It is the outermost solid part of the earth. It is brittle in nature. The thickness of the crust varies under the oceanic and continental areas.
Oceanic crust is thinner as compared to the continental crust. The thickness of the oceanic crust is 5km whereas that of the continental crust is around 30 km.
The continental crust is thicker in the areas of major mountain systems. It is as much as 70 to 100 km thick in the Himalayan region.
The upper part of the crust consists of granite rocks and forms the continents. Its main minerals constitute are silica and alumina. The lower part of the crust consists of basaltic rocks forming the ocean floors, comprising mainly silica, iron, and magnesium.
The outer covering of the crust is of sedimentary material (granitic rocks) and below that lie crystalline, igneous and metamorphic rocks which are acidic in nature. The lower layer of the crust consists of basaltic and ultra-basic rocks.
Mohorovicic (Moho) discontinuity forms the boundary between crust and asthenosphere.
The Asthenosphere is a weak zone of Earth’s Mantle and not of the Earth’s crust Its upper layer of the earth’s mantle, below the lithosphere, in which there is relatively low resistance to plastic flow and convection, is thought to occur.
Peridotite is a dark-colored igneous rock consisting mostly of olivine and pyroxene. It is an important rock type because the Earth’s mantle is predominantly composed of it.
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