Plate Tectonics You may not realize it, but the Earth's surface is constantly changing. Over a hundred years ago, a man named Alfred Wegener proposed a hypothesis called continental drift. His hypothesis stated that all the continents were once united into a super-continent called Pangea. At first, the majority of people who heard about his hypothesis did not believe it. They thought it was just a coincidence that South America and Africa seemed like they could come together. Several years later, however, fossils of the same type of dinosaur were discovered on both 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 the Earth's crust and upper mantle, called the lithosphere, are divided into sections called tectonic plates. There are approximately 13 tectonic plates floating on top of the asthenosphere, the plastic layer of the Earth's mantle. These tectonic plates move very gradually; about 3 centimeters per year. That's about how fast human nails grow! The place where two or more plates meet is called a plate boundary. There are three types of plate boundaries; converge, diverge and transform. These boundaries affect the Earth's surface and how it changes. Say no to plagiarism. Get a tailor-made essay on “Why Violent Video Games Should Not Be Banned”? Get the original essay Convergent boundaries occur when two tectonic plates move toward each other. When these two tectonic plates are continental (made up of land), they form mountain ranges. The Himalayan mountain range is formed by the collision of the Indian and Eurasian plates, both continental. This mountain range has the highest mountains in the world! When a continental plate and an oceanic plate collide, mountains, volcanoes and trenches form. At the boundaries of these plates, subduction also occurs. Subduction occurs when an oceanic plate and a continental plate collide and the oceanic plate is pushed beneath the continental plate. This happens because the density of continental crust is 2.8 grams/cm³ and that of oceanic crust is 3.0 grams/cm³. Because oceanic crust is denser than continental crust, it sinks into the land and continental crust rises above it. This is how the Andes were formed. When two ocean plates collide, the results are numerous. Subduction occurs here too, with trenches, volcanoes and island arcs. Island arcs are an arc-shaped chain of islands. They form when two ocean plates collide and the denser one sinks under the less dense one. Extreme heat from the mantle melts the subducting plate, resulting in the formation of underwater volcanoes. Eventually, these volcanoes erupt and grow until they reach the surface. The Caribbean islands were formed from island arcs, much like the Aleutian Islands of Alaska. The Mariana Trench is an ocean trench located in the western Pacific, off 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 ocean plates collided, one plate was pushed under the other, and both plates eventually moved down. A divergent border,.