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The rocks of the crust fall into two major categories – sial (aluminium silicate) and sima (magnesium silicate). [13] It is estimated that sima starts about 11 km below the Conrad discontinuity, [14] though the discontinuity is not distinct and can be absent in some continental regions. [15]
Because of the large pressures, over geologic time, the sima flows like a very viscous liquid, so, in a real sense, the sial floats on the sima, in isostatic equilibrium. [9] Mountains extend down as well as up, much like icebergs on the ocean; [9] so that on the continental plates, the sial runs between 5 km and 70 km deep. [10]
The sima has a higher density (2800 to 3300 kg/m 3) than the sial, which is due to larger amounts of iron and magnesium, and smaller amounts of aluminium. When the denser sima comes to the surface it forms mafic rocks, or rocks with mafic minerals. The most dense sima has less silica and forms ultramafic rocks.
The thickness of Earth's crust (km). The continental crust consists of various layers, with a bulk composition that is intermediate (SiO 2 wt% = 60.6). [5] The average density of the continental crust is about, 2.83 g/cm 3 (0.102 lb/cu in), [6] less dense than the ultramafic material that makes up the mantle, which has a density of around 3.3 g/cm 3 (0.12 lb/cu in).
Sial – Rocks rich in aluminium silicate minerals; Sima – Rocks rich in magnesium silicate minerals; Hydrosphere – Total amount of water on a planet; Lithosphere – Outermost shell of a terrestrial-type planet or natural satellite; Ocean – Body of salt water covering most of Earth; Plate tectonics – Movement of Earth's lithosphere
Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium. It is thinner than continental crust , or sial , generally less than 10 kilometers thick; however, it is denser, having a mean density of about 3.0 grams per cubic centimeter as opposed to continental crust which has a density of about 2.7 grams ...
Sial; Sima (geology) Earth’s surface; T. Transition zone (Earth) Travel to the Earth's center This page was last edited on 22 April 2024, at 08:02 (UTC). ...
Therefore, the seismologists of that time considered that the Conrad discontinuity should correspond to a sharply defined contact between the chemically distinct two layers, sial and sima. [2] Despite the fact that sial and sima are two solid layers, the lighter sial is thought to "float" on top of the denser sima layer.