Heat Sources
Temperature within the Earth increases with depth. Highly viscous or partially molten rock at temperatures between 650 to 1,200 °C (1,200 to 2,200 °F) is postulated to exist everywhere beneath the Earth's surface at depths of 80 to 100 kilometres (50 to 60 mi), and the temperature at the Earth's inner core/outer core boundary, around 3,500 kilometres (2,200 mi) deep, is estimated to be 5650 ± 600 kelvins. The heat content of the Earth is 1031 joules.
- Much of the heat is created by decay of naturally radioactive elements. An estimated 45 to 90 percent of the heat escaping from the Earth originates from radioactive decay of elements within the mantle.
- Heat of impact and compression released during the original formation of the Earth by accretion of in-falling meteorites.
- Heat released as abundant heavy metals (iron, nickel, copper) descended to the Earth's core.
- Latent heat released as the liquid outer core crystallizes at the inner core boundary.
- Heat may be generated by tidal force on the Earth as it rotates; since rock cannot flow as readily as water it compresses and distorts, generating heat.
- There is no reputable science to suggest that any significant heat may be created by electromagnetic effects of the magnetic fields involved in Earth's magnetic field, as suggested by some contemporary folk theories.
| Isotope | Heat release | Half-life | Mean mantle concentration | Heat release |
|---|---|---|---|---|
| 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 |
In Earth's continental crust, the decay of natural radioactive isotopes has had significant involvement in the origin of geothermal heat. The continental crust is abundant in lower density minerals but also contains significant concentrations of heavier lithophilic minerals such as uranium. Because of this, it holds the largest global reservoir of radioactive elements found in the Earth. Especially in layers closer to Earth's surface, naturally occurring isotopes are enriched in the granite and basaltic rocks. These high levels of radioactive elements are present because they cannot be readily accommodated by the Earth's mantle due to the high pressures that are present. The mantle is mostly made up of high density minerals with high contents of atoms that have relatively small atomic radii such as magnesium (Mg), titanium (Ti), and calcium (Ca).
Read more about this topic: Geothermal Gradient
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