Main article: Earth science
Further information: Earth physical characteristics tables
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 terrestrial planets in size and mass. Of
these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation,[65] and is probably the only one with active plate tectonics.[66]Shape
Main article: Figure of the Earth
Local topography deviates from this idealized spheroid, although on a global scale, these deviations are 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.[72] 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). Due to 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.[73][74][75]
| 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
See also: Abundance of elements on Earth
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.[77]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), with the other constituents occurring in minute quantities.[78]
Internal structure
Main article: Structure of the Earth
The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological)
properties, but unlike the other terrestrial planets, it has a distinct
outer and inner core. 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 (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 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 km 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.[79] 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.[80]
 Earth cutaway from core to exosphere. Not to scale. |
Depth[82] km |
Component Layer | Density g/cm3 |
|---|---|---|---|
| 0–60 | Lithosphere[note 8] | — | |
| 0–35 | Crust[note 9] | 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%).[83] The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.[84] At the center of the planet, the temperature may be up to 6,000 °C (10,830 °F),[85] and the pressure could reach 360 GPa.[86] 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 byr,[83] 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.[87]| Isotope | Heat release Wkg isotope |
Half-life years |
Mean mantle concentration kg isotopekg mantle |
Heat release Wkg 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 |
Tectonic plates
 |
|
| Plate name | Area 106 km2 |
|---|---|
| 103.3 | |
| 78.0 | |
| 75.9 | |
| 67.8 | |
| 60.9 | |
| 47.2 | |
| 43.6 | |
Main article: Plate tectonics
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.[93]
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,[94] 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. 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.[95][96] By comparison, the oldest dated continental crust is 4,030 myr.[97]
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[98] 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.[99]
Surface
Main articles: Landform and Extreme points of Earth
The Earth's terrain varies greatly from place to place. About 70.8%[13] of the surface is covered by water, with much of the continental shelf below sea level. This equates to 361.132 million km2 (139.43 million sq mi).[100] The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[69] 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 geomorphologies.The planetary surface undergoes reshaping over geological time periods due to 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[101] also act to reshape the landscape.
Present-day Earth altimetry and bathymetry. Data from the National Geophysical Data Center's TerrainBase Digital Terrain Model.
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.[14] 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.[106]
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.[107]
Besides being divided logically into Northern and Southern Hemispheres centered on the earths poles, the earth has been divided arbitrarily into Eastern and Western Hemispheres.
Hydrosphere
Main article: Hydrosphere
Elevation histogram of the surface of the Earth
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 3.618×108 km2 with a mean depth of 3,682 m, resulting in an estimated volume of 1.332×109 km3.[109] If all the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.[note 12] 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.[110]
The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5% salt).[111] Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[112] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[113] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[114] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.[115]
Atmosphere
Main article: Atmosphere of Earth
This is a picture of Earth in ultraviolet light, taken from the surface
of the Moon. The day-side (right) reflects a lot of UV light from the
Sun, but the night-side (left) shows bands of UV emission from the
aurora caused by charged particles.[116]
Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 bya, 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.[118] 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. Water vapor, carbon dioxide, methane and ozone are the primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the average surface would be −18 °C, in contrast to the current +15 °C, and life would likely not exist.[119]
Weather and climate
The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.[121] Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.[122]
Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation.[120] Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.[123]
The amount of solar energy reaching the Earth's decreases with increasing latitude. At higher latitudes the sunlight reaches the surface at lower angles and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4 °C per degree of latitude away from the equator.[124] The Earth can be subdivided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.[125] Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly used Köppen climate classification system (as modified by Wladimir Köppen's student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.[121]
Upper atmosphere
This view from orbit shows the full Moon partially obscured and deformed by the Earth's atmosphere. NASA image
See also: Outer space
Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.[118] Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere, where the Earth's magnetic fields interact with the solar wind.[126]
Within the stratosphere is the ozone layer, a component that partially
shields the surface from ultraviolet light and thus is important for
life on Earth. The Kármán line, defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.[127]Thermal energy causes some of the molecules at the outer edge of the Earth's atmosphere to increase their velocity to the point where they can escape from the planet's gravity. This causes a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate than other gasses.[128] The leakage of hydrogen into space contributes to the pushing of the Earth from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is believed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.[129] Hence the ability of hydrogen to escape from the Earth's atmosphere may have influenced the nature of life that developed on the planet.[130] In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.[131]
Magnetic field
Schematic of Earth's magnetosphere. The solar wind flows from left to right
Main article: Earth's magnetic field
The Earth's magnetic field is shaped roughly as a magnetic dipole,
with the poles currently located proximate to the planet's geographic
poles. At the equator of the magnetic field, the magnetic field strength
at the planet's surface is 3.05 × 10−5 T, with global magnetic dipole moment of 7.91 × 1015 T m3.[132] According to dynamo theory,
the field is generated within the molten outer core region where heat
creates convection motions of conducting materials, generating electric
currents. These in turn produce the Earth's magnetic field. The
convection movements in the core are chaotic; the magnetic poles drift
and periodically change alignment. This causes field reversals
at irregular intervals averaging a few times every million years. The
most recent reversal occurred approximately 700,000 years ago.[133][134]The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora.[135]
Aucun commentaire:
Enregistrer un commentaire