The earth is covered in a layer of earth called the crust, while this may seem like a solid sphere it is actually a bunch of 'plates' that float around on top of the mantle, like leaves on water except a lot slower. Where these plates meet it creates boundaries, there are three types of boundaries.
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convergent boundaries
Convergent boundaries are created when tectonic plates move towards each other and collide, creating volcanoes, mountain ranges, and subduction zones. Subduction zones are formed when a tectonic plates sinks underneath another, the heavier rock of the oceanic crust sinks beneath the lighter rock of the continental crust. when this happens, deep trenches are formed beneath the sea. The rocks of oceanic plates contain a lot of water so when they sink into the mantle the water heats auto and creates superheated steam, that is so hot it melts the surrounding rock creating magma, some of the magma rises up through cracks in the rock and may form volcanoes.
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Divergent boundaries
Divergent boundaries are formed when two plates move away from each other. Magma flows up from the crack and cools, when this happens on oceanic plates it creates underwater mountain ranges, deep-sea hydrothermal vents, and new seafloor, widening the ocean basin in a process called seafloor spreading. Most divergent boundaries are underneath the ocean, though there are a few on land such as the Great Rift Valley in Africa. Eventually, when the Rift Valley becomes wider, the Red Sea will flow into the valley dividing Africa into two land masses.
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transform boundaries
More commonly known a transform fault, a transform boundary is formed when two tectonic plates slide horizontally past each other. At transform boundaries the plates will frequently stick to each other and build up a lot of pressure, when the pressure builds up enough the plates will break free and slide past each other causing earthquakes. Most transform boundaries are on oceanic plates, though a few are on land. Transform faults do not create volcanoes.
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sea floor spreading
In the 1960's, Henry Hess proposed a hypothesis stating that new ocean floor is created where two plates move away from each other at mid-ocean ridges. As the two plates moved away from each other (at a divergent boundary) magma would flow up from the mantle and cool creating new seafloor, and further away older seafloor would go into the mantle in subduction zones, creating a loop of seafloor.
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