P2+-+Plate+Tectonics

=Plate Tectonics = The upper part of the earth's crust is broken into units called plates. A tectonic plate is like a segment of the Earth's lithosphere (uppermost 100km of the rigid shell) that moves independently of other regions. Today there are seven major plates: North America, South America, Eurasia, Africa, India, the Pacific, and Antarctica. The plates are divided by seismically active boundaries. The type of boundary depends on the motion occurring between the plates at that location. Plates at our planet’s surface move because of the intense heat in the Earth’s [|core] that causes molten rock in the [|mantle layer] to move.

History
Plate tectonic theory had its beginnings in 1915 when Alfred Wegener proposed his theory of "continental drift." He was not the first to notice that the earths surfaces changes.Continents that are separated now may have been joined together at one point in the past. Paleontologists had also found that there were fossils of similar species found on continents that are now separated by great geographic distance.That glaciers covered large areas of the world which also are now separated by great geographic distances. These observations seemed to indicate that the Earth's lithosphere had been moving over geologic time. At the time, many geologists believed that the features of the Earth were the result of the Earth going through cycles of heating and cooling, which causes expansion and contraction of the land masses.Paleomagnetic studies, which examine the Earth's past magnetic field, showed that the magnetic north pole seemingly wandered all over the globe. This meant that either the plates were moving, or else the north pole was. Since the north pole is essentially fixed, except during periods of magnetic reversals, this piece of evidence strongly supports the idea of plate tectonics. Earthquakes, volcanoes, and other active geologic features for the most part aligned along distinct belts around the world, and those belts defined the edges of tectonic plates.When the lava from spreading centers in the oceans forms and cools, these minerals align to the north pole. The Earth has undegone several magnetic reversals in the past, in which the north and south poles are reversed for a period of time. When geologists and geophysicists discovered that the crust in the ocean recorded these reversals, it was even more positive proof that the lithosphere had to be in motion, otherwise there would be no "stripes" of normal and reversed polarity crust. Since its emergence in the 1960's, plate tectonic theory has gained wide-spread acceptance as the model of Earth processes.

Plate Tectonics and Earth's Layers
The uppermost part of the mantle is more solid and cooler than the rest of the mantle. It combines with the thin, solid crust to form a layer called the lithosphere. It is this layer that has been broken into pieces, or tectonic plates. They are related because they are both part of the Earth and its layers.

Continental Drift
Continental drift is t he gradual movement and formation of the continents.The continental drift theory is the theory that once all the continents were joined in a super-continent, which scientists call Pangaea. Alfred Wegener first supported continental drift,Wegener’s explanation of continental drift in 1912 was that drifting occurred because of the earth’s rotation. The hypothesis that the continents had once formed a single landmass before breaking apart and drifting to their present locations was fully formulated by Alfred Wegener in 1912.[5] Although Wegener's theory was formed independently and was more complete than those of his predecessors, Wegener later credited a number of past authors with similar ideas:[6][7] Franklin Coxworthy (between 1848 and 1890),[8] Roberto Mantovani (between 1889 and 1909), William Henry Pickering (1907)[9] and Frank Bursley Taylor (1908).He announced his theory in 1912. Evidence for continental drift is now extensive. Similar plant and animal fossils are found around different continent shores, suggesting that they were once joined.