|MadSci Network: Earth Sciences|
Dear Krystal, The three types of plate boundaries are: Constructive plate boundaries. These are boundaries where the Earth’s tectonic plates are moving apart. For the most part, these boundaries are located on the ocean floors, where they form a continuous chain of volcanic mountains and rifts called mid-ocean ridges that extend throughout the Earth’s oceans. A good example is the Mid-Atlantic Ridge that runs down the middle of the Atlantic Ocean; the tectonic plates of Europe and Africa on the east side of the Mid-Atlantic Ridge are moving away from the North American and South American plates to the west. As the plates move apart magma wells up to fill the space between them, and this is why constructive plate boundaries are the sites of volcanic activity. The upwelling magma is formed by the partial melting of the rocks of the Earth’s mantle deep below, and the magma is basaltic in composition. As it cools and solidifies it creates rocks called gabbro and, where it erupts onto the sea floor, basalt. The gabbro and basalt form new oceanic crust, so constructive plate boundaries are sites where the Earth’s crust is growing. Destructive plate boundaries. If new crust is being created at constructive plate boundaries, then this growth must be balanced by the destruction of old crust somewhere else – if this were not the case, then the Earth would expand in diameter, and there is no evidence that it expands. The sites where crust is being destroyed are destructive plate boundaries, where two tectonic plates are in collision. At every destructive boundary the crust forming one of the plates is diving back down into the mantle beneath the other, overriding plate, a process called subduction. Destructive boundaries can occur where the two plates are both composed of oceanic crust, where one is composed of oceanic and the other of continental crust, or where both are composed of continental crust. Destructive plate boundaries are often the sites of major volcanoes. These volcanoes are the familiar stratocone volcanoes such as Mount Fuji in Japan. This volcanic activity is driven by the heating up of the plate that is descending back into the mantle, causing it and part of the overriding plate to melt. This melting typically occurs about 100 kilometers behind the actual plate boundary, so the line of volcanoes produced is also behind the actual boundary, located on the overriding plate. In collision between two pieces of oceanic crust the result is a chain of volcanic islands, of which Indonesia is a prime example. Where oceanic crust collides with a plate carrying a continent, the result is a chain of volcanoes on the continent, such as the Cascade volcanic chain in the Pacific Northwest of the USA and the Andes Mountains of South America. Where two plates carrying continents collide the result is a range of (usually non-volcanic) mountains where the continental crust is squeezed by the powerful forces driving the two plates together; the Himalayan Mountains are the prime modern example, and the Appalachian Mountains are an old chain created by a continent-continent collision about 300 million years. Transcurrent boundaries. The third type of plate boundary is called a transcurrent boundary. A transcurrent boundary is where two tectonic plates slide past one another, without either colliding or pulling apart. The most famous example active today is in Mexico and California, where the Pacific Plate is moving north past the North American plate. In this example the San Andreas Fault is the actual plate boundary. The San Andreas is a transcurrent or strike-slip fault, and earthquakes occur along it very frequently as the two plates move past one another. I hope this answers your question. Best wishes, David Scarboro
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