Plate tectonics: theory and mechanisms

¶ … continental drift to the present to explain the plate tectonics theory and how the Earth is forever shifting. Use some examples of past and present changes in the earth and the effect they caused. A newer theory in geological history, plate tectonics is used to explain many geological changes in the Earth, both past and present, and indicates how the Earth is forever adjusting and shifting, creating uplifts and cracks in the many plates that make up the Earth's interior surface. These plates are the cause of earthquakes, and so, they are ultimately the cause of some of the world's worst disasters.

Several theories of geologic process and scientific discovery helped lead to the discovery of plate tectonics in the 1920s, and the theory was generally accepted by the 1950s. The plate tectonic theory evolved from earlier theories, beginning with continental drift. The first time continental drift was mentioned was in 1908, by an American named Frank Bursey Taylor. However, the theory really did not gain acceptance until 1912 when German meteorologist and geophysicist Alfred Wegener detailed the theory and gave it more basis in fact. The continental drift theory believes that between 275 and 175 million years ago, all the continents were joined together in one land mass. Later, this massive continent broke apart into two gigantic landmasses in the north and south, those continents further divided about 100 million years ago into more currently recognizable shapes, and they began to drift apart during this time, too. Wegener used several examples from the current geological makeup of the continents to prove his theories, including the zigzag continental shelves of the Atlantic that would match up if mated, and features such as coal in the polar regions and glacial deposits in the temperate regions around the equator ("Continental Drift," 2000). Wegener also believed the oceans formed as a result of the continents shifting away from each other, and mountain ranges formed when the continents bumped into each other (Stallings, 1995, p. 114). However, Wegener's theories were not generally accepted at the time, and it took more study and theorizing for them to become openly accepted in most geologic and scientific circles ("Continental Drift," 2000). It seems by the 1970s, plate tectonics was widely accepted, and most all scientists and geologists believed it was the correct theory regarding continental shift and the resulting geological changes in the Earth.

Another important discovery that ultimately led scientists to believe the theory of plate tectonics was correct was the discovery of seafloor spreading. Until the development of new technologies in the 1950s, scientists really did not know much about the ocean floor and its makeup. Development of seismic technologies allowed scientists to plot the ocean floor, and Princeton geologist Harry Hess discovered the Mid-Atlantic Ridge in the 1960s. This Ridge basically split the Atlantic Ocean region in two, and then they discovered this ridge is just part of a vast system of undersea mountains that reaches around the globe. Some of the ridges are cut with rifts and jagged edges and offsets. The offsets are called "transform faults," and they are the origination point of most all shallow undersea earthquakes. The discovery of this central ridge led to the theory that the seafloor spreads from these central ridges, and is created by magma that rises through the ridges and spreads out to create new layers of the seafloor. The seafloor spreads out through magma release and far-field stresses and the mantel pushing upward against the spreading axis of the seafloor. These theories explain the deep oceanic trenches located near the continents, because this is where the oldest crust is reabsorbed into the mantle when earthquakes occur. In addition, the rock on the oceans' floors is much younger than rock on the continents and the sediment gets younger the closer it gets to the mid-ocean ridge.

What really helped fashion these two findings into the plate tectonics theory was the discovery that magnetic particles in many rock samples dated back to magnetic alignment of the Earth in previous time. Scientists do not know why, but the magnetic pole of the Earth flips back and forth between the South Pole and the North Pole. When scientists discovered bands of rock in the ocean floor that also pointed north, then alternately south, the plate tectonic theory took on additional meaning. One scientist writes, "What had happened was that hot lava, upon cooling to hard, basaltic rock, had frozen into it a permanent record of the reversals of the earth's magnetic field" (Morton, 1996, p. 19). Two Cambridge geologists, Frederick Vine and Drummond Matthews, discovered this in the 1950s, and realized that because the magnetic changes occurred in rocks around the world, that the continents had to have been connected at one time. This helped prove the continental drift and seafloor spreading concepts, and led to the discovery of plate tectonics, which explains how the continents were able to break apart and drift away from each other, and this also helped prove Hess' theory of seafloor spreading, which both led to further understand and acceptance of plate tectonics. Author Morton continues, "With the help of Vine and Matthews, Wegener's vision of drifting continents was introduced to Hess's geopoetry of seafloor spreading, and suddenly plate tectonics was a reality" (Morton, 1996, p. 20). Thus, it took several major discoveries that all blended together to ultimately agree with Wegener's idea that the continents drifted apart and are still drifting.

The plate tectonics theory states that the Earth's lithosphere (outer layer) is made up of about seven major plates, and perhaps as many as twelve or more smaller plates. This outer layer is about 100 kilometers deep (Wysession, 1995). These plates rest on a softer layer called the asthenosphere. The plates constantly change, because their sides are always being worn down or created anew, so the plates are never exactly the same. In addition, where the edges of the plates meet are often "hot spots" of continued volcanic action. Wysession continues, "These hot spots, which include Hawaii, Iceland, Tahiti, Galapagos, Yellowstone and many others, appear to be fixed relative to the deep mantle. Despite the vagaries of plate movements, the 40 or so major hot spots do not move relative to each other" (Wysession, 1995). This indicates these "hot spots" are anchored somewhere below the lithosphere and so are stable immovable. Some of the plates have continents imbedded in them, and because they are always changing, it makes it difficult for scientists to study them over any lengthy period of time. Scientists still do not know precisely what makes the plates move, but they believe it is related to transfer of heat energy or convection from the Earth's core. When and if the heat source is reduced, then the plates will probably stop moving and stabilize ("Plate Tectonics," 2000). Plate tectonics is actually the study of the plates and their drift, and is named after "Tekton," a carpenter in the "Iliad" by Homer, because Tekton was a "constructor," or put things together, and in tectonics, scientists study how the plated are put together. The largest plate is the California Plate that makes up Southern California and the Baja Peninsula, and the rest of the North American continent makes up the North American Plate (Morton, 1996, p. 16-17).