the forever shifting earth Essay Example For Students

the forever shifting earth Essay The land below us is always in motion. Plate tectonics studies these restless effects to give us a better understanding of the Earth and its past. New molten rocks are poured out in the form of magma from the mid-ocean ridges. The rock is recycled and re-entered back into the earth in deep ocean trenches through convection current. The convection current in the mantle drives plates around either against or away from each other. These collisions give rise to earthquakes, volcanoes, mountains, and continental drift. The crashing and spreading of the plates forms the landscape of the Earth as we see it today. The positions of the land masses today is a result of continental drift. During the Earths existance, the magnetic fields have never been stable. Solidified magma containing magnetic imprints reveal periods of time when the Earths magnetic fields have actually been reversed. Approximately 4.55 billion years ago, the Earth was just a ball of molten material. Since then, parts of the Earth have cooled forming the solid crust-mantle. This process has been occurring for roughly about 3.8 billion years. The mantle is about 2900 km. thick, which lies above a layer of molten magma that still exists today. The immense heat from the magma (approximately 2700(C) causes convection in the mantle (Figure 1). Convection is caused by non-uniform temperature in a fluid and density differences. This continuous convection is the cause of plate movement. Each complete cycle, called a convection cell, drives the plate in the direction of the cell. How does a solid mantle move? The mantle may be solid but, as with most solids, it will deform if long term stress is applied; like Silly Putty which seeps into the rug when left unattended, mantle material flows when subjected to small long-term stresses.1Presently, there are more than fourteen plates in the Earths crust (Figure 22). Upwelling hot magma flows out from mid-ocean ridges and then cools down when exposed to the cooled environment outside; the layer of cooled magma forms the lithosphere. When magma flows out from the ridges, the crust is fractured and a new ocean floor is built spreading perpendicularly away from the ridge. Because of this constant upwelling, the ocean is relatively shallow in these areas. Sea floor spreading and continental drift are the products of this continual upwelling. The cooled magma will, in time, sink back down into the Earth in the deep ocean trenches. The mantle sinking down produces subduction zones or Benioff zones. The deepest part of the ocean resides in these areas. There are three types of boundaries where plates meet: divergent boundaries the upwelling of magma; convergent boundaries where the plates collide producing mountains, volcanoes, and earthquakes; and transform boundaries lateral movement. Transform plates are caused by fracture zones. When a rift opens from the upwelling of magma it causes a crack in the crust. As new magma rises to the surface, the crack increases caused by the pressure, resulting in a horizontal faulting. The fractured plate pieces travel in the same direction as the original plate was traveling away from the ocean ridge. During the early 1900s, a theory of a super-continent was developed by Alfred Wegener. He was ridiculed for his ideas that continental drift produced the present positions of the continents from a single super-continent called Pangea. This theory is widely accepted today, however. There was abundant evidence for Wegener to believe in the existance of Pangea. The shape of the continents could be pieced together like a giant jigsaw puzzle suggesting that the continents were once glued together. The fossils found on the continents were not distinct to that particular land, but were also found in lands that were separated by thousands of kilometers of water. Fossils indicated that identical species existed in different continents. Geological structures also demonstrated that the continents were, in fact, one giant land mass; old mountain ranges from one continent matched with those from another (i.e., South America and Africa). Ocean spreading has always been moving the continents towards or away from each other. About 200 million years ago during the Jurassic period, Pangea began to separate (Figure 33). Pangeas continental crust was subjected to many faults and rifts. Hot magma would flow out, splitting the land apart and creating a rift valley. When this valley became deep enough, water flowed in. In time, the rift expanded so much that a sea began to form between thus creating two continents. About 135 million years ago, because of sea floor spreading, Pangea separated into two large land masses: Laurasia (containing North America, Europe, and Asia) to the north, and Gondwana (containing South America, Africa, Australia, Antarctica, and India) to the south. About 180 million years ago, Gondwana started to break up into South America-Africa, Australia-Antarctica, and India. About 130 million years ago, the Atlantic started separating South America and Africa while India sailed towards Asia, crashing into it about 30 million years ago. Australia and Antarctica split about 45 million years ago and North America separated from Europe 5-10 million years later. To this day, the continents are continually moving and will still be moving until the liquid inner core cools and solidifies. With the use of a highly-accurate distance-measuring device known as a geodimeter, the speed at which the continents are moving and the speed of ocean spreading could be measured. A geodimeter uses a helium-neon laser that acts like radar to measure distances. The average speed of sea floor spreading is about 2 cm. per year. Africa, today, is traveling towards Europe and Asia, causing the Mediterranean to close in; in due time, this sea will vanish. India, which is cemented to Asia, is an example of continental collision. Indias drift speed is about 17 cm. per year; this collision is shown physically by the Himalayan mountains. In the far future, North America will, most likely, be placed more the west, possibly colliding with Asia; and Australia will drift north, colliding with South Asia. Another possibility may be that, in a few hundred million years, all t he continents may join together, creating another super-continent.One of the most destructive forces the plates generate are earthquakes. There are earthquakes occurring every day of different intensity and magnitude, from 500,000 per year at a Richter scale of 1, to one every few years at a Richter scale of about 8. Faults are produced when rock strata are stressed beyond their limits, forming cracks in the crust. These cracks are fault zones where crustal movement is taking place. There are three types of faults shown in Figure 4: normal, reverse, and strike slip. Normal faults, also called tension faults, move up and down, caused by two plates pulling away at divergent boundaries. These vertical movements cause one side of the land to slide downwards along a plane that is slanted. This kind of downward-fault produces trench-like valleys called grabens similar to the Rhine Valley on the border of France and West Germany. Reverse faults, or compression faults, are caused by the col lision of two plates at convergent boundaries. Most faults are produced by this compressional force. Like normal faults, these faults also cause vertical movements where one side is pushed upwards vertically on an inclined plane. These faults produce high vertical upward-fault structures called horst. Harem: The Power Within EssayRadioactive dating along with magnetic reversals provides a means to record the speed at which the ocean floor is spreading. The youngest crust is where the magma flows out from the ridges and the oldest being where the crust flows back in the trenches. Figure 66 shows the age of the oceanic crust. Deep sea drilling and the art of radioactive dating could tell us when the magnetic field was reversed. Ships equipped with hollow drills would obtain samples of the ocean floor from various places around the ridge. The procedure most widely used to date the ocean floor is the Potassium-Argon dating method. It relies on any present radioactive material, Potassium-40. Potassium-40 decays slowly (1250 x 106 years) but not as slow as Uranium, which decays too slowly for this purpose and Carbon, which decays too fast. Potassium-40 decays into Argon-40 and Calcium-40. By measuring the amount of decay, the age of the ocean floor can be determined. Knowing the time and distance, the velocity of the ocean floor spreading can then be determined. It takes about 50 to 150 million years for the crust to travel from its origin to where it will circulate back below. The crust is relatively new because it is always being renewed. Using the magnetic orientation of rocks, more evidence could be deduced that backs of the theory of Pangea: it is possible, using simple trigonometry, to determine the latitude at which the rock was formed and the past orientation of the continent upon which it lay.7 This practice is called paleomagnetism. The readings can give the position of the magnetic North pole in any time period. If the readings from a single continent is plotted, a smooth curve called the polar wander curve, could be attained. The plot shows the curve leading away from the present pole. This is only possible if either the magnetic pole moved or the continent moved. When readings were calculated for other continents, the curves did not converge at a point. This means that there was only one magnetic North pole at any one time and indicated that the continents moved in respect to each other. Magnetic reversals are still a mystery, but many suggested hypothesis exists. One reason was that collisions with meteorites or comets may have caused the reversals. In fact, there was recent evidence that the Earth in fact, collided with a huge meteor. This hypothesis corresponded to the periods of mass extinctions; of the eight species that vanished from the cores during the 2.5 million years for which the record was most complete, six disappeared close to the time of a reversal, as recorded in the magnetic particles of the same core.8 Tektites, glassy fragments from meteorites containing large amounts of iron and magnesium were scattered over large sections of the Earth which corresponded to the last major reversal. The meteorites provided some proof to this hypothesis. This theory is just one of the many scientists have come up with. Others believed that the anomalies were formed by the compression of rocks the same kind of compression that existed during mountain building. The drifting of plates could cause devastation or wonder. Convection cells that propel the plates produces Earths surface dynamics. Murderous earthquakes and violent storms of volcanoes are a result from these ever-dynamic floating plates. Earths crust juts out as high as the sky along with the deep valleys that are being produced from the crashing and spreading of these plates. Upwelling of hot magma separates the land and continents similar to the separation of Pangea, but in time the continents will meet yet again to form another super-continent. The ever new sea floor containing magnetic footprints shows us of a time of magnetic field reversals. These reversals could explain continental drift and its velocity. There has been extensive study in tectonic plates, but there are still unsolved mysteries for one to discover. ENDNOTES1.Wallace S. Broecker, How to Build a Habitable Planet. (Palisades, New York: Eldigio Press, 1985), p. 147. 2.Robert W. Christopherson, Geosystems. 2nd. ed. (New York: MacMillan College Publishing Company, 1994), p. 341. 3.Ibid., p. 336-337. 4.Wallace S. Broecker, How to Build a Habitable Planet. (Palisades, New York: Eldigio Press, 1985), p. 155-156. 5.Walter Sullivan, Continents in Motion. 2nd. ed. (New York: McGraw Hill Book Company, 1991), p. 97. 6.Wallace S. Broecker, How to Build a Habitable Planet. (Palisades, New York: Eldigio Press, 1985), p. 159. 7.Peter J. Smith, The Earth. (New York: MacMillan Publishing Company, 1986), p. 13. 8.Waltus Sullivan, Continents in Motion. 2nd. ed. (New York: McGraw Hill Book Company, 1991), p. 96. BIBLIOGRAPHYBird, John M. and Isacks, Bryan, ed., Plate Tectonics. Washington American Geophysical Union, 1972. Broecker, Wallace S. How to Build a Habitable Planet. Palisades, New York: Eldigio Press, 1985. Christopherson, Robert W. Geosystems. 2nd. ed. New York: MacMillan College Publishing Company, 1994. Erickson, Jon Volcanoes and Earthquakes. Blue Ridge Summit: Tab Books Inc., 1988. Smith, Peter J. The Earth. New York: MacMillan Publishing Company, 1986. Sullivan, Walter Continents in Motion. 2nd. ed. New York: McGraw Hill Book Company, 1992. Category: History