spots
Mantle Plumes
Origin of Life Theories
Global Climate
Other Worlds
Welcome to Your World

Mountain Ranges
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 ranges.

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.

India collides with Asia and the Himalayas are bornHere's 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.

Fossilized 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 range? Unlikely.

Had the entire planet, Himalayas and all, at some point in Earth’s 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 world’s 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.

Andes Mountain RangeGrowing Mountains
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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