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Plates: Earths
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According to the theory, the surface of the Earth is broken into large plates. The size and position of these plates change over time. The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building. Plate tectonics is a combination of two earlier ideas, continental drift and sea-floor spreading. Continental drift is the movement of continents over the Earth's surface and in their change in position relative to each other. Sea-floor spreading is the creation of new oceanic crust at mid-ocean ridges and movement of the crust away from the mid-ocean ridges.
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The zone between two plates sliding horizontally past one another is called a transform-fault boundary, or simply a transform boundary. The concept of transform faults originated with Canadian geophysicist J. Tuzo Wilson, who proposed that these large faults or fracture zones connect two spreading centers (divergent plate boundaries) or, less commonly, trenches (convergent plate boundaries). Most transform faults are found on the ocean floor. They commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes. However, a few occur on land, for example the San Andreas fault zone in California. This transform fault connects the East Pacific Rise, a divergent boundary to the south, with the South Gorda -- Juan de Fuca -- Explorer Ridge, another divergent boundary to the north.
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Seafloor spreading is the movement of two oceanic plates away from each other (at a divergent plate boundary), which results in the formation of new oceanic crust (from magma that comes from within the Earth's mantle) along a a mid-ocean ridge. Where the oceanic plates are moving away from each other is called a zone of divergence. Ocean floor spreading was first suggested by Harry Hess and Robert Dietz in the 1960's.
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This plays a key role in the development of plate tectonic theory because it was the first positive proof that the plates were indeed moving and had been for most of geologic time. By using the magnetic reversal information preserved in the minerals of the mid-ocean ridge basalts, scientists were able to prove that the plates were moving, and that new crust was being formed and old crust was being destroyed in a continuous process that had been going on for most of Earth's history. The oldest crust in the ocean dates back to the early Cretaceous, 100-65 million years ago, which is relatively recent in geologic time.
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Like many features on the Earth's surface, plates change over time. Those composed partly or entirely of oceanic lithosphere can sink under another plate, usually a lighter, mostly continental plate, and eventually disappear completely. This process is happening now off the coast of Oregon and Washington. The small Juan de Fuca Plate, a remnant of the formerly much larger oceanic Farallon Plate, will someday be entirely consumed as it continues to sink beneath the North American Plate.
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The Earth’s tectonic plates can move apart, collide, or slide past each other. The Mid-Ocean Ridge system — the Earth’s underwater mountain range — arises where the plates are moving apart. As the plates part, the seafloor cracks. Cold seawater seeps down into these cracks, becomes super-heated by magma, and then bursts back out into the ocean, forming hydrothermal vents.
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