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Plate Boundaries
Earth's crust is fractured into around 12 major tectonic plates. These plates float atop the planet's mantle, a layer of molten rock. Convection within the mantle causes the plates to move, a process known as continental drift. As the plates slowly travel across the planet's surface, they interact with each other along what are called plate boundaries. Areas where plates are moving apart are known as divergent boundaries. Areas where plates are moving laterally across each other are called transform boundaries. Areas where plates are colliding are called convergent zones.
Oceanic versus Oceanic
When two oceanic plates collide along a convergent boundary, the older, denser plate is forced underneath. This process is known as subduction. These convergent boundaries result in the formation of two principal geological formations: trenches and volcanic islands. For example, the Mariana trench, the deepest place on the planet, is being formed by the subduction of the Pacific plate under the Philippine plate. As the subducting plate descends into the asthenosphere, it melts. The resulting magma then rises to the surface, creating chains of volcanic islands.
Oceanic versus Continental
When an oceanic plate collides with a continental plate, the oceanic plate is always subducted under the more buoyant continental plate. This tectonic process produces trenches, volcanic arcs and mountains. For example, the Atacama Trench, also known as the Peru-Chili Trench, is the world's longest oceanic trench. It is being formed by the subduction of the Nazca plate under the South American plate. The process has also produced the Andes Mountains, the world's longest mountain range. Mt. St. Helens, located in the northwestern United States, is an example of a subduction zone volcano.
Continental versus Continental
When two continental plates collide, neither plate is able to subduct under the other. Instead, the plates are crushed together under enormous pressure. This process produces towering mountains and plateaus. This tectonic process is occurring where the Indian and Eurasian plates are colliding. The ongoing collision has caused a massive uplift of the Eurasian plate, known as the Tibetan Plateau. Known as the "Roof of the World," this plateau is most likely the largest and highest area ever to have existed on the surface of the planet. The plateau has an average elevation of 16,400 feet and spans an area equal to half the continental United States. The collision has also formed the Himalayan and Karakaram Mountains, home to all fourteen of the world's highest peaks. The tallest of these peaks, Mount Everest, rises to 29,029 feet above sea level.