Thesis: Plate Tectonics

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PLATE TECTONICS: EVOLUTION & EXPANSION OF KNOWLEDGE OVER TIME

The objective of this work is to identify a specific scientific concept of phenomenon for which the understanding has changed over time. For the purpose of this specific work plate tectonics will be examined.

The work of Mian (1993) entitled: "Understanding Why Earth is a Planet with Plate Tectonics" states that earth scientists "now organize their thinking about our planet with in a picture of Earth as a dynamically active planet where even the basic geography of its surface appears to change radically. The consolidation of this perception, often described as the plate tectonics revolution, has overlapped the era of exploration of the other bodies in the solar system." (p.441) Mian states that the difficulty arises in the science of plate tectonics is that there is no known planet by which to make comparison of earth to in the area of plate tectonics and that this is because scientific experimentation involves singling out one specific factor for study comparison and there is not another planet by which to compare the changes that occur in the plate tectonics of the earth.

I. Difficulties and Challenges in the Study of Earth Plate Tectonics

Mian (1993) state that there has been a suggestion of "active, local spreading on Venus and a low level of active volcanism seems necessary to maintain the SO2 content of the Venusian atmosphere for more than a few million years, but it is clear there is no global-scale network of surface structures similar to those associated with the plate tectonics movements occurring on Earth." (p.442) Mian states that the surface of the Earth is "...obviously different in all those factors which determine habitability but it is unique among the terrestrial plants in having a pattern of large-scale relative motions of its surface material." (1993, p.442)

II. Plate Tectonics Theory

Plate tectonics is stated to Mian (1993) to have appeared "as a synthesis of ideas about continental motion or drift and sea-floor spreading, the central element being lateral motion of large areas of sea-floor away from ocean ridges (spreading centers) towards ocean trenches (subduction zones)." (p.442) However, the description given for this motion described as 'rigid plates' is stated to have "involved something of a misnomer because what was really meant was that areas of the surface moved as if they were 'rigid bodies'." (Mian, 1993, p.442)

However, since the sites of spreading and subduction are mobile in nature and the speeds of movement in some places can be compared with the speed of movement of the 'plates' the surface rock "...deformability...[and]...their propensity to break at spreading centers and bend at subduction zones" creates a problem therefore "coming to grips with the character of this problem requires an appreciation of the deformability of materials under stresses applied on very widely differing timescales." (Mian, 1993, p.442)

III. Formulation of Continental Drift Theory (1915)

Alfred Wegener is stated to have formulated continental drift in 1915 which was inclusive of "convective motions, driven by the heat generated through radioactive decay, as a possible cause of this drift in the 1929 edition of his book "The Origin of Continents and Oceans." (Mian, 1993, p.443) However, it is reported that a dilemma ensued between "the seismologists picture of a 'solid-solid Earth', where a solid was exactly that and had nothing in common with a fluid, and the view deriving from Wegener's (among others) more subtle idea of Earth's interior as a high viscosity 'liquid' scarcely distinguishable from a 'solid' under laboratory conditions of time, temperature and pressure..." (Mian, 1993, p.443)

IV. Lithosphere/Asthenosphere Boundary Marking Transition

If the original idea is accepted and specifically that of a "lithosphere/asthenosphere boundary marking transition from quasi-elastic to quasi-viscous behavior at a particular timescale of deformation" the question that arises is how does the "concept have any utility in explaining the few cm years movements of the surface, and if so, where is the boundary located?" (Mian, 1993, p.443) Mian (1993) holds that it is not possible to detach this problem from "views on the causes of superficial movement and its interaction with the classic problem of Earth's internal temperature -- it was the failure to see this connection which led in part to the misuse of seismological data..." (p.444)

V. Lord Kelvin (1862) and Earth Rigidity

In fact, since Lord Kelvin in 1862 demonstrated that the Earth "at tidal frequencies of deformation was 'more rigid than steel'. There has been some difficulty in explaining the source and cause of Earth's active volcanoes." (Mian, 1993, p.444) This demonstration of the apparent rigidity proved that heat was rapidly conducted to the surface of the earth however, the problem arises in accounting simultaneously for the "eruption of magma" which is liquefied rock. (Mian, 1994, paraphrased)

It is apparent that the problem arises from the fundamental trouble with the assumption "that heat would be conducted throughout Earth material because it had a rigidity at tidal or seismic deformation frequencies. It is clear that the convection of heat can be a potent factor controlling the internal temperature of planet-sized systems when their effective viscosities are enormous..." (Mian, 1993, p.444)

VI. Sea-Floor Spreading and Plate Tectonics (1960s)

Tilling, Heliker and Wright (1987) report that the related concepts of "sea-floor spreading and plate tectonics, in the early 1960s...emerged as powerful new hypotheses that geologists used to interpret the features and movements of the Earth's surface layer." (in: USGS, nd, p.1) According to the theory of plate tectonics the surface of the Earth is comprised by approximately one dozen "rigid slabs or plates, each average at least 50 miles thick. These plates move relative to one another at average speeds of a few inches per year..." (Tilling, Heliker and Wright, 1987 in: USGS, nd, p.1) Three common types of boundaries are now recognized by scientist as existing between these moving plates as follows:

(1) Divergent or spreading -- adjacent plates pull apart, such as at the Mid-Atlantic Ridge, which separates the North and South American Plates from the Eurasian and African Plates. This pulling apart causes "sea-floor spreading" as new material is added to the oceanic plates;

(2) Convergent -- plates moving in opposite directions meet and one is dragged down (or subducted) beneath the other. Convergent plate boundaries are also called subduction zones and are typified by the Aleutian Trench, where the Pacific Plate is being subducted under the North American Plate; and (3) Transform fault -- one plate slides horizontally past another. The best known example is the earthquake-prone San Andreas fault zone of California, which marks the boundary between the Pacific and North American Plates. (Tilling, Heliker and Wright, 1987 in: USGS, nd, p.1)

VII. Knowledge on a Continuous Course of Growth

The work of Kious and Tilling (1996) entitled: "This Dynamic Earth: The Story of Plate Tectonics" reported that states that history which has been recorded relates that "explosive eruptions at subduction-zone (convergent-boundary) volcanoes have posed the greatest hazard to civilizations. Yet scientists have estimated that about three quarters of the material erupted on Earth each year originates at spreading mid-ocean ridges. However, no deep submarine eruption has yet been observed 'live' by scientists." (in USGS, nd, p.1)

However, as of December 17, 2009, this is no longer the case because as reported by Fox News on that date it is reported that scientists "...have recorded the deepest erupting undersea volcano ever seen, capturing for the first time video of fiery molten lava bubbles exploding 4,000 feet beneath the Pacific Ocean." (Fox News, 2009, p.1) For the first time ever scientists were able to witness the creation of "a material called boninite, which had previously been found only in samples at least a million years old. In the past, boninite lavas had only been found on extinct volcanoes..." (Fox News, 2009, p.1)

Summary and… [END OF PREVIEW]

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