Plate Tetonics Essay Research Paper Plate TectonicsPlate

Plate Tetonics Essay, Research Paper

Plate Tectonics

Plate tectonics are a relatively new theory that has revolutionized the way geologists think about the Earth. 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.

The Earth is divided into three chemical layers: the core, the mantle and the crust. The core is composed of mostly iron and nickel and remains very hot, even after 4.5 billion years of cooling. The core is divided into two layers: a solid inner core and a liquid outer core. The middle layer of the Earth, the mantle, is made of minerals rich in the elements iron, magnesium, silicon, and oxygen. The crust is rich in the elements oxygen and silicon with lesser amounts of aluminum, iron, magnesium, calcium, potassium, and sodium. There are two types of crust. Basalt is the most common rock on Earth. Oceanic crust is made of relatively dense rock called basalt.

The outermost layers of the Earth can be divided by their physical properties into lithosphere and asthenosphere. It is made of lower density rocks, such as andesite and granite. The lithosphere is the rigid outermost layer made of crust and uppermost mantle. The lithosphere is the “plate” of the plate tectonic theory. The asthenosphere is part of the mantle that flows, a characteristic called plastic behavior. It might seem strange that a solid material can flow. The flow of the asthenosphere is part of mantle convection, which plays an important role in moving lithosphere plates.

Alfred Wegener, a German meteorologist, originally proposed continental drift in 1912. This geologist Wegener used the fit of the continents, the distribution of fossils, a similar sequence of rocks at numerous locations, ancient climates, and the apparent wandering of the Earth’s polar regions to support his idea. Wegener used his observations to hypothesize that all of the present-day continents were once part of a single super continent called Pangaea. Fossils of the same species were found on several different continents. Wegener proposed that the species dispersed when the continents were connected and later carried to their present positions as the continents drifted. For example, the specie Glossopteris, a fern, was found on the continents of South America, Africa, India, and Australia. The distribution of other species can also be accounted for by initially spreading across Pangaea, followed by the breakup of the super continent, and movement of the continents to their present positions.

Rock sequences in South America, Africa, India, Antarctica, and Australia show remarkable similarities. Wegener showed that the same three layers occur at each of these localities. The bottom (oldest) layer is called tillite and is thought to be a glacial deposit. The middle layer is composed of sandstone, shale, and coal beds. Glossopteris fossils are in the bottom and middle layers. The top (youngest) layer is lava flows. The same three layers are in the same order in areas just separated by great distances. Wegener proposed that the rock layers were made when all the continents were part of Pangaea. Thus, they formed in a smaller contiguous area that was later broken and drifted apart.

Glaciers covered all or part of each of these continents during the same time period in the geologic past. If the continents were in their present position, a major glaciation event that covered nearly all of the continents and extended north of the equator would be required. Geologists have found no evidence of glacial action in the northern hemisphere during this time period. In fact, during this time period, the climate in North America was warm. Wegener proposed that the continents were adjacent to each other during the glacial event. Therefore, glaciers spread over a much smaller area in the southern hemisphere and probably did not influence the climate of the northern hemisphere. Wegener used the distribution of specific rock types to determine the distribution of climate zones in the geologic past. For example, glacial till and striations, sand dunes, and coral reefs, indicate polar, desert, and tropical climates, respectively. Using the distribution of rock types, Wegener reconstructed the distribution of climates zones at specific times in the geologic past. He found that, unlike the present distribution, in which zones parallel the equator, the past zones occupied very different positions. This implies that the rotational pole was in very different locations relative to today. Wegener proposed an alternative interpretation. He believed that the climate zones remained stationary and the continents drifted to different locations. The drift of the continents caused the apparent movement of the climate zones. Wegener used the distribution of climate zones to determine the location of the poles at diffe