Earth
Earth
Earth is the third planet from the Sun and is the largest of the terrestrial planets, in terms of both diameter and mass. Earth is also referred to as "the Earth", "Planet Earth", "Gaia", "Terra", or "the World".
The Earth is the first planet known to have liquid water on the surface and is the only place in the universe known to harbor life. Earth has a magnetic field that, together with a primarily nitrogen-oxygen atmosphere, protects the surface from radiation that is harmful to life. The atmosphere also serves as a shield that causes smaller meteors to burn up before they strike the surface.
The Earth formed around 4.57 billion years[1] ago and its only known natural satellite, the Moon, began orbiting it around 4.53 billion years ago. At present the Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis (which is equal to 365.26 solar days), a period known as the sidereal year. The Earth's axis of rotation is tilted 23.4° relative to the Sun, producing seasonal variations on the planet's surface.
Atmospheric conditions on Earth have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 71% of the surface is covered in salt-water oceans, and the remainder consists of continents and islands. The outer surface is divided into several tectonic plates that gradually migrate across the surface over geologic time spans. The interior of the planet remains active, with a thick layer of convecting yet solid mantle, a liquid outer core that generates a magnetic field, and a solid-iron inner core.
The space environment interacts with the Earth to a significant degree. The relatively large moon provides ocean tides, stabilizes the axial tilt and has gradually modified the length of the planet's rotation period. A cometary bombardment during the early history of the planet played a role in the formation of the oceans. Later, asteroid impacts caused significant changes to the surface environment. Long term periodic changes in the orbit of the planet are believed to have caused the ice ages that have covered significant portions of the surface in glacial sheets.
History
Current scientists have been able to reconstruct detailed information about the planet's past. Earth formed 4.57 billion years ago[1] out of the solar nebula, along with the Sun and the other planets. Initially molten, the outer layer of the planet 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 with about 10% of the Earth's mass,[2] known as Theia, impacting the Earth in a glancing blow.[3] Some of this object's mass merged with the Earth and a portion was ejected into space, but enough material survived to form an orbiting moon.
Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered by comets, produced the oceans.[4] 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.[5]
The development of photosynthesis allowed the sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and gave rise to the ozone layer. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[6] 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.[7]
Over hundreds of millions of years, continents formed and broke up as the surface of Earth continually reshaped itself. The continents have migrated across the surface of the Earth, 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.[8]
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.[9]
Following the Cambrian explosion, about 535 mya, there have been five mass extinctions.[10] 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 mya, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright.[11] 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, 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.
Composition and structure
Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant such as Jupiter. It is the largest of the four solar terrestrial planets, both in terms of size and total mass. Of these four planets, Earth also has the highest density, the highest surface gravity and the strongest magnetic field.
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 metres (327 ft).[14] The average diameter of the reference spheroid is about 12,742 km (7,913 mi). More approximately the distance is 40,000 km/π because the metre was originally defined as 1/10,000,000 of the distance from the equator to the north pole through Paris, France.
The rotation of the Earth creates the equatorial bulge so that the equatorial diameter is 43 km (27 mi) larger than the pole to pole diameter. The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m [29,028 ft] above local sea level) and the Mariana Trench (10,911 m [35,798 ft] 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.[15] Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador.
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 aluminum (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.[17]
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.
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.[19]
The geologic component layers of the Earth[20] are at the following depths below the surface:[21]
Depth Layer Density
g/cm3
Kilometres Miles
0–60 0–37 Lithosphere (locally varies between 5 and 200 km) —
0–35 0–22 ... Crust (locally varies between 5 and 70 km) 2.2–2.9
35–60 22–37 ... Uppermost part of mantle 3.4–4.4
35–2890 22–1790 Mantle 3.4–5.6
100–700 62–435 ... Asthenosphere —
2890–5100 1790–3160 Outer core 9.9–12.2
5100–6378 3160–3954 Inner core 12.8–13.1
The internal heat of the planet is most likely produced by the radioactive decay of potassium-40, uranium-238 and thorium-232 isotopes. All three have half-life decay periods of more than a billion years.[22] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.[23] 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.
Surface
The Earth's terrain varies greatly from place to place. About 70.8%[29] 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 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 due to the effects 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[30] also act to reshape the landscape.
As the continental plates migrate across the planet, the ocean floor is subducted under the leading edges. At the same time, upwellings of mantle material create a divergent boundary along mid-ocean ridges. The combination of these processes continually recycles the ocean plate material. Most of the ocean floor is less than 100 million years in age. The oldest ocean plate is located in the western Pacific, and has an estimated age of about 200 million years. By comparison, the oldest fossils found on land have an age of about 3 billion years.[31][32]
The continental plates consist 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.[33] 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.[34] 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.[35] Common carbonate minerals include calcite (found in limestone) and dolomite.
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.[36] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 3.3 × 109 acres of cropland and 8.4 × 109 acres of pastureland.[37]
Elevation histogram of the surface of the Earth—approximately 71% of the Earth's surface is covered with water.
Elevation histogram of the surface of the Earth—approximately 71% of the Earth's surface is covered with water.
The elevation of the land surface of the Earth varies from the low point of −418 m (−1,371 ft) at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m (29,028 ft) at the top of Mount Everest. The mean height of land above sea level is 686 m (426 ft).
Hydrosphere
The abundance of water on Earth 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 m (35,798 ft or 6.78 mi).[39] The average depth of the oceans is 3,794 m (12,447 ft), more than five times the average height of the continents.[38]
The mass of the oceans is approximately 1.35 × 1018 tonnes, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386 × 109 km³. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km (approximately 1.7 mi).[40] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.[41]
About 3.5% of the total mass of the oceans consists of salt. Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[42] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[43] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[44] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.
Atmosphere
The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 6 km. It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The atmosphere protects the Earth's life forms by absorbing ultraviolet solar radiation, moderating temperature, transporting water vapor, and providing useful gases.[45]
In a phenomenon known as the greenhouse effect, trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the net 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.
Weather and climate
The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km (about 4 mi) of the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that drives the weather and climate through redistribution of heat energy.[46]
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°.[47] However, ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes heat energy from the equatorial oceans to the polar regions.
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.[46] Most of the water is then transported back to lower elevations by river systems, usually returning to the oceans or being 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 metres of water per year to less than a millimetre. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.[48]
The Earth can be sub-divided 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.[49] 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.
Earth is the third planet from the Sun and is the largest of the terrestrial planets, in terms of both diameter and mass. Earth is also referred to as "the Earth", "Planet Earth", "Gaia", "Terra", or "the World".
The Earth is the first planet known to have liquid water on the surface and is the only place in the universe known to harbor life. Earth has a magnetic field that, together with a primarily nitrogen-oxygen atmosphere, protects the surface from radiation that is harmful to life. The atmosphere also serves as a shield that causes smaller meteors to burn up before they strike the surface.
The Earth formed around 4.57 billion years[1] ago and its only known natural satellite, the Moon, began orbiting it around 4.53 billion years ago. At present the Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis (which is equal to 365.26 solar days), a period known as the sidereal year. The Earth's axis of rotation is tilted 23.4° relative to the Sun, producing seasonal variations on the planet's surface.
Atmospheric conditions on Earth have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 71% of the surface is covered in salt-water oceans, and the remainder consists of continents and islands. The outer surface is divided into several tectonic plates that gradually migrate across the surface over geologic time spans. The interior of the planet remains active, with a thick layer of convecting yet solid mantle, a liquid outer core that generates a magnetic field, and a solid-iron inner core.
The space environment interacts with the Earth to a significant degree. The relatively large moon provides ocean tides, stabilizes the axial tilt and has gradually modified the length of the planet's rotation period. A cometary bombardment during the early history of the planet played a role in the formation of the oceans. Later, asteroid impacts caused significant changes to the surface environment. Long term periodic changes in the orbit of the planet are believed to have caused the ice ages that have covered significant portions of the surface in glacial sheets.
History
Current scientists have been able to reconstruct detailed information about the planet's past. Earth formed 4.57 billion years ago[1] out of the solar nebula, along with the Sun and the other planets. Initially molten, the outer layer of the planet 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 with about 10% of the Earth's mass,[2] known as Theia, impacting the Earth in a glancing blow.[3] Some of this object's mass merged with the Earth and a portion was ejected into space, but enough material survived to form an orbiting moon.
Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered by comets, produced the oceans.[4] 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.[5]
The development of photosynthesis allowed the sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and gave rise to the ozone layer. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[6] 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.[7]
Over hundreds of millions of years, continents formed and broke up as the surface of Earth continually reshaped itself. The continents have migrated across the surface of the Earth, 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.[8]
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.[9]
Following the Cambrian explosion, about 535 mya, there have been five mass extinctions.[10] 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 mya, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright.[11] 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, 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.
Composition and structure
Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant such as Jupiter. It is the largest of the four solar terrestrial planets, both in terms of size and total mass. Of these four planets, Earth also has the highest density, the highest surface gravity and the strongest magnetic field.
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 metres (327 ft).[14] The average diameter of the reference spheroid is about 12,742 km (7,913 mi). More approximately the distance is 40,000 km/π because the metre was originally defined as 1/10,000,000 of the distance from the equator to the north pole through Paris, France.
The rotation of the Earth creates the equatorial bulge so that the equatorial diameter is 43 km (27 mi) larger than the pole to pole diameter. The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m [29,028 ft] above local sea level) and the Mariana Trench (10,911 m [35,798 ft] 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.[15] Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador.
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 aluminum (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.[17]
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.
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.[19]
The geologic component layers of the Earth[20] are at the following depths below the surface:[21]
Depth Layer Density
g/cm3
Kilometres Miles
0–60 0–37 Lithosphere (locally varies between 5 and 200 km) —
0–35 0–22 ... Crust (locally varies between 5 and 70 km) 2.2–2.9
35–60 22–37 ... Uppermost part of mantle 3.4–4.4
35–2890 22–1790 Mantle 3.4–5.6
100–700 62–435 ... Asthenosphere —
2890–5100 1790–3160 Outer core 9.9–12.2
5100–6378 3160–3954 Inner core 12.8–13.1
The internal heat of the planet is most likely produced by the radioactive decay of potassium-40, uranium-238 and thorium-232 isotopes. All three have half-life decay periods of more than a billion years.[22] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.[23] 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.
Surface
The Earth's terrain varies greatly from place to place. About 70.8%[29] 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 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 due to the effects 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[30] also act to reshape the landscape.
As the continental plates migrate across the planet, the ocean floor is subducted under the leading edges. At the same time, upwellings of mantle material create a divergent boundary along mid-ocean ridges. The combination of these processes continually recycles the ocean plate material. Most of the ocean floor is less than 100 million years in age. The oldest ocean plate is located in the western Pacific, and has an estimated age of about 200 million years. By comparison, the oldest fossils found on land have an age of about 3 billion years.[31][32]
The continental plates consist 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.[33] 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.[34] 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.[35] Common carbonate minerals include calcite (found in limestone) and dolomite.
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.[36] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 3.3 × 109 acres of cropland and 8.4 × 109 acres of pastureland.[37]
Elevation histogram of the surface of the Earth—approximately 71% of the Earth's surface is covered with water.
Elevation histogram of the surface of the Earth—approximately 71% of the Earth's surface is covered with water.
The elevation of the land surface of the Earth varies from the low point of −418 m (−1,371 ft) at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m (29,028 ft) at the top of Mount Everest. The mean height of land above sea level is 686 m (426 ft).
Hydrosphere
The abundance of water on Earth 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 m (35,798 ft or 6.78 mi).[39] The average depth of the oceans is 3,794 m (12,447 ft), more than five times the average height of the continents.[38]
The mass of the oceans is approximately 1.35 × 1018 tonnes, or about 1/4400 of the total mass of the Earth, and occupies a volume of 1.386 × 109 km³. If all of the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km (approximately 1.7 mi).[40] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. The majority of the fresh water, about 68.7%, is currently in the form of ice.[41]
About 3.5% of the total mass of the oceans consists of salt. Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[42] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[43] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[44] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.
Atmosphere
The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 6 km. It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The atmosphere protects the Earth's life forms by absorbing ultraviolet solar radiation, moderating temperature, transporting water vapor, and providing useful gases.[45]
In a phenomenon known as the greenhouse effect, trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the net 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.
Weather and climate
The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km (about 4 mi) of the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that drives the weather and climate through redistribution of heat energy.[46]
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°.[47] However, ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes heat energy from the equatorial oceans to the polar regions.
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.[46] Most of the water is then transported back to lower elevations by river systems, usually returning to the oceans or being 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 metres of water per year to less than a millimetre. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.[48]
The Earth can be sub-divided 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.[49] 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.
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