Earth's crust is broken into segments called Tectonic Plates. The major and minor plates move very slowly, carrying along the continents and ocean floor that lie on top of them.

Jigsaw puzzle - that's what may people think of when they look at a map of the world. Push Europe and Africa over across the Atlantic Ocean, and the coastlines fit - fit very well - against the coastlines of the Americas. Move India, Australia, and Antarctica around, and their coastlines also fit together - like piece of a giant jigsaw puzzle.

When the Americans were discovered and mapped some five centuries ago, scientist noticed that the opposing coast along the Atlantic Ocean had shapes that would fit together. They proposed that, early in Earth's history, the continents had been joined, and that later they had been violently torn apart. in the 19th century, this idea was supported by studies of geology and life-forms on  both sides of the Atlantic. The studies revealed many similarities between species, suggesting that they had been intermixing and interbreeding across the continents as recently as 150 million years ago. Studies such as the led Alfred Lothar Wegener, a German meteorologist, to propose in 1912 the theory of CONTINENTAL DRIFT.

Wegner believed that the opening of the Atlantic and Indian oceans was not due to an earlier cataclysm, but rather had occurred slowly and gradually. He bolstered his argument with measurements from recent surveys of the distance between Greenland and Europe. The measurement suggested that the two landmasses were moving away from each other at a perceivable rate. Wegner further theorizes that, because Earth is rotating sphere, there exists a force that pushes the continents toward the equator.

The continents, he believed, plow through the rocks of the seafloor like ship through water. As a continents moves, he added, coastal mountain range pile up like low waves along the land's leading edge.

By 1965, investigations led to the proposal that Earth's surface was broken into 7 large plates and several small plates. It was further suggested that these plates are rigid, and that their boundaries are marked by earthquakes and volcanic activity. In recent years, satellite pictures have documented the existence of plate boundaries. An especially visible example is the San Andreas Fault in California.

Plates interact with one another at their boundaries by moving toward, away, or along side each other. Faults are examples of boundaries where two plates slide horizontally past each other. Mid-ocean ridges mark boundaries where plates are forced apart as new ocean floor is being created between them. Mountains, volcanic-island arcs, and ocean trenches occur at the boundaries where plates are colliding, causing one plate to slide beneath the other. The network of crustal plates and the geologic activity caused by their movements is referred to as PLATE TECTONICS.

The theory of plate tectonics may also be used to explain circular island arc and oceanic trenches. Where ocean floor is being carried down freely into the interior, it is likely to do so along circular arcs. Geometry dictates this pattern, a concept illustrated when a person pushes his or her thumb into a dead tennis ball: the resulting depression is circular. This principles may explain the origin of circular volcanic-island arcs, such as the Aleutians.

If, on the other hand, a continental plate pushes past and over an oceanic plate, the ocean floor will be forced down into the interior of the Earth along an offshore trench. The deep trenches of the coasts of Peru and Chile are prime example.

The Theory of plate tectonics also explains how and when mountains ranges such as ANDES and the CASCADES arose.

Both this ranges lie at the converging boundaries of 2 plates. As these 2 plates collide, portion of Earth's crust are uplifted and folded, causing a great compressing and thrusting of rock. Earthquakes and volcanoes continue to occur int the area of these new and active mountain ranges.


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