Scientific understanding of the internal structure ofEarth is based on observations oftopography andbathymetry,observations ofrock inoutcrop, samples brought to the surface from greater depths byvolcanoes or volcanic activity, analysis of theseismic waves that pass through Earth, measurements of thegravitational andmagnetic fields of Earth, and experiments with crystalline solids at pressures and temperatures characteristic of Earth's deep interior.
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).[1]
Measurements of the force exerted byEarth's gravity can be used to calculate itsmass. Astronomers can also calculateEarth's mass by observing the motion of orbitingsatellites. Earth's averagedensity can be determined through gravimetric experiments, which have historically involvedpendulums. The mass of Earth is about6×1024 kg.[4] The average density of Earth is5.515 g/cm3.[5]
The structure of Earth can be defined in two ways: by mechanical properties such asrheology, or chemically. Mechanically, it can be divided intolithosphere,asthenosphere,mesospheric mantle,outer core, and theinner core. Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core.[6] The geologic component layers of Earth are at increasing depths below the surface.[6]: 146
Earth's crust ranges from 5 to 70 kilometres (3.1–43.5 mi)[7] in depth and is the outermost layer.[8] The thin parts are theoceanic crust, which underlies the ocean basins (5–10 km) and ismafic-rich[9] (dense iron-magnesiumsilicate mineral origneous rock).[10] The thicker crust is thecontinental crust, which is less dense[11] and isfelsic-rich (igneous rocks rich in elements that formfeldspar andquartz).[12] The rocks of the crust fall into two major categories –sial (aluminium silicate) andsima (magnesium silicate).[13] It is estimated that sima starts about 11 km below theConrad discontinuity,[14] though the discontinuity is not distinct and can be absent in some continental regions.[15]
Earth's lithosphere consists of the crust and the uppermostmantle.[16] The crust-mantle boundary occurs as two physically different phenomena. TheMohorovičić discontinuity is a distinct change ofseismic wave velocity. This is caused by a change in the rock's density[17] – immediately above the Moho, the velocities of primary seismic waves (P wave) are consistent with those throughbasalt (6.7–7.2 km/s), and below they are similar to those throughperidotite ordunite (7.6–8.6 km/s).[18] Second, in oceanic crust, there is a chemical discontinuity betweenultramafic cumulates and tectonizedharzburgites, which has been observed from deep parts of the oceanic crust that have beenobducted onto the continental crust and preserved asophiolite sequences.[clarification needed]
Many rocks making up Earth's crust formed less than 100million years ago; however, the oldest known mineral grains are about 4.4billion years old, indicating that Earth has had a solid crust for at least 4.4 billion years.[19]
Earth's crust and mantle,Mohorovičić discontinuity between bottom of crust and solid uppermost mantle
Earth's mantle extends to a depth of 2,890 km (1,800 mi), making it the planet's thickest layer.[21][This is 45% of the 6,371 km (3,959 mi) radius, and 83.7% of the volume - 0.6% of the volume is the crust].The mantle is divided intoupper andlower mantle[22] separated by atransition zone.[23] The lowest part of the mantle next to thecore-mantle boundary is known as the D″ (D-double-prime) layer.[24] Thepressure at the bottom of the mantle is ≈140 GPa (1.4 Matm).[25] The mantle is composed ofsilicate rocks richer in iron and magnesium than the overlying crust.[26] Although solid, the mantle's extremely hot silicate material canflow over very long timescales.[27]Convection of the mantle propels themotion of the tectonic plates in the crust. Thesource of heat that drives this motion is the decay ofradioactive isotopes in Earth's crust and mantle combined with the initial heat from the planet's formation[28] (from thepotential energy released by collapsing a large amount of matter into agravity well, and thekinetic energy 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 1024pascal-second, increasing with depth.[29] In comparison, the viscosity of water at 300 K (27 °C; 80 °F) is 0.89 millipascal-second[30] andpitch is (2.3 ± 0.5) × 108 pascal-second.[31]
A diagram of Earth's geodynamo and magnetic field, which could have been driven in Earth's early history by the crystallization ofmagnesium oxide,silicon dioxide, andiron(II) oxide
Earth's outer core is a fluid layer about 2,260 km (1,400 mi) in height (i.e. distance from the highest point to the lowest point at the edge of the inner core) [36% of the Earth's radius, 15.6% of the volume] and composed of mostlyiron andnickel that lies above Earth's solidinner core and below itsmantle.[33] Its outer boundary lies 2,890 km (1,800 mi) beneath Earth's surface. The transition between the inner core and outer core is located approximately 5,150 km (3,200 mi) beneath Earth's surface. Earth's inner core is the innermostgeologic layer of the planetEarth. It is primarily a solid ball with a radius of about 1,220 km (760 mi), which is about 19% ofEarth's radius [0.7% of volume] or 70% of theMoon's radius.[34][35]
The inner core was discovered in 1936 byInge 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.[36] 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.[37][38] The laser studies create plasma,[39] 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 calledplanetary differentiation (see also theiron catastrophe), while less-dense materials would have migrated to thecrust. The core is thus believed to largely be composed of iron (80%), along withnickel and one or more light elements, whereas other dense elements, such aslead anduranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (seefelsic materials). Some have argued that the inner core may be in the form of a single ironcrystal.[40][41]
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.[42][43]
The composition of Earth bears strong similarities to that of certainchondrite meteorites, and even to some elements in the outer portion of the Sun.[44][45] Beginning as early as 1940, scientists, includingFrancis 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 abundantenstatite 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.[citation needed]
Dynamo theory suggests that convection in the outer core, combined with theCoriolis effect, gives rise toEarth's magnetic field. The solid inner core is too hot to hold a permanent magnetic field (seeCurie 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 2.5 milliteslas (25 gauss), 50 times stronger than the magnetic field at the surface.[46]
The magnetic field generated by core flow is essential to protect life from interplanetary radiation and prevent the atmosphere from dissipating in thesolar wind. The rate of cooling by conduction and convection is uncertain,[47] 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.[48][better source needed]
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 toSnell's law, 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.
^Hess, H. (1955-01-01)."The oceanic crust".Journal of Marine Research.14 (4): 424.It has been common practice to subdivide the crust into sial and sima. These terms refer to generalized compositions, sial being those rocks rich in Si and Al and sima those rich in Si and Mg.
^Edgeworth, R.; Dalton, B.J.; Parnell, T."The Pitch Drop Experiment". The University of Queensland Australia. Archived fromthe original on 28 March 2013. Retrieved15 October 2007.
^"MANTO",Silvae, Harvard University Press, pp. 2–29, 2004-07-30, retrieved2025-10-09
^"Earth's Interior".Science & Innovation. National Geographic. 18 January 2017. Archived fromthe original on 18 January 2019. Retrieved14 November 2018.
^Benuzzi-Mounaix, A.; Koenig, M.; Husar, G.; Faral, B. (June 2002). "Absolute equation of state measurements of iron using laser driven shocks".Physics of Plasmas.9 (6): 2466.Bibcode:2002PhPl....9.2466B.doi:10.1063/1.1478557.
^Schneider, Michael (1996)."Crystal at the Center of the Earth".Projects in Scientific Computing, 1996. Pittsburgh Supercomputing Center.Archived from the original on 5 February 2007. Retrieved8 March 2019.
![[icon]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aWiki_letter_w_cropped.svg)
.jpg)
.svg)
