Reality → Matter → Earth → Crust, mantle, core
| Crust | Mantle | ||
|---|---|---|---|
| Oxides | |||
| silicon | 60% | 45% | |
| aluminum | 15% | 4% | |
| iron | 7% | 7% | |
| calcium | 6% | 3% | |
| magnesium | 5% | 38% |
Earth’s crust is a layer of igneous, sedimentary, and metamorphic rocks [1] , with a thickness of 5 to 10 km underneath the oceans and 30 to 50 km under the continents. Even the thicker portions of the continental crust measure less than 0.5 % of the distance to Earth’s center. All mining of minerals, ores, and hydrocarbons takes place only in the uppermost tenth of the crust [2] . Temperature and pressure increase with depth (temperature by about 30 centigrades and pressure by about 300 bars per kilometer of depth). Because of the high pressure, the rock at the bottom of the crust stays solid, though the melting temperature at atmospheric pressure is exceeded. The boundary between crust and mantle is marked by a seismic discontinuity [3] . The mantle, extending over a depth of about 2,900 km, is about 100 times thicker than the crust and accounts for about 80 % of Earth’s volume. The mantle is believed to be solid and similar to the crust in composition and physical properties [4] . The boundary between lower mantle and upper core is again defined by a seismic discontinuity. The core is believed to consist of an iron-nickel alloy. A distinction is made between outer and inner core, based on seismic studies that lead to the assumption of a liquid outer and a solid inner core [5] .
Igneous rock is formed by cooling and solidifying magma. Sedimentary rock is formed by sedimentation and lithification of weathered mineral particles or organisms. Metamorphic rock is formed by transformation of igneous or sedimentary rock under high pressure and temperatures. The formation and transformation of rock is still ongoing, albeit at geological (very slow) speed and is closely linked with volcanism, subsidence, plate tectonics, and continental drift. The heat of the underlying mantle is the motor for movements within the crust (rocks of the mantle are semi-plastic in geologic timeframe and show movements similar to a lava lamp). Much of the heat is believed to be generated by the continuing radioactive decay of elements heavier than iron.
More than two thirds of the world mineral production is extracted from open pits at the surface (maximum depth 700 m). Underground mining, under extreme conditions, extends to a maximum of 3,900 m (deepest point of a South African gold mine). The deepest offshore oil and gas wells go down about 10,000 m (including water depth).
The discontinuity is an abrupt increase of the velocity of compressional seismic waves, discovered by Mohorovicic in the early 1900s. The sudden velocity increase is caused by the higher density of rocks from the mantle.
Though little is known about the mantle as a whole, fragments of dense 'mafic' rocks (believed to originate from the outer mantle) are found in igneous rocks of the crust and give an indication of the mantle’s composition. Overall, rocks from the mantle are denser and more 'mafic' than the 'felsic' rocks from the crust.
Below the boundary of mantle and core, no transversal shear waves are observed, indicating that the outer core is liquid. However, though very difficult to detect, transversal seismic waves from earthquakes have been observed in the inner core. It is therefore assumed that only in the inner core the pressure has increased to a level that causes solidification, despite temperatures believed to be in the order of 5,000 °C (similar to the Sun’s surface).