Earth’s Crust: Thickness, Composition, Lithosphere, Asthenosphere and Pedosphere
Earth’s crust is the outermost layer composed of various types of rocks. The boundary between the crust and mantle is generally called the Mohorovičić discontinuity.
Thickness of Continental and Oceanic Crust
It varies in thickness from around 5 to 100 kilometers. The continental crust is thicker in comparison to oceanic crust. The oceanic crust ranges from 5 to 10 kilometers {average 7 km} while continental crust ranges from 25 to 100 kilometers {average 30-35 km}. Thickest continental crust regions are under large mountain ranges.
Difference in composition and density
Oceanic crust is made of dark rocks having more of Iron and Magnesium and are more basaltic. Continental crust is made of lighter rocks, having more of Silica and are more felsic. The below table shows the key differences between Oceanic and continental rocks.
Lithosphere, Asthenosphere and Pedosphere
Lithos means rock. Lithium is an alkali metal and its name is also derived from Lithos. Lithosphere is the upper 80 Kilometers layer composed of both the crust and part of the upper mantle. However, overall, it is cool enough to be tough and elastic than the molten mantle. The Oceanic lithosphere is associated with Oceanic crust and exists in the ocean basins, while the Continental lithosphere is associated with Continental crust. The Oceanic lithosphere is denser than the continental lithosphere.
Lithosphere is obviously thinner under the oceans and volcanically active continental regions than the other landmasses. The entire lithosphere is physically broken up into the brittle, moving plates containing the world’s continents and oceans. These lithospheric plates appear to “float” and move around on the more ductile asthenosphere. The asthenosphere is the relatively narrow, moving zone in the upper mantle located between 72 to 250 kilometers beneath the Earth’s surface. It is composed of a hot, semi-solid material that is soft and flowing after being subjected to high temperatures and pressures. The asthenosphere boundary is closer to the surface-within a few kilometers under oceans and near mid-ocean ridges than it is below the landmasses. The upper section of the asthenosphere is thought to be the area in which the lithospheric plates move, “carrying” the continental and oceanic plates also known as Tectonic Plates.
Further, the uppermost part of the Lithosphere that reacts with the atmosphere, biosphere and Hydrosphere is called as pedospehere. Pedos means soil. Pedospehere is composed of soil and it is the cradle of all the chemical and biogeochemical reactions which leads to soil development.
Composition of Earth Crust
Almost half of Earth’s crust is made of oxygen, while a quarter of it is made of silicon. Since silicon and Oxygen react to make silica, around 48.6% of Earth’s crust is made of silica.
Major elements in Earth’s crust are Oxygen (47%), Silicon (28%), Aluminum (8%), Iron (5%), Calcium (3.5%), Sodium (2.5%), Potassium (2.5%), Magnesium (2.2%) and other elements such as Hydrogen, Carbon, Phosphorus, Sulphur etc.
Major compounds in Earth’s crust are shown in below table.
| Compound | Formula | Continental | Oceanic |
|---|---|---|---|
| Silica | SiO2 | 60.2% | 48.6% |
| Alumina | Al2O3 | 15.2% | 16.5% |
| Lime | CaO | 5.5% | 12.3% |
| Magnesia | MgO | 3.1% | 6.8% |
| Iron(II) Oxide | FeO | 3.8% | 6.2% |
| Sodium Oxide | Na2O | 3.0% | 2.6% |
| Potassium Oxide | K2O | 2.8% | 0.4% |
| Iron(III) Oxide | Fe2O3 | 2.5% | 2.3% |
| Water | H2O | 1.4% | 1.1% |
| Carbon Dioxide | CO2 | 1.2% | 1.4% |
| Titanium Dioxide | TiO2 | 0.7% | 1.4% |
| Phosphorus Pentoxide | P2O5 | 0.2% | 0.3% |
| Total | #colspan# | 99.6% | 99.9% |
Thus, most of the rocks in Earth’s crust are all oxides. The principal oxides are silica, alumina, iron oxides, lime, magnesia & potash. There are not many iron loving compounds in Earth Crust because they were depleted and relocated deeper. Further, more meteoritic content is found in Earth’s Crust.
Conrad Discontinuity
Conrad discontinuity (named after the seismologist Victor Conrad) is considered to be the border between the upper continental crust and the lower one. It is not as pronounced as the Mohorovičić discontinuity, and absent in some continental regions.