Plate Tectonics

Intro to Plate Tectonic Theory

Plate tectonics is the theory that Earth's outer layer is made up of plates, which have moved throughout Earth's history. The theory explains the how and why behind mountains, volcanoes, and earthquakes, as well as how, long ago, similar animals could have lived at the same time on what are now widely separated continents.
You probably wouldn't recognize the Earth if you could see it 225 million years ago. Back then, all the major continents formed one giant supercontinent, called Pangaea.
Perhaps initiated by heat building up underneath the vast continent, Pangaea began to rift, or split apart, around 200 million years ago. Oceans filled the areas between these new sub-continents. The land masses continued to move apart, riding on separate plates, until they reached the positions they currently occupy. These continents are still on the move today.

Exactly what drives plate tectonics is not known. One theory is that convection within the Earth's mantle pushes the plates, in much the same way that air heated by your body rises upward and is deflected sideways when it reaches the ceiling.
Another theory is that gravity is pulling the older, colder, and thus heavier ocean floor with more force than the newer, lighter seafloor.
Whatever drives the movement, plate tectonic activity takes place at four types of boundaries: divergent boundaries, where new crust is formed; convergent boundaries, where crust is consumed; collisional boundaries, where two land masses collide; and transform boundaries, where two plates slide against each other.

Go directly to Plate Tectonics activity
(47K - requires Shockwave)

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Take a hard-boiled egg and crack its shell. Does the egg remind you of anything? The Earth, perhaps? 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.
Even though the theory of continental drift was proposed in 1912 by Alfred Wegener, the idea of moving continents wasn't generally accepted until the early 1960s. That's when Wegener's theory was resurrected by Harry Hess, Robert Dietz, Fred Vine, and Drummond Matthews. The ensuing theory, known as plate tectonics, has had a major impact on Earth Sciences. It represents a scientific revolution as significant to geology as relativity was to physics.
This activity lets you manipulate tectonic plates. Pull the plates apart and push them together and watch what happens to the Earth.

http://www.pbs.org/wgbh/aso/tryit/tectonics/shockwave.html

Text Version of Mountain Maker, Earth Shaker
Build mountains. Trigger volcanoes. Create new sea floor. You now have the power to change the landscape with the slightest push of your mouse. Four types of plate tectonic activity are demonstrated in this feature. Keep an eye on the map to see where in the world the activity takes place.

Divergent Boundary
Also known as spreading boundary, a divergent boundary occurs where two plates move apart, allowing magma, or molten rock, to rise from the Earth's interior to fill in the gap. The two plates move away from each other like two conveyor belts moving in opposite directions.
For more on divergent boundary, go to The Sea Floor Spread.

Convergent Boundary
Also known as subduction boundary, a convergent boundary occurs where one plate slides under another as the two are pushed together. If there is land at the edge of one of these plates, the ocean plate will subduct, or slide under that plate.
For more information, go to The Continental Slide.


Collisional Boundary
A collisional boundary occurs where two land masses on plates are pushed together. Trying to occupy the same space, the land masses buckle and fold, creating mountain ranges.
For more information, go to The Continental Crush.


Transform Boundary
A transform boundary occurs where two plates slide against each other. But rather than sliding smoothly, the plates build up tension, then release the tension with a spurt of movement. This movement is felt as an earthquake.
For more information, go to Slippin' and a Slidin'.

The 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.


As the two sides of the mountain move away from each other, magma wells up from the Earth's interior. It then solidifies into rock as it is cooled by the sea, creating new ocean floor.
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.


The Continental Slide



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.
Deep below the Earth's surface, subduction causes partial melting of both the ocean crust and mantle as they slide past one another. This melting generates magma that makes its way to the surface, producing volcanoes, such as Mt. St. Helens. Most of the subducting plate continues into the mantle, perhaps to reappear much later at a distant divergent boundary.

The Continental Crush



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.

 Slippin' and a Slidin'



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.
Los Angeles, located on the Pacific plate, is now 340 miles south of San Francisco, located on the North American plate. In 16 million years, the plates will have moved so much that Los Angeles will be north of San Francisco!