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Continental Drift and Plate Tectonics Theories
It is by now universally recognized that the continents and other land masses on the earth are constantly moving, albeit at a very slow rate and have been on the move for millions of years. The land masses have collided, broken apart and drifted across the planet while floating on the fiery mantle beneath the outer layer of its crust. The Continental Drift and Plate Tectonic theories indicate that about 250 million years ago there was only one continent on earth named Pangaea
This great land mass fragmented and its parts began to move away from one another forming the great oceans in between the continents. As an extension of these theories, it can now be predicted with a fair degree of confidence that the moving Continents will one day come together again to form a single giant land mass. This paper describes the Continental drift and Plate Tectonic theories and traces their history, discusses the key players who were involved in developing the theories and examines the current state of scientific knowledge about Earth's geology and paleontology.
Theory of the Continental Drift
The idea that the earth's continents have drifted has a long history. As long ago as 1596 the Dutch map maker Abraham Ortelius in his work Thesaurus Geographicus Ortelius suggested that the Americas were "torn away from Europe and Africa . . . By earthquakes and floods." He had been the first (and certainly not the last) to notice the apparent "jig-saw" fit of the bulge of eastern South America into the bight of Africa. (Kious and Tilling, 1999) In around 1850 A.D., a French scientist Antonio Snider-Pellegrini, while researching the similarity of the fossil plants and coal deposits in North American and European and concluded that the phenomenon could only be explained if the two continents had once been connected. In 1908 Frank B. Taylor of the United States invoked the notion of continental collision to explain the formation of some of the world's mountain ranges.
Alfred Wegener (1880-1930)
Alfred Wegener, a German meteorologist, was a brilliant inter-disciplinary scientist who first proposed the theory of the Continental Drift, which was ridiculed at the time, but was later accepted by the scientist community and gave rise to one of the most important geological theories, i.e., the theory of plate tectonics.
Wegener had always believed that only by combining all the findings and evidence of all earth sciences could man learn the truth about the earth's past. In 1911, he came across a scientific paper listing fossils of identical plants and animals on either side of the Atlantic. At that time, the accepted theory in science to explain such similarity was that the land bridges (now sunken) had at one time connected the continents. But Wegener had been intrigued by the close fit between the coastlines of Africa and South America and was convinced that the continents had been joined together at one time. He proceeded to gather scientific evidence to prove the theory. He soon found that several geographical features on either side the Atlantic matched closely, e.g., the mountains in eastern North America and the Scottish highlands. Moreover, he discovered that the fossils found in a certain place often indicated that the animal / plant had existed in a climate utterly different of the area from where it was found. (Waggoner, 1996)
Wegener published his theory of Continental Drift in his book The Origin of Continents and Oceans in 1915. The theory claimed that about 300 million years ago, all the continents had formed a single mass, called Pangaea which later split into pieces and started to move away -- the movement continuing to this day. Despite having presented considerable evidence in support of the theory, the scientific community received the theory with derision. Apart from a natural resistance to revolutionary ideas, the opposition was also due to the fact that Wegener could not explain adequately how the continents moved. He contended that centrifugal and tidal forces forced the continents to move through the earth's crust like icebreakers plowing through ice sheets. Such an explanation was flawed since the centrifugal and tidal forces were far too weak to move the continents and some scientists demonstrated that it was physically impossible for a large rock (the continent) to plow through the ocean floor without breaking up. Hence, despite scattered support for Wegener's theory of the Continental Drift, the majority of scientists continued to believe in the old theory about the existence of "land bridges" between continents in the past.
Support for the Theory
Holmes' Convection Theory:
While most scientists had rejected Wegener's theory of Continental Drift, a few started to build upon his ideas. In 1929, Arthur Holmes -- an English geologist suggested that convection currents within the Earth's mantle, driven by radioactive heat, might furnish the mechanism for the continental drift theory. The idea was based on the fact that as a substance is heated its density decreases and rises to the surface until it is cooled and sinks again. Holmes suggested that this repeated heating and cooling results in a thermal convection that could break apart a continent and then force the broken continent in opposite directions carried by the convection currents. The idea did not attract much attention but would be revisited later. (Weil, 1997 ." The Rocky History of an Idea,")
Revival of Interest in the Continental Drift:
Beginning in the 1950s, new evidence emerged to revive interest in Wegener's discredited Continental Drift theory. A British geophysicist Stanley K. Runcorn showed that the north magnetic pole had wandered from its original position in the past, and continental drift was a reasonable explanation for the fact.
Other developments that led to further demonstration of Wegener's ideas included more accurate mapping of the ocean floor that revealed the presence of a great mountain range on the ocean floor (named the global mid-ocean ridge) virtually encircled the Earth "like the seam on a baseball." (Kious and Tilling, 1999: "Developing the Theory") It was also revealed that the sediment layer on the floor of the Atlantic was much thinner than originally thought -- indicating it could not have been formed 4 billion years ago, as was previously thought.
How the mid-ocean ridge was formed was an intriguing question. In the 1960s evidence began to emerge that the mid-ocean ridge marks the structurally weak zones in the ocean floor from which new magma from deep within the Earth rose and settled along the crest of the ridges to create new oceanic crust. This process was later called seafloor spreading.
American geologist Harry H. Hess was the first to understand the implication of seafloor spreading. He used Holmes' Convection Theory to explain the expansion of the Earth's crust along the oceanic ridges without an increase in the Earth's size. Hess reasoned that the new oceanic crust which was continuously spreading away from the ridges in a conveyor belt-like motion, millions of years later, eventually descends into the oceanic trenches -- the deep, narrow canyons along the rim of the Pacific Ocean basin.
Hence, according to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. Hess's theory explains "why the Earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks." (Ibid.)
Plate Tectonic Theory
The above mentioned developments eventually evolved into the Plate Tectonic Theory, the main features of which are:
The Earth's surface (called the lithosphere) is covered by a series of crustal plates.
The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated.
Convection currents beneath the plates move the crustal plates in different directions.
The source of heat driving the convection currents is radioactivity deep in the Earths mantle. (Weil, 1999, "Plate Tectonics: The Mechanism")
The Plate Tectonic theory, in its current state of development, satisfactorily explains most developments that take place on the earth's surface such as earthquakes and the formation of mountains. According to the theory the lithosphere (outer crust of the Earth) consists of about a dozen large plates and several small ones. The lithosphere rests on and slides over an underlying, weaker layer of partially molten rock known as the asthenosphere. The crust of the continents is made from a lighter material than the crust ocean floor and is much thicker (30 to 40 kilometers) as compared to the oceanic crust (6 to 7 kilometers). The greater buoyancy of the continents causes them to float much higher on the layer of molten rock than the oceanic crust; hence the resulting difference between the levels of the land mass and the ocean floors.
The earth's surface plates move relative to each other and interact at their boundaries, where they diverge, converge, or sometimes slip relatively harmlessly past one another. These interactions of the plates at the 'fault lines' are believed to be responsible for most…[continue]
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Plate Tectonics Theory The story of Plate Tectonics is the story of continents drifting from place to place, breaking apart, colliding, and grinding against each other (Story pp). It is also the story of terrestrial mountain ranges rising up while being pushed together, of oceans opening and closing, of undersea mountain chains girdling the planet like seams on a baseball, and of violent earthquakes and fiery volcanoes (Story pp). Plate Tectonics
plate tectonics is responsible for changing continental landmasses through geological occurrences. Thousands of years ago the earth's surface has been hypothesized as one big landmass. The Earth's surface has been constant motion. "Fragmented into giant sheets of solid rock that glide atop a layer of hotter, more pliable material, the globe's appearance is forever changing." [Cowen, 1999]. These plates are semi-rigid, floated on flow of mantle. The plates measured around