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Sudeshjeewan

Joined 19 October 2009

--Sudeshjeewan (talk) 06:29, 19 October 2009 (UTC)sudesh jeewan gunarathna

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Earth

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Main articles: Earth, Earth science, Structure of the Earth, Plate tectonics, and Geology
File:Sudeshjeewan.jpg
View of Earth, taken in 1972 by the Apollo 17 crew. This image is the only photograph of its kind to date, showing a fully sunlit hemisphere of the Earth.

Earth (or, "the earth") is the only planet presently known to support life, and as such, its natural features are the subject of many fields of scientific research. Within the solar system, it is third nearest to the sun; it is the largest terrestrial planet and the fifth largest overall. Its most prominent climatic features are its two large polar regions, two relatively narrow temperate zones, and a wide equatorial tropical to subtropical region.[1] Precipitation varies widely with location, from several metres of water per year to less than a millimetre. About 70 percent of the surface is covered by salt-water oceans. The remainder consists of continents and islands, with most of the inhabited land in the Northern Hemisphere.

Earth has evolved through geological and biological processes that have left traces of the original conditions. The outer surface is divided into several gradually migrating tectonic plates, which have changed relatively quickly several times. The interior remains active, with a thick layer of molten mantle and an iron-filled core that generates a magnetic field.

The atmospheric conditions have been significantly altered from the original conditions by the presence of life-forms,[2] which create an ecological balance that stabilizes the surface conditions. Despite the wide regional variations in climate by latitude and other geographic factors, the long-term average global climate is quite stable during interglacial periods,[3] and variations of a degree or two of average global temperature have historically had major effects on the ecological balance, and on the actual geography of the Earth.[4][5]

Historical perspective

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Main articles: History of Earth and Evolution
 
Phylum Pediastrumboryanum. Plankton have existed on Earth for at least 2 billion years.[6]

Earth is estimated to have formed 4.55 billion years ago from the solar nebula, along with the Sun and other planets.[7] The moon formed roughly 20 million years later. Initially molten, the outer layer of the planet cooled, resulting in the solid crust. Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, most or all of which came from ice delivered by comets, produced the oceans and other water sources.[8] The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago.[9]

Continents formed, then broke up and reformed as the surface of Earth reshaped over hundreds of millions of years, occasionally combining to make a supercontinent. Roughly 750 million years ago, the earliest known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia which broke apart about 540 million years ago, then finally Pangaea, which broke apart about 180 million years ago.[10]

 
Land-based plants and fungi have been part of nature on Earth for about the past 400 million years. These have needed to adapt and move many times as the continents and climates changed.[11][12]

There is significant evidence, still being discussed among scientists, that a severe glacial action during the Neoproterozoic era covered much of the planet in a sheet of ice. This hypothesis has been termed the "Snowball Earth", and it is of particular interest as it precedes the Cambrian explosion in which multicellular life forms began to proliferate about 530–540 million years ago.[13]

Since the Cambrian explosion there have been five distinctly identifiable mass extinctions.[14] The last mass extinction occurred some 65 million years ago, when a meteorite collision probably triggered the extinction of the non-avian dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the past 65 million years, mammalian life diversified.[15]

Several million years ago, a species of small African ape gained the ability to stand upright.[16] The subsequent advent of human life, and the development of agriculture and further civilization allowed humans to affect the Earth more rapidly than any previous life form, affecting both the nature and quantity of other organisms as well as global climate. By comparison, the oxygen catastrophe, produced by the proliferation of algae during the Siderian period, required about 300 million years to culminate.)

The present era is classified as part of a mass extinction event, the Holocene extinction event, the fastest ever to have occurred.[17][18] Some, such as E. O. Wilson of Harvard University, predict that human destruction of the biosphere could cause the extinction of one-half of all species in the next 100 years.[19] The extent of the current extinction event is still being researched, debated and calculated by biologists.[20]

Geology

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Main article: Geology
 
Three types of plate boundary.

Geology (from Greek: γη, gê, "earth"; and λόγος, logos, "speech" lit. to talk about the earth) is the science and study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed. The field is a major academic discipline, and is also important for mineral and hydrocarbon extraction, knowledge about and mitigation of natural hazards, some engineering fields, and understanding past climates and environments.

Geological evolution of an area

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The geology of an area evolves through time as rock units are deposited and inserted and deformational processes change their shapes and locations.

Rock units are first emplaced either by deposition onto the surface or intrude into the overlying rock. Deposition can occur when sediments settle onto the surface of the Earth and later lithify into sedimentary rock, or when as volcanic material such as volcanic ash or lava flows, blanket the surface. Igneous intrusions such as batholiths, laccoliths, dikes, and sills, push upwards into the overlying rock, and crystallize as they intrude.

After the initial sequence of rocks has been deposited, the rock units can be deformed and/or metamorphosed. Deformation typically occurs as a result of horizontal shortening, horizontal extension, or side-to-side (strike-slip) motion. These structural regimes broadly relate to convergent boundaries, divergent boundaries, and transform boundaries, respectively, between tectonic plates.

  1. ^ "World Climates". Blue Planet Biomes. Retrieved 2006-09-21.
  2. ^ "Calculations favor reducing atmopshere for early Earth". Science Daily. 2005-09-11. Retrieved 2007-01-06.
  3. ^ "Past Climate Change". U.S. Environmental Protection Agency. Retrieved 2007-01-07.
  4. ^ Hugh Anderson, Bernard Walter (March 28, 1997). "History of Climate Change". NASA. Archived from the original on 2008-01-23. Retrieved 2007-01-07.
  5. ^ Weart, Spencer (June 2006). "The Discovery of Global Warming". American Institute of Physics. Retrieved 2007-01-07.{{cite web}}: CS1 maint: date and year (link)
  6. ^ Margulis, Lynn (1995). What is Life?. New York: Simon & Schuster. ISBN 0684813262. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ Dalrymple, G. Brent (1991). The Age of the Earth. Stanford: Stanford University Press. ISBN 0-8047-1569-6.
  8. ^ Morbidelli, A.; Chambers, J.; Lunine, J. I.; Petit, J. M.; Robert, F.; Valsecchi, G. B.; Cyr, K. E. (2000). "Source Regions and Time Scales for the Delivery of Water to Earth". Meteoritics & Planetary Science. 35 (6): 1309–1320. Bibcode:2000M&PS...35.1309M. doi:10.1111/j.1945-5100.2000.tb01518.x.{{cite journal}}: CS1 maint: date and year (link)
  9. ^ "Earth's Oldest Mineral Grains Suggest an Early Start for Life". NASA Astrobilogy Institute. 2001-12-24. Retrieved 2006-05-24.
  10. ^ Murphy, J.B. (2004). "How do supercontinents assemble?". American Scientist. 92 (4): 324. doi:10.1511/2004.4.324. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  11. ^ Colebrook, Michael. "Chronology of Earth History". Cosmology and The Universe Story. Retrieved September 21 2006. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |lay-format= ignored (help)
  12. ^ Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman. ISBN 0-7167-2882-6.
  13. ^ Kirschvink, J.L. (1992). "Late Proterozoic Low-Latitude Global Glaciation: The Snowball Earth" (PDF). In J.W. Schopf, C. Klein eds. (ed.). The Proterozoic Biosphere. Cambridge: Cambridge University Press. pp. 51–52. ISBN 0-521-36615-1. {{cite book}}: |editor= has generic name (help)
  14. ^ Raup, David M.; Sepkoski, J. John (1982). "Mass extinctions in the marine fossil record". Science. 215 (4539): 1501–1503. doi:10.1126/science.215.4539.1501. PMID 17788674. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: date and year (link)
  15. ^ Margulis, Lynn (1995). What is Life?. New York: Simon & Schuster. p. 145. ISBN 0-684-81326-2. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  16. ^ Margulis, Lynn (1995). What is Life?. New York: Simon & Schuster. ISBN 0-684-81326-2. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  17. ^ Diamond J (1989). "The present, past and future of human-caused extinctions". Philos Trans R Soc Lond B Biol Sci. 325 (1228): 469–76, discussion 476–7. doi:10.1098/rstb.1989.0100. PMID 2574887.
  18. ^ Novacek M, Cleland E (2001). "The current biodiversity extinction event: scenarios for mitigation and recovery". Proc Natl Acad Sci USA. 98 (10): 5466–5470. doi:10.1073/pnas.091093698. PMC 33235. PMID 11344295.
  19. ^ "The mid-Holocene extinction of silver fir (Abies alba) in the ..." pdf
  20. ^ See, e.g. [1], [2], [3]
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