The Theory of Plate Tectonics and the Three Types of Plate Boundaries

You may not realize it, but Earth's surface is always changing. Over a hundred years ago, a man named Alfred Wegener came up with a hypothesis called continental drift. His hypothesis stated that all of the continents were once joined together into a super-continent called Pangaea. At first, the majority of people who heard about his hypothesis didn't believe it. They thought it was simply a coincidence that South America and Africa seemed as if they would fit together. Many years later, however, fossils of the same type of dinosaur were found in both of these continents.

Scientists were convinced that Wegener may have been right. Millions of years ago, the continents were positioned differently than they are today, but how? Plate tectonics is the theory that Earth's crust and upper mantle, called the lithosphere, is broken up into sections called tectonic plates. There are around 13 tectonic plates that float around on the asthenosphere, the plastic-like layer of Earth's mantle. These tectonic plates move very gradually; about 3 centimeters per year. That is about how fast human fingernails grow! The place where two or more plates meet is called a plate boundary. There are three types of plate boundaries; convergent, divergent, and transform. These boundaries affect Earth's surface and the way it changes.

Convergent boundaries occur when two tectonic plates move towards each other. When both of these tectonic plates are continental (made up of land), they form mountain ranges. The Himalaya mountain range is formed by the collision of the Indian and Eurasian plates, which are both continental. This mountain range has the highest mountains in the world! When a continental and an oceanic plate collide, mountains, volcanoes, and trenches form. At these plate boundaries, subduction also occurs. Subduction is when an oceanic and a continental plate collide, and the oceanic plate is pushed below the continental plate.

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This happens because the density of continental crust is 2.8 grams/cm3, and the density of oceanic crust is 3.0 grams/cm3. Since oceanic crust is denser than continental crust, it sinks down into the earth, and the continental crust goes on top. This is how the Andes Mountains formed. When two oceanic plates collide, there are many results. Subduction happens here too, along with trenches, volcanoes, and island arcs. Island arcs are a chain of islands shaped like an arc. They are formed when two oceanic plates collide, and the denser one sinks down underneath the less dense one.

The extreme heat of the mantle causes the subducting plate to melt, resulting in undersea volcanoes. Eventually, these volcanoes erupt and build up, until they reach the surface. The Caribbean Islands were formed by island arcs, and so were the Aleutian Islands of Alaska. The Marianas Trench is an ocean trench located in the West Pacific, off the coast of the island of Guam. At its deepest point, it is seven miles underwater, making it the deepest place on Earth! This trench was formed when two oceanic plates collided, one plate was forced beneath the other, and both plates ended up going downwards.

A divergent boundary is another type of boundary that occurs when two tectonic plates are moving away from each other. These two plates can both be either continental, or oceanic, but not one of each. When two oceanic plates are moving away from each other, seafloor spreading takes place. Seafloor spreading is Harry Hess's theory that new sea floor is formed when magma; hot, less dense material, is forced upwards towards Earth's surface at a mid-ocean ridge.

An example of a mid-ocean ridge would be the Mid-Atlantic Ridge. While new seafloor is being created here, older seafloor that has been around for a long time is being destroyed at ocean trenches, such as the Marianas Trench. As time goes on, the Atlantic Ocean is getting bigger and bigger due to seafloor spreading, and the Pacific Ocean is gradually getting smaller and smaller, because of subduction at ocean trenches. When two continental plates move apart, rift valleys are formed. Rift valleys are simply valleys formed from two continental plates splitting apart. The Great Rift Valley in East Africa is the largest rift valley in the world! Underwater, oceanic plates moving apart form ocean trenches, while continental plates moving apart form rift valleys.

The third boundary type is a transform boundary. Transform boundaries are where two tectonic plates are either sliding past each other, or moving in the same direction at different rates. At these plate boundaries, many earthquakes occur. The San Andreas fault, located in Southern California, is a located along the Ring of Fire. The Ring of Fire is a place where many earthquakes happen. It is located along the west coast of South America, North America, all the way up to Alaska, and around to the east coast of Asia.

As you can see, plate tectonics is a very complex theory. The three boundary types; convergent, divergent, and transform, affect the way Earth's surface shifts around. Subduction, trenches, mountains, volcanoes, rift valleys, island arcs, seafloor, spreading, and many other landforms are results of Earth's ever-changing surface. The tectonic plates that make up Earth's surface cause many natural disasters around the planet. Most of the time, scientists cannot predict when these disasters will strike, but they have learned how to prepare for them. Evidence has shown that Earths' surface has changed over millions of years, and it will continue to change in the future, affecting the lives of everyone who lives on it.

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