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 Earth's existance, the magnetic fields have never been stable. Solidified magma containing magnetic imprints reveal periods of time when the Earth's 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."1
Presently, there are more than fourteen plates in the Earth's 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 1900's, 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). Pangea's 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. India's 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 the 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 collision 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.
Strike slip or transform faults are lateral movements of faults at the transformed boundaries. Strike-slip faults do not produce any cliffs but they can produce rift valleys. Tectonic forces deform the rocks on both sides of the fault. At this point, rocks are bending and storing potential energy. Finally, when the force exceeds the frictional force between the two rocks, the plates suddenly slip at the most vulnerable place. The initial slip causes more slippage along the fault which in turn causes energy to be released. The released energy produces vibrations called seismic waves which originate at the epicenter. The San Andreas Fault is a well-known example of this released energy from a transform fault. At this location, an almost straight valley is produced by the parallel fractures. The Pacific plate, in Canada, is sliding northwards and thus, in the future, California may end up where Vancouver is, today.
One of the most prominent signs that molten material resides below the crust and mantle is the display of volcanoes. Magma seeks out weak spots on the crust where it could seep out. Volcanoes are mostly present at fault lines especially at the ocean ridges where new magma is constantly being poured out. This accounts for about 81% of all magma that escapes to the surface. The other 19% rises at certain points rather than along fissures. On of the most famous examples of volcanic activity is The Ring of Fire, located around the Pacific Plate. There, a continuous 'ring' of volcanoes exists. 'Island arcs' are formed there by many volcanoes developing islands in the form of a curve. The longest island arc is the Aleutian Islands stretching more than 3000 miles from Alaska to Asia. One explanation for this arc is that the Pacific plate is rotating very slowly. The westward-moving plate moves away from the source of volcanic activity making the volcanoes arise in an arc due to the rotation of the plate.
One of the beauties plate collisions could offer are mountains. There are three types of tectonic mountains: volcanoes, block fault, and folding. One way mountains are formed are by volcanoes such as the aforementioned island arcs. In time, after numerous eruptions, more and more sediments are layered and compressed, forming mountainous islands. Block fault mountains occur when two plates collide, causing one to climb up. This is known also as a horst mentioned before. Mountains such as the Sierra Nevada Range is a large tilted fault block. Folding mountains occur when two converging plates bring two land masses together. When a continent is pushing its way towards another, the oceanic crust sinks into the subduction zone. As it moves down the zone, the sediment that makes up the crust is scraped off by the other continent. With the continental crusts pushing together, the sedimentary rocks are compressed into complex folds where the folds themselves fold as well. This process forms the high alpine mountains such as the Himalayas which were caused by India crashing into Asia.
If the mantle is always being convected back down into the depths of the Earth, then why doesn't the continents disappear in the deep ocean trenches as well? The crust contains two different crusts: the granite continental crust and the basaltic oceanic crust. Only the basaltic crust is thrust back into the Earth while the granite crust floats on top of it. This is due to the difference in densities. The granite crust is less dense (2.7 g/cm3) and thicker than the basaltic crust (2.8 g/cm3) making it seem as if the land is actually floating, instead of one big solid mass that extends down to the Earth. Using Broecker analogy:
"...swimming pool with 4 x 4 hardwood beams and part with 8 x 8 softwood beams. The softwood beams would float higher for two reasons: they are thicker and they are less dense."4
As new crust is formed from upwelling magma, the ocean floor spreads away from the source. Because molten magma contains metallic substances such as iron, the cooled rock will possess a magnetic field parallel to the direction of the Earth's field. The magnetic imprint occurs when certain substances cool after intense heating within a magnetic field. The rock cools to the temperature when the magnetic field of the rock becomes permanent; this is called the Curie temperature. During the history of the Earth, this 'normal' magnetic field (North pole to true North) has not been constant. Over the past 110 million years, the Earth's magnetic field has reversed about 80 times with North becoming South and vice versa. Figure 55 shows the chronological reversals of Earth's magnetic field over the past 4.5 million years. The last major reversal was approximately 700,000 years ago called the Brunhes Epoch. These magnetic reversals are symmetrical to either side of the ridge. The reversals are also random with no determined period of time.
Radioactive 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 Earth's surface dynamics. Murderous earthquakes and violent storms of volcanoes are a result from these ever-dynamic floating plates. Earth's 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.
ENDNOTES
1. 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.
BIBLIOGRAPHY
Bird, 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.
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