Plate Tectonics class
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I hope you'll like this class and enjoy geology as much as I do!
according to the American Heritage Dictionary of the English Language, Third Edition 1992
A theory of global dynamics having to do with the movement of a small number of semirigid sections of the earths crust, with seismic activity and volcanism occurring primarily at the margins of these sections. The movement has resulted in a continental drift and changes in the shape and size of ocean basins and continents.
A Brief History of Continental Drift
People started to look at the phenomena of plate tectonics for the first time in the beginning of the 20th century. A man named Alfred Wegener noticed that the continents on earth are just like pieces of an enormous jigsaw puzzle. He was the first who came up with the idea that maybe those seperate continents were once one big continent, which he called Pangea. Wegener went searching for evidence for his continental drift theory.
One argument for his theory was that at continents - that are now far away from each other like Europe and North America - people found the same fossils (see the Paleontology
class) of animals that could never have traveled across those big oceans that separate them today. His theory also gave a good explanation for the existence of mountains, because that were places where the plates collided. A good example is the Himalayas, they formed when India collided with Asia.
Wegener had one problem: how did those continents move?
It was Arthur Holmes who came with the idea of mantle convection. That works something like this :
Underneath the earth’s crust (see below) we find the mantle and eventually the core (the center of the earth). The core of the earth is hot and the mantle material is capable of flow. If we combine these facts we can easily predict that mantle material that is near the core will be warmer than mantle material near the crust. The hotter material is less dense and wants to rise upward (just like hot air in a room), the colder material sinks down again. That’s how the convection is established within the mantle. Think about it as big cell-like conveyor belts that circle from core to crust through the mantle. These belts can drag the continents that ‘float’ on the ‘liquid’ mantle.
Through time theories have been tested and remodeled and tested again. Because research methods are getting better and we’re capable of doing a lot more then in the 20th century, we’re able to design better theories as well.
That's why we don't believe that it's only the continents that drift, but that the whole crust is moving. It’s also commonly stated that besides the convection in the mantle, the plates themselves also cause the movement, through the so called slab pull force, which is created when an oceanic plate sinks into the mantle at a subduction zone.
Now that we know how it started, let’s look at the tectonics itself.
Common Tectonic Language
The crust of the earth – roughly the upper 30 km – consists of two kinds of crust. The oceanic crust, which is heavy and thin and the continental crust, which is light and thick. The boundaries of these kinds of crust aren’t given by the appearance of oceans and continents, but to keep it simple we assume that they are. Think of the crust as a layer which is ‘floating’ on the mantle.
The crust isn’t whole, it’s devided into different parts, which form the plates. There are some major plates and some small ones. Think of them as pieces of a spherical jigsaw puzzle. The eight biggest plates are:
The African Plate, The Antarctic Plate, The Australian-Indian Plate, The Eurasian Plate, The North American Plate, The South American Plate.
It’s not hard to imagine that if the plates drift, they can collide to or rip apart. We have three different major margins of interaction between the plates:
- Divergent margins (spreading zones), this is where the crust breaks apart and new crust is formed, like at the mid ocean ridges.
- Convergent margins, this is where two plates move toward eachother. We have subduction zones, where the oceanic plate sinks beneath the continental plate (creating that slab pull force) and we have a collision zone, where two continental plates collide and create mountains for example.
- Transform fault margins, this is where two plates slide past each other, the San Andreas fault is a nice example here.
Plate Tectonics Related to Earthquakes and Volcanism
Earthquakes and volcanism are strongly related to plate tectonics, because both occur at the plate boundaries and are generated by the movement of the plates.
They usually occur at transform fault boundaries, because the plates won’t slide past eachother without any resistence. The movement is ‘shocklike’, the plate wants to move but can’t because the other plate is in it’s way, so the pressure is building up inside that plate, until it’s big enough to overcome the resistence and moves; then you have an earthquake.
This occurs at both divergent margins and convergent margins (mostly subduction zones).
At divergent margins we have a really thin crust that is ripped slowly apart. That’s why the mantle material (magma) is just ‘dripping out’, creating really nice pillow basalts at the mid ocean ridges.
At convergent margins we have the volcanoes like most of us know them. Because the oceanic plate sinks beneath the continental plate, it warms up and eventually melts. This melt wants to move upward and will create a volcano behind the subduction zone.
There are some really nice websites about tectonics that you could visit.
A really beautiful site where you can find out almost everything about geology.
The Dynamic Earth, need I say more?
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