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TEORY OF PLATE TECTONICS
THEORY OF PLATE TECTONICS
Open Geography Education by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License.
The surface layer of the earth is called the crust and it makes up only 1 percent of Earth's mass. The crust is subdivided into two components: oceanic and continental crust. Again referring back to the image on the right, note that the oceanic crust is only about 3 miles thick, but is slightly more dense than continental crust. Most of this oceanic rock is called basalt and is a dark, dense rock. Continental crust is much thicker than oceanic crust (averages between 20 to 25 miles thick), but is actually slightly less dense than oceanic crust. The main type of rock on continents is called granite. So if these two types of crust were to collide into each other, what do you think would happen to the oceanic crust? As a whole, notice that the crust is lighter than the mantle. It is sometimes said that the crust "floats" on the mantle like an iceberg in water and that is not too far from the truth and is called isostacy. Finally, the crust is the coldest, most rigid, and brittle layer with lots of folds and fractures.
THEORY OF PLATE TECTONICS
The driving force of earthquakes and volcanoes is described in the theory of plate tectonics. The theory states that the earth is made of several tectonic plates along with several smaller plates. Each tectonic plate consists of oceanic and continental crust that move around the earth's surface like bumper cars because of convection within the mantle.
The theory also explains that the majority of earth's earthquakes and volcanoes occur along the boundaries of these tectonic plates as they either grind past or underneath each other.
There are three major types of tectonic plate boundares: convergent, divergent, and transform. Let's first look at convergent plate boundaries, which can be broken down into three subcategories.
Recall that oceanic crust is denser than continental rock like granite. Thus when two tectonic plates collide, the denser oceanic crust will subduct underneath the lighter continental crust. If the subducting rock becomes stuck, vast amounts of energy builds up. But once the pressure and energy is too great, the rock will rupture creating powerful earthquakes. As the subducted material sinks further, it will begin to melt under great heat and pressure, becoming less dense as it melts, and rise up as magma to form dangerous composite volcanoes. Mountain ranges created by oceanic-to-continental convergence are the Andes mountains in South America, the Cascades in the western United States, and the Ring of Fire in the Pacific Ocean.
Below is a Google Earth image showing a series of oceanic-to-oceanic subduction zones within the Pacific Ring of Fire. You can visibly see the subduciton zones that create the volcanic and powerful Aleutian Islands and the converging subduction plates that make of volcanic islands of Japan.
With oceanic-to-oceanic convergence, the heavier of the two will subduct down beneath the other. Just like continental-to-oceanic convergence, this plate boundary can generate powerful earthquakes and volcanoes; but instead of volcanoes on land, volcanic islands form such as Japan, the Aleutian Islands of Alaska, and Indonesia. The great earthquake in Indonesia in 2004, which produced the devastating tsunami, was created by this process along with the 2011 earthquake and tsunami in Japan.
When two continental plates converge, instead of subduction, the two similar tectonic plates will buckle up to create large mountain ranges like a massive car pile-up. This is called continental-to-continental convergence, and geologically creates intense folding and faulting rather than volcanic activity. Examples of mountain ranges created by this process are the Himalayan mountains (taken from the International Space Station) as India is colliding with Asia, the Alps in Europe, and the Appalacian mountains in the United States as the North American plate collided with the African plate when Pangea was forming. The Kashmir India earthquake of 2005 that killed over 80,000 people occurred because of this process. And most recently, the 2008 earthquake in China which killed nearly 85,000 people before the Summer Olympics was because of this tectonic force.
When two tectonic plates move away from each other, or when a tectonic plate tears itself apart, divergent boundaries can form. As divergence occurs, shallow earthquakes can occur along with volcanoes along the rift areas. When the process begins, a valley will develop such as the Great Rift Valley in Africa. Over time that valley can fill up with water creating linear lakes. If divergence continues, a sea can form like the Red Sea and finally an ocean like the Atlantic Ocean. Check out the eastern half of Africa and notice the lakes that look linear. Eastern Africa is tearing apart from these linear lakes, to the Great Rift Valley, and up to the Red Sea. Notice how the Red Sea looks like it could be put back together again. The ultimate divergent boundary is the Atlantic Ocean, which began when Pangaea broke apart.
Below are two satellite images using Google Earth, both focusing on parts of Africa. On the left yo can see rift valleys that have filled in with water to form linear lakes. On the right in northern Africa, you can see the Red Sea with a rift valley in the center, which use to be a linear lake that grew into a sea. If the Red Sea continues to grow, it could form an ocean similar to the Atlantic Ocean with the mid-Atlantic Ridge.
Transform boundaries occurs when two tectonic plates slide (or grind) past parallel to each other. The most famous transform boundary is the San Andreas Fault where the Pacific plate (that Los Angeles and Hawaii are on) is grinding past the North American plate (that San Francisco and the rest of the United States is on) at the rate of 3 inches a year. Recently, geologists have stated that San Francisco should expect another disastrous earthquake in the next 30 years. Another important transform boundary is the North Anatolian Fault in Turkey. This powerful fault last ruptured in 1999 in Izmit, Turkey which killed 17,000 people in 48 seconds.
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