When discussing the composition of the Earth as a whole, geologists conceptually divide the Earth into several layers. One of these layers is the crust, which is the outermost part of the planet. The lithosphere is not an individual layer, but rather a zone made up of two of the layers of the Earth, which includes the crust.
The Layers of the Earth
The Earth consists of three layers: the crust, the mantle and the core. The core, the innermost layer, is rich in iron and very dense. It can be further subdivided into the inner and outer core. The mantle is the intermediate layer of the Earth and can be subdivided into the inner and outer mantle. Most of the mantle is a thick fluid that moves in currents, but the very outermost portion of the outer mantle is solid. This portion and the solid crust make up the lithosphere.
The Mantle and the Lithosphere
The mantle is made up of molten rock called magma. This magma circulates in currents determined by the cooling and sinking of heavier minerals and the heating and rising of lighter minerals. All but the very uppermost portion of the mantle is part of the asthenosphere, which refers to the liquid zone of the inner Earth. The uppermost portion of the mantle makes up the bottom portion of the lithosphere. On average, it is 30 kilometers thick, but its thickness depends on the age of that portion of the lithosphere and temperature and pressure conditions. The mantle consists largely of heavy ultramafic rock like olivine.
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The Crust and the Lithosphere
The crust makes up the upper portion of the lithosphere. It is made up of lighter materials than the mantle and core, comprising mainly mafic and felsic rocks like granite. While it is the thinnest layer of the Earth at only 60 to 70 kilometers thick, it makes up the majority of the lithosphere and is the portion of the Earth that supports life. The crust surface is shaped by characteristics of the lithosphere that cause formations like mountains and fault lines. The part of the crust that makes up continents is formed of lighter minerals than the part of the crust that makes up the oceanic floor.
The Importance of the Lithosphere
The lithosphere, unlike the layers of the Earth, is defined not by composition but by behavior. The lithosphere is cold, relative to the fluid asthenosphere at least, and solid. It floats freely on top of the liquid magma of the upper mantle and is divided into discrete sections known as tectonic plates. The thickness of the lithosphere can be variable, with older portions being thicker, but tends to average a height of 100 kilometers. Young portions of the lithosphere are formed by the downward movement and melting of one tectonic plate beneath another at a boundary known as a subduction zone. These boundaries between tectonic plates have a profound effect on the shape of the surface of the earth. A boundary that moves longitudinally is known as a transform fault line and causes earthquakes. Volcanic activity occurs at subduction zones and forms continental landmasses, while divergent boundaries cause a magma upwelling that forms the ocean floor.