Final#2 Plate Techtonics

This discussion topic submitted by Brian "Barnacle" Coy ( coybe@miamioh.edu) at 11:06 am on 7/1/00. Additions were last made on Wednesday, May 7, 2014.

 The theory of plate tectonics, formulated during the late 1960s,rests on a broad synthesis of geologic and geophysical data. It is nowalmost universally accepted and has had a major impact on thedevelopment of the Earth sciences. Its adoption represents a truescientific revolution, analogous in its consequences to the Rutherfordand Bohr atomic models in physics or the discovery of the genetic codein biology. (Heirtzler, J. R., 1968) Incorporating the much older ideaof continental drift, the theory of plate tectonics has made the studyof the Earth more difficult by doing away with the notion of fixedcontinents, but it has at the same time provided the means ofreconstructing the past geography of continents and oceans. While itsimpact has, to a considerable degree, run its course in marine geologyand shows signs of reaching the limits of usefulness in the study ofmountain-building processes, its influence on the scientificunderstanding of the Earth's history, of ancient oceans and climates,and of the evolution of life is only beginning to be felt. (Dott etal., 1976) The plate tectonics theory has a long and tortuous history.Yet, the theory itself is elegantly simple. The surface layer of theEarth, from 50 to 100 kilometers (31 to 62 miles) thick, is assumed tobe composed of a set of large and small plates, which togetherconstitute the rigid lithosphere. The lithosphere rests on and slidesover an underlying, weaker layer of partially molten rock known as theasthenosphere. The constituent lithospheric plates move across theEarth's surface, driven by forces as yet not fully agreed upon, andinteract along their boundaries, diverging, converging, or slippingpast each other. While the interiors of the plates are presumed toremain essentially undeformed, their boundaries are the sites of manyof the principal processes that shape the terrestrial surface,including earthquakes, volcanism, and orogeny (i.e., the deformationthat builds mountain ranges). (Dott et al., 1976) The most conspicuous feature of the Earth's surface is itsdivision into continents and ocean basins, a division that owes itsexistence to differences in thickness and composition between thecontinental and the oceanic crust. The continents have a crust ofgranitic composition and hence are somewhat lighter than the basalticocean floor. (Dietz et al., 1970) Also, they are 30 to 40 kilometersthick as compared to the oceanic crust, which measures only 6 to 7kilometers in thickness. (Dott et al., 1976) Their greater buoyancycauses them to float much higher in the mantle than does the oceaniccrust, thus accounting for the difference between the two principallevels of the Earth's surface. The boundary between the continental oroceanic crust and the underlying mantle, the Mohorovicicdiscontinuity, has been clearly defined by seismic studies. (Hurley,P.M., 1968) As conceived by the theory of plate tectonics, the lithosphericplates are much thicker than the oceanic or the continental crust;their boundaries do not usually coincide with those between oceans andcontinents; and their behavior is only partly influenced by whetherthey carry oceans, continents, or both. (Dott et al., 1976) ThePacific Plate, for example, is purely oceanic, but most of the otherscontain continents. At a divergent plate boundary, magma wells up from below as therelease of pressure produces partial melting of the underlying mantleand generates new crust. (Dott et al., 1976) Because the partial meltis basaltic in composition, the new crust is oceanic. Consequently,diverging plate boundaries, even if they originate within continents,eventually come to lie in ocean basins of their own making. In fact,most divergent plate boundaries seem to have formed within continentsrather than in oceans, probably because a hot, weak layer, sandwichedat a depth of about 15 kilometers between two stronger ones, rendersthe continental crust more vulnerable to fragmentation than itsoceanic counterpart. The creation of the new crust is accompanied bymuch volcanic activity and by many shallow tension earthquakes as thecrust repeatedly rifts, heals, and rifts again. (Dott et al., 1976) The continuous formation of new crust produces an excess thatmust be disposed of elsewhere. This is accomplished at convergentplate boundaries where one plate descends--i.e., is subducted--beneaththe other. At depths between 300 and 700 kilometers, the subductedplate melts and is recycled into the mantle. (Heirtzler, J. R., 1968)Because the plates form an integrated system that completely coversthe surface of the Earth, it is not necessary that new crust formed atany given divergent boundary be completely compensated at the nearestsubduction zone, as long as the total amount of crust generated equalsthat destroyed. (Hurley, P.M., 1968) It is in subduction zones that the difference between platescarrying oceanic and continental crust can be most clearly seen. Ifboth plates have oceanic edges, either one may dive beneath the other;but, if one carries a continent, the greater buoyancy prevents thisedge from sinking. Thus, it is invariably the oceanic plate that issubducted. (Heirtzler, J. R., 1968) Continents are permanently preserved in this manner, while theocean floor continuously renews itself. If both plates possess acontinental edge, neither can be subducted and a complex sequence ofevents from crumpling to under- and overthrusting raises loftymountain ranges. Much later, after these ranges have been largelyleveled by erosion, their remains continue as a reminder that this isthe "suture" where continents were once fused. (Hurley, P.M., 1968) The subduction process, which involves the descent into themantle of a slab of cold rock about 100 kilometers thick, is marked bynumerous earthquakes along a plane inclined 30-60 into the mantle--theBenioff zone. Most earthquakes in this planar dipping zone result fromcompression, and the seismic activity extends 300-700 kilometers belowthe surface. At a depth of 100 kilometers or more the subductedoceanic sediments, together with part of the upper basaltic crust,melt to an andesitic magma, which rises to the surface and gives birthto a line of volcanoes a few hundred kilometers behind the subductingboundary. (Dott et al., 1976) This boundary is usually marked by anoceanic deep, or trench, where the overriding plate scrapes off theupper crust of the lower plate to create a zone of highly deformed,largely sedimentary rock. If both plates are oceanic, the deformedsediments and volcanoes form two island arcs parallel to the trench.If one plate is continental, the sediments are usually accretedagainst the continental margin and the volcanoes form inland, as theydo in Mexico or western South America. (Dietz et al., 1970) Along the third type of plate boundary, two plates movelaterally and pass each other without creating or destroying crust.Large earthquakes are common along such strike-slip, or transform,boundaries. Also known as fracture zones, these plate boundaries areperhaps best exemplified by the San Andreas fault in California andthe North Anatolian fault system in Turkey. (Dott et al., 1976) Most of the seismic and volcanic activity on Earth is thereforeconcentrated along plate boundaries where mid-ocean ridges, trencheswith island arcs, and mountain ranges are generated. Some seismic andvolcanic activity also occurs within plates. Interesting examples ofthis interplate activity are linear volcanic chains in ocean basins,such as the Hawaiian Islands and their westward continuation as astring of reefs and submerged seamounts. An active volcano usuallyexists at one end of an island chain of this type, with progressivelyolder extinct volcanoes occurring along the rest of the chain. Suchtopographic features have been explained by J. Tuzo Wilson of Canadaand W. Jason Morgan of the United States as the product of "hotspots," magma-generating centers of controversial origin located deepin the mantle far below the lithosphere. A volcano builds at thesurface of a plate positioned above a hot spot. As the plate moveson, the volcano dies, is eroded, and eventually sinks below thesurface of the sea, while a new one forms above the hot spot. Hotspot volcanism is not restricted to the ocean basins; othermanifestations occur within continents, as in the case of YellowstoneNational Park in western North America. (Hurley, P.M., 1968) The movement of a plate across the surface of the Earth can bedescribed as a rotation around a pole, and it may be rigorouslydescribed with the theorem of spherical geometry formulated by theSwiss mathematician Leonhard Euler during the 18th century.(Heirtzler, J. R., 1968) Similarly, the motions of two plates withrespect to each other may be described as rotations around a commonpole, provided that the plates retain their shape. The requirementthat plates are not internally deformed has become one of thepostulates of plate tectonics. (Heirtzler, J. R., 1968) It is nottotally supported by evidence, but it appears to be a reasonableapproximation of what actually happens in most cases. It is needed topermit the mathematical reconstruction of past plate configurations. It is, of course, conceivable that the entire lithosphere mightslide around over the asthenosphere like a loose skin, altering thepositions of all plates with respect to the spin axis of the Earth and theequator. (Hurley, P.M., 1968) To determine the true geographic positions ofthe plates in the past investigators have to define their motions, relativenot to each other but rather to this independent frame of reference. Thehot spot island chains serve this purpose, their trends providing thedirection of motion of a plate; the speed of the plate can be inferred fromthe increase in age of the volcanoes along the chain. It is assumed, ofcourse, that the hot spots themselves remain fixed with respect to theEarth, an assumption that appears to be reasonably accurate for at leastsome hot spots. The theory of plate techtonics is originally credited to AlfredWegener. (Dietz et al., 1970) Wegener was by training and profession ameteorologist (he was highly respected for his work in climatology andpaleoclimatology), but he is best remembered for the foray into geologythat led to his formulation of the concept of continental drift. In 1910,because of the geography of the Atlantic coastlines, Wegener came toconsider the existence of a single supercontinent during the late Paleozoicera (about 350 to 225 million years ago) and named it Pangaea. He searchedthe geologic and paleontological literature for evidence attesting to thecontinuity of geologic features across the Indian and Atlantic oceans,which he assumed had formed during the Mesozoic era (about 245 to 66.4million years ago). His efforts proved rewarding, and he presented the ideaof continental drift and some of the supporting evidence in a lecture in1912. This was followed in 1915 by his major work, Die Entstehung derKontinente und Ozeane (The Origin of Continents and Oceans). (Dietz et al.,1970) Much was thus to be said for the idea that the continents werejoined together in the Paleozoic, and supporting evidence hascontinued to accumulate to this day. (Dott et al., 1976) The opposingAtlantic shores match well, especially at the 1,000-metre (3,300-foot)depth contour, which is a better approximation of the edge of thecontinental block than the present shoreline, as Sir Edward Bullardcogently demonstrated in 1964 with the aid of computer analysis.Similarly, the structures and stratigraphic sequences of Paleozoicmountain ranges in eastern North America and northwestern Europe canbe matched in detail. This fact was already known to Wegener and hasbeen strengthened substantially in subsequent years. (Dott et al.,1976) The existing theory of plate tectonics leaves many unansweredquestions. Unforeseen natural occurrences occur everyday which oftencontradict once believed theories. Luckily for us though, scientistssuch as Alfred Wegener have paved the way in reaching a betterunderstanding of the natural environment and the way we can growsymbiotically with it.

REFERENCES

Dietz, R. S., and J. C. Holden. “The Breakup of Pangaea.” Scientific American, October 1970, pp. 30-41.

Dott, R., and R. Batten. Evolutions of the Earth. New York: McGraw- Hill, 1976.

Heirtzler, J. R. “Sea Floor Spreading.” Scientific American, December 1968, pp. 60-70.

Hurley, P. M. “The Confirmation of Continental Drift.” Scientific American, April 1968, pp. 52-64.


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