Structure of the Earth
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The internal structure of Earth are the layers of the
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
, excluding its
atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
and
hydrosphere The hydrosphere () is the combined mass of water found on, under, and above the Planetary surface, surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to ch ...
. The
structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...
consists of an outer
silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...
solid crust, a highly viscous
asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between c. below the surface, and extends as deep as . However, the lower boundary of the asthenosphere i ...
, and solid mantle, a liquid
outer core Earth's outer core is a fluid layer about thick, composed of mostly iron and nickel that lies above Earth's solid Earth's inner core, inner core and below its Earth's mantle, mantle. The outer core begins approximately beneath Earth's surface ...
whose flow generates the
Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...
, and a solid
inner core Earth's inner core is the innermost internal structure of Earth, geologic layer of the planet Earth. It is primarily a solid ball (mathematics), ball with a radius of about , which is about 20% of Earth's radius or 70% of the Moon's radius. T ...
. Scientific understanding of the internal structure of
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
is based on observations of
topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...
and bathymetry, observations of rock in
outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth and other terrestrial planets. Features Outcrops do not cover the majority of the Earth's land surface because in most p ...
, samples brought to the surface from greater depths by
volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...
es or volcanic activity, analysis of the
seismic wave A seismic wave is a mechanical wave of acoustic energy that travels through the Earth or another planetary body. It can result from an earthquake (or generally, a quake), volcanic eruption, magma movement, a large landslide and a large ma ...
s that pass through Earth, measurements of the gravitational and magnetic fields of Earth, and experiments with crystalline solids at pressures and temperatures characteristic of Earth's deep interior.


Global properties

Note: In chondrite model (1), the light element in the core is assumed to be Si. Chondrite model (2) is a model of chemical composition of the mantle corresponding to the model of core shown in chondrite model (1).Measurements of the force exerted by
Earth's gravity The gravity of Earth, denoted by , is the net acceleration that is imparted to objects due to the combined effect of gravitation (from mass distribution within Earth) and the centrifugal force (from the Earth's rotation). It is a vector qu ...
can be used to calculate its
mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
. Astronomers can also calculate Earth's mass by observing the motion of orbiting
satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...
s. Earth's average
density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...
can be determined through gravimetric experiments, which have historically involved
pendulum A pendulum is a device made of a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate i ...
s. The mass of Earth is about .''M''E = 5·9722×1024 kg ± 6×1020 kg.
2016 Selected Astronomical Constants
" in
The average density of Earth is .


Layers

The structure of Earth can be defined in two ways: by mechanical properties such as
rheology Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applie ...
, or chemically. Mechanically, it can be divided into
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...
,
asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between c. below the surface, and extends as deep as . However, the lower boundary of the asthenosphere i ...
, mesospheric mantle,
outer core Earth's outer core is a fluid layer about thick, composed of mostly iron and nickel that lies above Earth's solid Earth's inner core, inner core and below its Earth's mantle, mantle. The outer core begins approximately beneath Earth's surface ...
, and the
inner core Earth's inner core is the innermost internal structure of Earth, geologic layer of the planet Earth. It is primarily a solid ball (mathematics), ball with a radius of about , which is about 20% of Earth's radius or 70% of the Moon's radius. T ...
. Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core. The geologic component layers of Earth are at increasing depths below the surface.


Crust and lithosphere

Earth's crust ranges from in depth and is the outermost layer. The thin parts are the
oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramaf ...
, which underlies the ocean basins (5–10 km) and is mafic-rich (dense iron-magnesium
silicate mineral Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, the crystalline forms of silica (silicon dio ...
or
igneous rock Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...
). The thicker crust is the
continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...
, which is less dense and is
felsic In geology, felsic is a grammatical modifier, modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted ...
-rich (igneous rocks rich in elements that form
feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...
and
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
). The rocks of the crust fall into two major categories – sial (aluminium silicate) and sima (magnesium silicate). It is estimated that sima starts about 11 km below the Conrad discontinuity, though the discontinuity is not distinct and can be absent in some continental regions. Earth's lithosphere consists of the crust and the uppermost mantle. The crust-mantle boundary occurs as two physically different phenomena. The Mohorovičić discontinuity is a distinct change of
seismic wave A seismic wave is a mechanical wave of acoustic energy that travels through the Earth or another planetary body. It can result from an earthquake (or generally, a quake), volcanic eruption, magma movement, a large landslide and a large ma ...
velocity. This is caused by a change in the rock's density – immediately above the Moho, the velocities of primary seismic waves ( P wave) are consistent with those through
basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
(6.7–7.2 km/s), and below they are similar to those through
peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...
or dunite (7.6–8.6 km/s). Second, in oceanic crust, there is a chemical discontinuity between
ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...
cumulates and tectonized harzburgites, which has been observed from deep parts of the oceanic crust that have been obducted onto the continental crust and preserved as ophiolite sequences. Many rocks making up Earth's crust formed less than 100
million years Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a unit of time equal to (i.e. years), or approximately 31.6 teraseconds. Usage Myr is in common use in fields such as Earth science and cosmology. Myr is also used ...
ago; however, the oldest known mineral grains are about 4.4 billion years old, indicating that Earth has had a solid crust for at least 4.4 billion years.


Mantle

Earth's mantle extends to a depth of , making it the planet's thickest layer. his is 45% of the radius, and 83.7% of the volume - 0.6% of the volume is the crust The mantle is divided into upper and lower mantle separated by a transition zone. The lowest part of the mantle next to the core-mantle boundary is known as the D″ (D-double-prime) layer. The
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
at the bottom of the mantle is ≈140 G Pa (1.4 M atm). The mantle is composed of
silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...
rocks richer in iron and magnesium than the overlying crust. Although solid, the mantle's extremely hot silicate material can flow over very long timescales.
Convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...
of the mantle propels the motion of the tectonic plates in the crust. The source of heat that drives this motion is the decay of radioactive isotopes in Earth's crust and mantle combined with the initial heat from the planet's formation (from the
potential energy In physics, potential energy is the energy of an object or system due to the body's position relative to other objects, or the configuration of its particles. The energy is equal to the work done against any restoring forces, such as gravity ...
released by collapsing a large amount of matter into a gravity well, and the
kinetic energy In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...
of accreted matter). Due to increasing pressure deeper in the mantle, the lower part flows less easily, though chemical changes within the mantle may also be important. The viscosity of the mantle ranges between 1021 and 1024
pascal-second Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for example, syrup h ...
, increasing with depth. In comparison, the viscosity of water at is 0.89 millipascal-second and pitch is (2.3 ± 0.5) × 108 pascal-second.


Core

Earth's outer core is a fluid layer about in height (i.e. distance from the highest point to the lowest point at the edge of the inner core) 6% of the Earth's radius, 15.6% of the volumeand composed of mostly
iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
and
nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...
that lies above Earth's solid
inner core Earth's inner core is the innermost internal structure of Earth, geologic layer of the planet Earth. It is primarily a solid ball (mathematics), ball with a radius of about , which is about 20% of Earth's radius or 70% of the Moon's radius. T ...
and below its mantle. Its outer boundary lies beneath Earth's surface. The transition between the inner core and outer core is located approximately beneath Earth's surface. Earth's inner core is the innermost geologic layer of the planet
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
. It is primarily a solid ball with a radius of about , which is about 19% of Earth's radius .7% of volumeor 70% of the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
's radius. The inner core was discovered in 1936 by Inge Lehmann and is composed primarily of iron and some nickel. Since this layer is able to transmit shear waves (transverse seismic waves), it must be solid. Experimental evidence has at times been inconsistent with current crystal models of the core. Other experimental studies show a discrepancy under high pressure: diamond anvil (static) studies at core pressures yield melting temperatures that are approximately 2000 K below those from shock laser (dynamic) studies. The laser studies create plasma, and the results are suggestive that constraining inner core conditions will depend on whether the inner core is a solid or is a plasma with the density of a solid. This is an area of active research. In early stages of Earth's formation about 4.6 billion years ago, melting would have caused denser substances to sink toward the center in a process called planetary differentiation (see also the iron catastrophe), while less-dense materials would have migrated to the crust. The core is thus believed to largely be composed of iron (80%), along with
nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...
and one or more light elements, whereas other dense elements, such as
lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...
and
uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see felsic materials). Some have argued that the inner core may be in the form of a single iron
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
. Under laboratory conditions a sample of iron–nickel alloy was subjected to the core-like pressure by gripping it in a vise between 2 diamond tips ( diamond anvil cell), and then heating to approximately 4000 K. The sample was observed with x-rays, and strongly supported the theory that Earth's inner core was made of giant crystals running north to south. The composition of Earth bears strong similarities to that of certain
chondrite A chondrite is a stony (non-metallic) meteorite that has not been modified by either melting or planetary differentiation, differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar Syste ...
meteorites, and even to some elements in the outer portion of the Sun. Beginning as early as 1940, scientists, including Francis Birch, built geophysics upon the premise that Earth is like ordinary chondrites, the most common type of meteorite observed impacting Earth. This ignores the less abundant enstatite chondrites, which formed under extremely limited available oxygen, leading to certain normally oxyphile elements existing either partially or wholly in the alloy portion that corresponds to the core of Earth.
Dynamo theory In physics, the dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field. The dynamo theory describes the process through which a rotating, convection, convecting, and electrically conductin ...
suggests that convection in the outer core, combined with the Coriolis effect, gives rise to
Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...
. The solid inner core is too hot to hold a permanent magnetic field (see Curie temperature) but probably acts to stabilize the magnetic field generated by the liquid outer core. The average magnetic field in Earth's outer core is estimated to measure , 50 times stronger than the magnetic field at the surface. The magnetic field generated by core flow is essential to protect life from interplanetary radiation and prevent the atmosphere from dissipating in the
solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
. The rate of cooling by conduction and convection is uncertain, but one estimate is that the core would not be expected to freeze up for approximately 91 billion years, which is well after the Sun is expected to expand, sterilize the surface of the planet, and then burn out.


Seismology

The layering of Earth has been inferred indirectly using the time of travel of refracted and reflected seismic waves created by earthquakes. The core does not allow shear waves to pass through it, while the speed of travel ( seismic velocity) is different in other layers. The changes in seismic velocity between different layers causes refraction owing to
Snell's law Snell's law (also known as the Snell–Descartes law, the ibn-Sahl law, and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing th ...
, like light bending as it passes through a prism. Likewise, reflections are caused by a large increase in seismic velocity and are similar to light reflecting from a mirror.


See also

* Hollow Earth * Geological history of Earth * Large low-shear-velocity provinces * Lehmann discontinuity * Rain-out model * Seismic tomography – technique for imaging the subsurface of Earth using seismic waves *
Travel to the Earth's center Travelling to the Earth's center is a popular List of science fiction themes, theme in science fiction. Some subterranean fiction involves traveling to the Earth's center and finding either a hollow Earth or Inner core, Earth's molten core. pla ...
* Solid earth


References


Further reading

* * * *


External links

* {{DEFAULTSORT:Structure Of The Earth Geology Geophysics Vertical distributions