Misplaced Pages

This is an old revision of this page, as edited by 155.239.56.97 (talk) at 14:36, 12 March 2009. The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Definition of tectonic plates

Plate tectonics (from the Greek τέκτων; tektōn, meaning "builder" or "mason") describes the large scale motions of Earth's lithosphere. The theory encompasses the older concepts of continental drift, developed during the first decades of the 20th century by Alfred Wegener, and seafloor spreading, understood during the 1960s.

The outermost part of the Earth's interior is made up of two layers: the lithosphere and the asthenosphere.

• Above is the lithosphere, comprising of the crust and the rigid uppermost part of the mantle.
• Below the lithosphere lies the asthenosphere. Although solid, the asthenosphere has relatively low viscosity and shear strength and can flow like a liquid on geological time scales. The deeper mantle below the asthenosphere is more rigid again due to the higher pressure.

The lithosphere is broken up into what are called tectonic plates. In the case of Earth, there are eight major and many minor plates (see list below). The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: convergent, or collisional boundaries; divergent boundaries, also called spreading centers; and transform boundaries. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 50–100 mm annually.

Synopsis of the development of the theory

In the late 19th and early 20th centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as mountain ranges could be explained by vertical crustal movement, through geosynclinal theory. It was observed as early as 1596 that the opposite coasts of the Atlantic Ocean—or, more precisely, the edges of the continental shelves—have similar shapes and seem to have once fitted together. Since that time many theories were proposed to explain this apparent compatibility, but the assumption of a solid earth made the various proposals difficult to explain.

The discovery of radium and its associated heating properties in 1896 prompted a re-examination of the apparent age of the Earth, since this had previously been estimated by its cooling rate and assumption the Earth's surface radiated like a black body. Those calculations implied that, even if it started at red heat, the Earth would have dropped to its present temperature in a few tens of millions of years. Armed with the knowledge of a new heat source, scientists reasoned it was credible that the Earth was much older, and also that its core was still sufficiently hot to be liquid.

Plate tectonic theory arose out of the hypothesis of continental drift proposed by Alfred Wegener in 1912 and expanded in his 1915 book The Origin of Continents and Oceans. He suggested that the present continents once formed a single land mass that drifted apart, thus releasing the continents from the Earth's core and likening them to "icebergs" of low density granite floating on a sea of denser basalt.<ref>{{cite book |author=Alfred Wegener |title=The Origin of Continents and Oceans |translator=Biram John |year=1966 |publisher=Courier Dover |pages=246 |isbn=0486617084

1. Read HH, Watson Janet (1975). Introduction to Geology. New York: Halsted. pp. 13–15.
2. Kious WJ, Tilling RI. "Historical perspective". This Dynamic Earth: the Story of Plate Tectonics (Online edition ed.). U.S. Geological Survey. ISBN 0160482208. Retrieved 2008-01-29. Abraham Ortelius in his work Thesaurus Geographicus ... suggested that the Americas were "torn away from Europe and Africa ... by earthquakes and floods ... The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three ." {{cite book}}: |edition= has extra text (help); External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help); Unknown parameter |origmonth= ignored (help)
3. Cite error: The named reference Frankel was invoked but never defined (see the help page).
4. Joly J (1909). Radioactivity and Geology: An Account of the Influence of Radioactive Energy on Terrestrial History. London: Archibald Constable. p. 36. ISBN 1402135777.
5. Thomson W (1863). "On the secular cooling of the earth". Philosophical Magazine. 4 (25): 1–14. doi:10.1080/14786435908238225.
6. Hughes Patrick. "Alfred Wegener (1880-1930): A Geographic Jigsaw Puzzle". On the Shoulders of Giants. Earth Observatory, NASA. Retrieved 2007-12-26. ... on January 6, 1912, Wegener ... proposed instead a grand vision of drifting continents and widening seas to explain the evolution of Earth's geography.