The shape of the Earth is very close to that of an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator.[58] This bulge results from the rotation of the Earth, and causes the diameter at the equator to be 43 km larger than the pole to pole diameter.[59] The average diameter of the reference spheroid is about 12,742 km, which is approximately 40,000 km/π, as the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.[60]
Local topography deviates from this idealized spheroid, though on a global scale, these deviations are very small: Earth has a tolerance of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in billiard balls.[61] The largest local deviations in the rocky surface of the Earth are Mount Everest (8848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Because of the equatorial bulge, the surface locations farthest from the center of the Earth are the summits of Mount Chimborazo in Ecuador and Huascarán in Peru.[62][63][64]
| Compound | Formula | Composition | |
|---|---|---|---|
| Continental | Oceanic | ||
| silica | SiO2 | 60.2% | 48.6% |
| alumina | Al2O3 | 15.2% | 16.5% |
| lime | CaO | 5.5% | 12.3% |
| magnesia | MgO | 3.1% | 6.8% |
| iron(II) oxide | FeO | 3.8% | 6.2% |
| sodium oxide | Na2O | 3.0% | 2.6% |
| potassium oxide | K2O | 2.8% | 0.4% |
| iron(III) oxide | Fe2O3 | 2.5% | 2.3% |
| water | H2O | 1.4% | 1.1% |
| carbon dioxide | CO2 | 1.2% | 1.4% |
| titanium dioxide | TiO2 | 0.7% | 1.4% |
| phosphorus pentoxide | P2O5 | 0.2% | 0.3% |
| Total | 99.6% | 99.9% | |
Chemical composition
The mass of the Earth is approximately 5.98 × 1024 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.[66]
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.[67]
Internal structure
The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties. The outer layer of the Earth is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. 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. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are comprised. Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 kilometers below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core.[68] The inner core may rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.[69]
 Earth cutaway from core to exosphere. Not to scale. | Depth[71] km | Component Layer | Density g/cm3 |
|---|---|---|---|
| 0–60 | Lithosphere[note 11] | — | |
| 0–35 | Crust[note 12] | 2.2–2.9 | |
| 35–60 | Upper mantle | 3.4–4.4 | |
| 35–2890 | Mantle | 3.4–5.6 | |
| 100–700 | Asthenosphere | — | |
| 2890–5100 | Outer core | 9.9–12.2 | |
| 5100–6378 | Inner core | 12.8–13.1 |
Heat
Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[72] The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.[73] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.[74] Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. This extra heat production, twice present-day at approximately 3 billion years ago,[72] would have increased temperature gradients within the Earth, increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatiites that are not formed today.[75]
| Isotope | Heat release W/kg isotope | Half-life years | Mean mantle concentration kg isotope/kg mantle | Heat release W/kg mantle |
|---|---|---|---|---|
| 238U | 9.46 × 10−5 | 4.47 × 109 | 30.8 × 10−9 | 2.91 × 10−12 |
| 235U | 5.69 × 10−4 | 7.04 × 108 | 0.22 × 10−9 | 1.25 × 10−13 |
| 232Th | 2.64 × 10−5 | 1.40 × 1010 | 124 × 10−9 | 3.27 × 10−12 |
| 40K | 2.92 × 10−5 | 1.25 × 109 | 36.9 × 10−9 | 1.08 × 10−12 |
Total heat loss from the Earth is 4.2 × 1013 watts.[77] A portion of the core's thermal energy is transported toward the crust by mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.[78] More of the heat in the Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs in the oceans because the crust there is much thinner than that of the continents.[77]
Tectonic plates
 | |
| Plate name | Area 106 km2 |
|---|---|
| African Plate[note 13] | 78.0 |
| Antarctic Plate | 60.9 |
| Indo-Australian Plate | 47.2 |
| Eurasian Plate | 67.8 |
| North American Plate | 75.9 |
| South American Plate | 43.6 |
| Pacific Plate | 103.3 |
The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: Convergent boundaries, at which two plates come together, Divergent boundaries, at which two plates are pulled apart, and Transform boundaries, in which two plates slide past one another laterally. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.[80] The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates,[81] and their motion is strongly coupled with convection patterns inside the Earth's mantle.
As the tectonic plates migrate across the planet, the ocean floor is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes continually recycles the oceanic crust back into the mantle. Because of this recycling, most of the ocean floor is less than 100 million years in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about 200 million years.[82][83] By comparison, the oldest dated continental crust is 4030 million years old.[84]
Other notable plates include the Indian Plate, the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between 50 and 55 million years ago. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/yr[85] and the Pacific Plate moving 52–69 mm/yr. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/yr.[86]
Surface
The Earth's terrain varies greatly from place to place. About 70.8%[87] of the surface is covered by water, with much of the continental shelf below sea level. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[59] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.
The planetary surface undergoes reshaping over geological time periods because of tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts[88] also act to reshape the landscape.
The continental crust consists of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.[89] Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust.[90] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine.[91] Common carbonate minerals include calcite (found in limestone) and dolomite.[92]
The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.[10] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3 × 107 km2 of cropland and 3.4 × 107 km2 of pastureland.[93]
The elevation of the land surface of the Earth varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 840 m.[94]
Hydrosphere
The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the Solar System. The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911.4 m.[note 14][95]
The mass of the oceans is approximately 1.35 × 1018 metric tons, or about 1/4400 of the total mass of the Earth. The oceans cover an area of 361.8 × 106 km2 with a mean depth of 3,682 m, resulting in an estimated volume of 1.332 × 109 km3.[96] If all the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.[note 15] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is currently ice.[97]
The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (35 ‰).[98] Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[99] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[100] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[101] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.[102]
Atmosphere
The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.[3] It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation resulting from weather and seasonal factors.[103]
Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 billion years ago, forming the primarily nitrogen-oxygen atmosphere of today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[104] This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Carbon dioxide, water vapor, methane and ozone are the primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C and life would likely not exist
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