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 Mohorovicic 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]
Geologic layers of the Earth[81]
Earth-crust-cutaway-english.svg
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]
Present-day major heat-producing isotopes[88]
Isotope Heat release
W
kg isotope
Half-life
years Mean mantle concentration
kg isotope
kg mantle
Heat release
W
kg 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
The mean heat loss from the Earth is 87 mW m-2, for a global heat loss of 4.42 × 1013 W.[89] 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.[90] More of the heat in the 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 in the oceans because the crust there is much thinner than that of the continents.[91]
Tectonic plates
Geologic layers of the Earth[81]
Earth-crust-cutaway-english.svg
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]
Present-day major heat-producing isotopes[88]
Isotope Heat release
W
kg isotope
Half-life
years Mean mantle concentration
kg isotope
kg mantle
Heat release
W
kg 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
The mean heat loss from the Earth is 87 mW m-2, for a global heat loss of 4.42 × 1013 W.[89] 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.[90] More of the heat in the 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 in the oceans because the crust there is much thinner than that of the continents.[91]
Tectonic plates
