Earth
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Template:About
Template:Pp-semi-indef
Template:Infobox Planet
Earth (pronounced Template:Audio-IPA)<ref>Earth (PLANET), entry in the Cambridge Advanced Learner's Dictionary, Cambridge University Press, online. Accessed 14-II-2008.</ref> is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the Earth, Planet Earth, the World, and Terra.<ref>Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather than for the planet Earth. C.f.:
Template:Cite web</ref>
Home to millions of species,<ref>{{#if:May
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}}{{#if:Science
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}}</ref> including humans, Earth is the only place in the universe where life is known to exist. Scientific evidence indicates that the planet formed 4.54 billion years ago,<ref name="age_earth">Template:Cite book</ref><ref>Template:Cite web</ref><ref>{{#if:Dalrymple
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|{{#ifeq:The age of the Earth in the twentieth century: a problem (mostly) solved||Citation is missing a title.
Either specify one, or click here and a bot will try to complete the citation details for you. }}[{{#if:http://sp.lyellcollection.org/cgi/content/abstract/190/1/205|http://sp.lyellcollection.org/cgi/content/abstract/190/1/205|http://www.pubmedcentral.gov/articlerender.fcgi?tool=pmcentrez&artid={{{pmc}}}}} The age of the Earth in the twentieth century: a problem (mostly) solved]
|The age of the Earth in the twentieth century: a problem (mostly) solved
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}}</ref><ref>Template:Cite web</ref> and life appeared on its surface within a billion years. Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful radiation, permitting life on land.<ref>Template:Cite book</ref>
Earth's outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt-water oceans, the remainder consisting of continents and islands; liquid water, necessary for all known life, is not known to exist on any other planet's surface.<ref>Other planets in the solar system are either too hot or too cold to support liquid water. However, it is confirmed to have existed on the surface of Mars in the past, and may still appear today. See: Template:Cite newsTemplate:Cite news</ref><ref>As of 2007, water vapor has been detected in the atmosphere of only one extrasolar planet, and it is a gas giant. See: {{#if:G. Tinetti et al.
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|Water vapour in the atmosphere of a transiting extrasolar planet
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}}</ref> Earth's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.
Earth interacts with other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days.<ref>The number of solar days is one less than the number of sidereal days because the orbital motion of the Earth about the Sun results in one additional revolution of the planet about its axis.</ref> The Earth's axis of rotation is tilted 23.4° away from the perpendicular to its orbital plane,<ref>Ahrens, Global Earth Physics: A Handbook of Physical Constants, p. 8.</ref> producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days). Earth's only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation. A cometary bombardment during the early history of the planet played a role in the formation of the oceans.<ref name="comet">{{#if:Morbidelli, A.; Chambers, J.; Lunine, J. I.; Petit, J. M.; Robert, F.; Valsecchi, G. B.; Cyr, K. E.
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Either specify one, or click here and a bot will try to complete the citation details for you. }}[{{#if:http://adsabs.harvard.edu/abs/2000M&PS...35.1309M|http://adsabs.harvard.edu/abs/2000M&PS...35.1309M|http://www.pubmedcentral.gov/articlerender.fcgi?tool=pmcentrez&artid={{{pmc}}}}} Source regions and time scales for the delivery of water to Earth]
|Source regions and time scales for the delivery of water to Earth
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}}{{#if:Meteoritics & Planetary Science
|. Meteoritics & Planetary Science
}}{{#if:35
| 35
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Contents |
History
Template:Seealso Scientists have been able to reconstruct detailed information about the planet's past. Earth and the other planets in the Solar System formed 4.54 billion years ago<ref name="age_earth" /> out of the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun. Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed soon afterwards, possibly as the result of a Mars-sized object (sometimes called Theia) with about 10% of the Earth's mass<ref>Template:Cite conference</ref> impacting the Earth in a glancing blow.<ref>{{#if:R. Canup and E. Asphaug
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|Origin of the Moon in a giant impact near the end of the Earth's formation
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}}</ref> Some of this object's mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to form the Moon.
Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects produced the oceans.<ref name="comet"/> The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later, the last common ancestor of all life existed.<ref>{{#if:Doolittle
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The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and resulted in a layer of ozone (a form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.<ref>{{#if:Berkner, L. V.; Marshall, L. C.
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|On the Origin and Rise of Oxygen Concentration in the Earth's Atmosphere
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|{{#if:1965
|{{#if:
| ({{{month}}} 1965)
| (1965)
}}
}}
}}
}}{{#if:Journal of Atmospheric Sciences
|. Journal of Atmospheric Sciences
}}{{#if:22
| 22
}}{{#if:3
| (3)
}}{{#if:225–261
|: 225–261
}}{{#if:
|. {{#if:
|{{{location}}}:
}}{{{publisher}}}
}}{{#if:10.1175/1520-0469(1965)022<0225:OTOARO>2.0.CO;2
|. doi:{{#if: | {{#tag:nowiki|10.1175/1520-0469(1965)022<0225:OTOARO>2.0.CO;2}} (inactive [[]]) {{#ifeq: | | [[Category:Pages with DOIs broken since {{#time: Y | }}]] }} | }}
}}{{#if:
|. ISSN {{{issn}}}
}}{{#if:
|. PMID {{{pmid}}}
}}{{#if:
| {{#if:http://adsabs.harvard.edu/abs/1965JAtS...22..225B |. PMC:{{{pmc}}}}}
}}{{#if:
|. Bibcode: {{{bibcode}}}
}}{{#if:
|. OCLC {{{oclc}}}
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|. {{{id}}}
}}{{#if:2007-03-05
|. Retrieved on 2007-03-05{{#if: | , [[{{{accessyear}}}]] }}
}}{{#if:
| Retrieved on {{{accessmonthday}}}, {{{accessyear}}}
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| Retrieved on {{{accessdaymonth}}} {{{accessyear}}}
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|. [{{{laysummary}}} Lay summary]{{#if: | – {{{laysource}}}}}
}}{{#if:
| ([[{{{laydate}}}]])
}}.{{#if:
| “{{{quote}}}”
}}</ref> True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.<ref>Template:Cite web</ref>
Beginning with almost no dry land, the total amount of surface lying above the oceans has steadily increased. During the past two billion years, for example, the total size of the continents has doubled.<ref name=ward_brownlee>Ward and Brownlee (2002)</ref> As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago (mya), the earliest known supercontinent, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which broke apart 180 mya.<ref>{{#if:Murphy, J. B.; Nance, R. D.
|{{#if:
|[[{{{authorlink}}}|{{#if:
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|, {{{first}}}
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|Murphy, J. B.; Nance, R. D.
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|{{#if:
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|, {{{first}}}
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|Murphy, J. B.; Nance, R. D.
}}
}}
}}{{#if:Murphy, J. B.; Nance, R. D.
|{{#if:
| ; {{{coauthors}}}
}}{{#if:
| ({{{date}}})
|{{#if:1965
|{{#if:
| ({{{month}}} 1965)
| (1965)
}}
}}
}}
}}{{#if:Murphy, J. B.; Nance, R. D.
| .
}}{{#if:Murphy, J. B.; Nance, R. D.1965
|
}}{{#ifeq:
| no
|
| {{#if: |“|"}}
}}{{#if:http://scienceweek.com/2004/sa040730-5.htm
|{{#ifeq:How do supercontinents assemble?||Citation is missing a title.
Either specify one, or click here and a bot will try to complete the citation details for you. }}[{{#if:http://scienceweek.com/2004/sa040730-5.htm|http://scienceweek.com/2004/sa040730-5.htm|http://www.pubmedcentral.gov/articlerender.fcgi?tool=pmcentrez&artid={{{pmc}}}}} How do supercontinents assemble?]
|How do supercontinents assemble?
}}{{#ifeq:
| no
|
| {{#if:|”|"}}
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}}{{#if:
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|
| {{#if:
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|{{#if:1965
|{{#if:
| ({{{month}}} 1965)
| (1965)
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}}
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}}{{#if:American Scientist
|. American Scientist
}}{{#if:92
| 92
}}{{#if:
| ({{{issue}}})
}}{{#if:324–33
|: 324–33
}}{{#if:
|. {{#if:
|{{{location}}}:
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}}{{#if:10.1511/2004.4.324
|. doi:{{#if: | {{#tag:nowiki|10.1511/2004.4.324}} (inactive [[]]) {{#ifeq: | | [[Category:Pages with DOIs broken since {{#time: Y | }}]] }} | }}
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|. {{{id}}}
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Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.<ref>Template:Cite book</ref>
Following the Cambrian explosion, about 535 mya, there have been five mass extinctions.<ref>{{#if:Raup, D. M.; Sepkoski, J. J.
|{{#if:
|[[{{{authorlink}}}|{{#if:
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}}{{#if:Raup, D. M.; Sepkoski, J. J.
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| ; {{{coauthors}}}
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|{{#if:1982
|{{#if:
| ({{{month}}} 1982)
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}}
}}
}}{{#if:Raup, D. M.; Sepkoski, J. J.
| .
}}{{#if:Raup, D. M.; Sepkoski, J. J.1982
|
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| no
|
| {{#if: |“|"}}
}}{{#if:http://adsabs.harvard.edu/abs/1982Sci...215.1501R
|{{#ifeq:Mass Extinctions in the Marine Fossil Record||Citation is missing a title.
Either specify one, or click here and a bot will try to complete the citation details for you. }}[{{#if:http://adsabs.harvard.edu/abs/1982Sci...215.1501R|http://adsabs.harvard.edu/abs/1982Sci...215.1501R|http://www.pubmedcentral.gov/articlerender.fcgi?tool=pmcentrez&artid={{{pmc}}}}} Mass Extinctions in the Marine Fossil Record]
|Mass Extinctions in the Marine Fossil Record
}}{{#ifeq:
| no
|
| {{#if:|”|"}}
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| ({{{format}}})
}}{{#if:Raup, D. M.; Sepkoski, J. J.
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| {{#if:
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|{{#if:1982
|{{#if:
| ({{{month}}} 1982)
| (1982)
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}}
}}
}}{{#if:Science
|. Science
}}{{#if:215
| 215
}}{{#if:4539
| (4539)
}}{{#if:1501–1503
|: 1501–1503
}}{{#if:
|. {{#if:
|{{{location}}}:
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}}{{#if:10.1126/science.215.4539.1501
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}}{{#if:
|. ISSN {{{issn}}}
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|. PMID {{{pmid}}}
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| {{#if:http://adsabs.harvard.edu/abs/1982Sci...215.1501R |. PMC:{{{pmc}}}}}
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|. Bibcode: {{{bibcode}}}
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|. OCLC {{{oclc}}}
}}{{#if:
|. {{{id}}}
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|. Retrieved on 2007-03-05{{#if: | , [[{{{accessyear}}}]] }}
}}{{#if:
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|. [{{{laysummary}}} Lay summary]{{#if: | – {{{laysource}}}}}
}}{{#if:
| ([[{{{laydate}}}]])
}}.{{#if:
| “{{{quote}}}”
}}</ref> The last extinction event occurred 65 mya, 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 has diversified, and several mya, an African ape-like animal gained the ability to stand upright.<ref>{{#if:Gould
|{{#if:
|[[{{{authorlink}}}|{{#if:
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|Gould{{#if:
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|Gould{{#if:
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|, Stephan J.
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}}{{#if:Gould
|{{#if:
| ; {{{coauthors}}}
}}{{#if:
| ({{{date}}})
|{{#if:1994
|{{#if:October
| (October 1994)
| (1994)
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}}{{#if:Gould
| .
}}{{#if:Gould1994
|
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| no
|
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|The Evolution of Life on Earth
}}{{#ifeq:
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}}{{#if:Scientific American
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}}</ref> This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,<ref>{{#if:Wilkinson, B. H.; McElroy, B. J.
|{{#if:
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}}{{#if:Wilkinson, B. H.; McElroy, B. J.
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|{{#if:2007
|{{#if:
| ({{{month}}} 2007)
| (2007)
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}}{{#if:Wilkinson, B. H.; McElroy, B. J.
| .
}}{{#if:Wilkinson, B. H.; McElroy, B. J.2007
|
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| no
|
| {{#if: |“|"}}
}}{{#if:http://bulletin.geoscienceworld.org/cgi/content/abstract/119/1-2/140
|{{#ifeq:The impact of humans on continental erosion and sedimentation||Citation is missing a title.
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|The impact of humans on continental erosion and sedimentation
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}}{{#if:Bulletin of the Geological Society of America
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}}{{#if:119
| 119
}}{{#if:1–2
| (1–2)
}}{{#if:140–156
|: 140–156
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|. {{#if:
|{{{location}}}:
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}}{{#if:10.1130/B25899.1
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|. OCLC {{{oclc}}}
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|. Retrieved on 2007-04-22{{#if: | , [[{{{accessyear}}}]] }}
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| ([[{{{laydate}}}]])
}}.{{#if:
| “{{{quote}}}”
}}</ref> affecting both the nature and quantity of other life forms.
The present pattern of ice ages began about 40 mya, then intensified during the Pleistocene about 3 mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last ice age ended 10,000 years ago.<ref>Template:Cite web</ref>
Composition and structure
Template:See Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar terrestrial planets, both in terms of size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation.<ref>Template:Cite web</ref> It also is the only terrestrial planet with active plate tectonics.Template:Fact
Shape
The Earth's shape is very close to an oblate spheroid—a rounded shape with a bulge around the equator—although the precise shape (the geoid) varies from this by up to 100 meters.<ref>Template:Cite web</ref> The average diameter of the reference spheroid is about 12,742 km. More approximately the distance is 40,000 km/π because the meter was originally defined as 1/10,000,000 of the distance from the equator to the north pole through Paris, France.<ref>Template:Cite web</ref>
The rotation of the Earth creates the equatorial bulge so that the equatorial diameter is 43 km larger than the pole to pole diameter.<ref name="ngdc2006">Template:Cite web</ref> The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%, which is less than the 0.22% tolerance allowed in billiard balls.<ref>Template:Cite web</ref> Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador.<ref>{{#if:Senne
|{{#if:
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Senne
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}}
}}
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|{{#if:
| ; {{{coauthors}}}
}}{{#if:
| ({{{date}}})
|{{#if:2000
|{{#if:
| ({{{month}}} 2000)
| (2000)
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}}
}}
}}{{#if:Senne
| .
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|
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| no
|
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}}{{#if:http://www.profsurv.com/archive.php?issue=42&article=589
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Either specify one, or click here and a bot will try to complete the citation details for you. }}[{{#if:http://www.profsurv.com/archive.php?issue=42&article=589|http://www.profsurv.com/archive.php?issue=42&article=589|http://www.pubmedcentral.gov/articlerender.fcgi?tool=pmcentrez&artid={{{pmc}}}}} Did Edmund Hillary Climb the Wrong Mountain]
|Did Edmund Hillary Climb the Wrong Mountain
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}}{{#if:Professional Surveyor
|. Professional Surveyor
}}{{#if:20
| 20
}}{{#if:5
| (5)
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}}.{{#if:
| “{{{quote}}}”
}}</ref>
| Compound | Formula | Composition |
|---|---|---|
| silica | SiO2 | 59.71% |
| alumina | Al2O3 | 15.41% |
| lime | CaO | 4.90% |
| Magnesia | MgO | 4.36% |
| sodium oxide | Na2O | 3.55% |
| iron(II) oxide | FeO | 3.52% |
| potassium oxide | K2O | 2.80% |
| iron(III) oxide | Fe2O3 | 2.63% |
| water | H2O | 1.52% |
| titanium dioxide | TiO2 | 0.60% |
| phosphorus pentoxide | P2O5 | 0.22% |
| Total | 99.22% | |
Chemical composition
Template:Seealso The mass of the Earth is approximately 5.98Template:E kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.<ref>{{#if:Morgan, J. W.; Anders, E.
|{{#if:
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| (1980)
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}}{{#if:Morgan, J. W.; Anders, E.
| .
}}{{#if:Morgan, J. W.; Anders, E.1980
|
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|Chemical composition of Earth, Venus, and Mercury
}}{{#ifeq:
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|{{#if:1980
|{{#if:
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| (1980)
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}}
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}}{{#if:Proceedings of the National Academy of Science
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}}{{#if:71
| 71
}}{{#if:12
| (12)
}}{{#if:6973–6977
|: 6973–6977
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}}{{#if:16592930
|. PMID 16592930
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|. OCLC {{{oclc}}}
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|. {{{id}}}
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The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right.) All the other constituents occur only in very small quantities.<ref name=EB1911>Template:1911</ref>
Internal structure
The interior of the Earth, like that of the other terrestrial planets, is chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 km on the continents.<ref>Template:Cite book</ref>
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