Composition and structure

 

 

 

Shape

The shape of Earth approximates an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator.[60] This bulge results from the rotation of Earth, and causes the diameter at the equator to be 43 kilometres (27 mi) larger than the pole-to-pole diameter.[61] Thus the point on the surface farthest from Earth's center of mass is the Chimborazo volcano inEcuador.[62] The average diameter of the reference spheroid is about 12,742 kilometres (7,918 mi), which is approximately 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.[63]

Local topography deviates from this idealized spheroid, although on a global scale these deviations are small compared to Earth's radius: The maximum deviation of only 0.17% is at the Mariana Trench (10911 m below local sea level), whereasMount Everest (8,848 m above local sea level) represents a deviation of 0.14%. If Earth were shrunk to the size of a cue ball, some areas of Earth such as mountain ranges and oceanic trenches would feel like small imperfections, whereas much of the planet, including the Great Plains and the abyssal plains, would actually feel smoother than a cue ball.[64] Due to the equatorial bulge, the surface locations farthest from Earth's center are the summits of Mount Chimborazo in Ecuador andHuascarán in Peru.

 

Chemical composition

 

Earth's mass is approximately 5.97×1024 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.[70]

The geochemist F. W. Clarke calculated that a little more than 47% of Earth's crust consists of oxygen. The more common rock constituents of the crust are nearly all oxides; chlorine, sulfur and fluorine are the 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 most common 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), with the other constituents occurring in minute quantities.

 

Internal structure

 

Earth's interior, like that of the other terrestrial planets, is divided into layers by theirchemical or physical (rheological) properties, but unlike the other terrestrial planets, it has a distinct outer and inner core. The outer layer is a chemically distinct silicatesolid 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 (kilometers) 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 composed. 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 kmbelow 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.[72] The inner core may rotate at a slightly higher angular velocitythan the remainder of the planet, advancing by 0.1–0.5° per year.[73] The radius of the inner core is about one fifth of Earth's.

                                                                                                                                                                                     

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Heat

 

Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[76] The major heat-producing isotopes within Earth are potassium-40, uranium-238,uranium-235, and thorium-232.[77] At the center, the temperature may be up to 6,000 °C (10,830 °F),[78] and the pressure could reach 360 GPa.[79] Because much of the heat is provided by radioactive decay, scientists postulate that early in Earth's 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 byr,[76] would have increased temperature gradients with radius, increasing the rates of mantle convection and plate tectonics, and allowing the production of uncommon igneous rocks such as komatiites that are rarely formed today.

 

 

 

 

 

 

 

 

 

 

 

The mean heat loss from Earth is 87 mW m−2, for a global heat loss of 4.42 × 1013 W.[119] 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.[120] More of the heat in 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 under the oceans because the crust there is much thinner than that of the continents.[121]

 

Tectonic plates

The mechanically rigid outer layer of 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.[123] 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,[124] and their motion is strongly coupled with convection patterns inside the 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. Due to this recycling, most of the ocean floor is less than 100 myr old in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about 200 myr.[125][126] By comparison, the oldest dated continental crust is4030 myr.[127]

The seven major plates are the Pacific, North American, Eurasian, African, Antarctic,Indo-Australian, and South American. Other notable plates include 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 mya. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/year[128] and the Pacific Plate moving 52–69 mm/year. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/year.

 

Surface

Earth's terrain varies greatly from place to place. About 70.8%[14] of the surface is covered by water, with much of the continental shelfbelow sea level. This equates to361.132 million km2 (139.43 million sq mi).[130]The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[98] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% (148.94 million km2, or 57.51 million sq mi) not covered by water consists of mountains, deserts, plains, plateaus, and other landforms.

The planetary surface undergoes reshaping over geological time periods due totectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering and erosion from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts[131] also act to reshape the landscape.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Present-day Earth altimetry andbathymetry. Data from the National Geophysical Data Center'sTerrainBase Digital Terrain Model.

The continental crust consists of lower density material such as the igneous rocks granite andandesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.[132] Sedimentary rock is formed from the accumulation of sediment that becomes buried and compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the crust.[133] 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 Earth's surface include quartz, feldspars, amphibole,mica, pyroxene and olivine.[134] Common carbonate minerals include calcite (found in limestone) and dolomite.[135]

The pedosphere is the outermost layer of Earth's continental surface and is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. The total arable land is 13.31% of the land surface, with 4.71% supporting permanent crops.[15] Close to 40% of Earth's land surface is used for cropland and pasture, or an estimated 1.3×107 km2 of cropland and 3.4×107 km2 of pastureland.[136]

The elevation of the land surface 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.[137]

Besides being divided logically into Northern and Southern hemispheres centered on the poles, Earth has been divided arbitrarily into Eastern and Western hemispheres. Earth's surface is traditionally divided into seven continents and various seas. As people settled and organized the planet, nearly all the land was divided into nations.