Plate Tectonics Page
To have an understanding of volcanoes or earthquakes, you need to understand a little about the structure of the earth.
Take a hard-boiled egg and crack its shell. Does the egg remind you of anything? The egg could be seen as a tiny model of the Earth. The thin shell represents the Earth's crust, divided into plates; within the shell is the firm but slippery mantle. Move the pieces of shell around. Notice how the shell buckles in some places and exposes 'mantle' in other places. The same thing happens on Earth, but on Earth, this activity results in the formation of mountains, earthquakes, and new ocean floor.
The theory of continental drift was first proposed in 1912 by Alfred Wegener, his ideas wern't really accepted until the early 1960s. This theory, is known as plate tectonics. Below are some examples of what happens at different points on the earth's crust as plates collide and move apart, albeit at a very slow pace!
Sea Floor Spread
The Earth's longest mountain chain isn't the Andes in South America, or the Himalayas in Asia, or even North America's Rockies. It's an underwater chain of mountains 47,000 miles long. The chain runs down the middle of the Atlantic Ocean (surfacing at Iceland), around Africa, through the Indian Ocean, between Australia and Antarctica, and north through the Pacific Ocean. Running along the top of this chain of mountains is a deep crack, called a rift valley. It is here that new ocean floor is continuously created.
The speed at which new ocean floor is created varies from one location on the ocean ridge to another. Between North America and Europe, the rate is about 2.2 inches (3.6 cm) per year. At the East Pacific rise, which is pushing a plate into the west coast of South America, the rate is 12.6 inches (32.2 cm) per year.
New crust is continually being pushed away from divergent boundaries (where sea-floor spreading occurs), increasing Earth's surface. But the Earth isn't getting any bigger. What happens, then, to keep the Earth the same size? The answer is subduction.
In locations around the world, ocean crust subducts, or slides under, other pieces of Earth's crust. The boundary where the two plates meet is called a convergent boundary. Deep trenches appear at these boundaries, caused by the oceanic plate bending downward into the Earth.
At a depth between 190 and 430 miles (300 and 700 kilometers), the rock of the descending plate melts. Some of this molten ocean floor makes its way to Earth's surface, producing volcanoes. Most of it, though, becomes part of the Earth's mantle, perhaps to reappear much later at a distant divergent boundary.
Thc Collision Boundary
An ocean floor pushed toward a land mass will always slide under the land mass. This is because the land mass is more buoyant, or lighter, than the ocean floor. When two land masses meet, on the other hand, neither will slide under the other. Instead, the two crush together at what is known as a collisional boundary. They crumple and fold. Some pieces of land are thrust over or under other pieces. The result is a mountain range.
The Himalayas, the highest mountains in the world, were created this way. (In fact, they're still growing.) So were the European Alps. Even the Appalachian Mountains formed when two land masses came together. Although with the Appalachians, the crushing ended long ago -- all that's left now are the eroded remnants of a once high mountain range.
Transform boundaries neither create nor consume crust. Rather, two plates move against each other, building up tension, then releasing the tension in a sudden and often violent jerk. This sudden jerk creates an earthquake.
The San Andreas Fault is undoubtedly the most famous transform boundary in the world. To the west of the fault is the Pacific plate, which is moving northwest. To the east is the North American Plate, which is moving southeast.
All together now! The model below shows all of the oceanic and coastal tectonic processes with the exception of transform boundaries.
Thanks to: http://www.pbs.org/wgbh/aso/tryit/tectonics/ for the images
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