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

Tectonic Plates

Tectonic Plates
Earth's outer shell, the lithosphere, long thought to be a continuous, unbroken, crust is actually a fluid mosaic of many irregular rigid segments, or plates. Comprised primarily of cool, solid rock 4 to 40 miles thick,* these enormous blocks of Earth’s crust vary in size and shape, and have definite borders that cut through continents and oceans alike. *[Oceanic crust is much thinner and more dense than continental, or terrestrial crust].
There are nine large plates and a number of smaller plates. While most plates are comprised of both continental and oceanic crust the giant Pacific Plate is almost entirely oceanic, and the tiny Turkish-Aegean Plate is entirely land. Of the nine major plates, six are named for the continents embedded in them: the North American, South American, Eurasian, African, Indo-Australian, and Antarctic. The other three are oceanic plates: the Pacific, Nazca, and Cocos.

The relative small size of the numerous other plates neither diminishes their significance, nor their impact on the surface activity of the planet. The jostling of the tiny Juan de Fuca Plate, for example, sandwiched between the Pacific and North American Plate near the state of Washington, is largely responsible for the frequent tremors and periodic volcanic eruptions in that region of the country.
   
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