The Earth’s mantle is a thick layer of compressed and heated rock that accounts for 70% of the Earth’s volume. It is largely composed of oxide compounds and differs in chemical ratios from the crust. The upper mantle flows smoothly and convection causes continental drift. The entire oceanic crust is recycled approximately every 100 million years.
The Earth’s mantle is a shell of compressed and heated rock approximately 1,800 miles (2,900 km) thick, starting under the Earth’s crust (lithosphere), extending 3.1 miles (5 km) below the ocean floor and from 19 to 31 miles (30 to 50 km) below the continents. It accounts for 70% of the Earth’s volume, compared to the Earth’s crust, which makes up less than 1% of the total. In fact, the crust is just a thin layer of frozen rock that protects the mantle from outer space. The two layers are separated by a transition area called the Mohorovičić discontinuity (the “Moho”) where a certain type of seismic wave accelerates rapidly during the transit.
Like the crust, the mantle is largely composed of oxide compounds such as olivine, pyroxenes, spinel, garnet, peridotite, and eclogite. However, this layer differs in its chemical ratios from the crust. It is composed of approximately 45% oxygen, 23% magnesium, 22% silicon, 6% iron, 2% aluminum, 2% calcium, with traces of sodium, potassium and other elements. Like the crust, the mantle can generally be considered a silicate. Beneath it are the Earth’s outer core and inner core, which make up about 29% of the Earth’s volume and are composed primarily of molten (outer core) or solid (inner core) iron and nickel.
The upper mantle (esthenosphere) has a low density compared to the rest of this layer and flows smoothly, like a plastic. Conditions get hotter and denser as it descends, until the rock melts completely where the bottom ends and the inner core begins. Convection in the upper mantle causes continental drift. The main driver of this convection is the overlying lithosphere sinking into the mantle through the subduction zones of the ocean edges. By subducting crust across ocean edges and regenerating it at divergent boundary areas (where the plates move apart) such as the Mid-Atlantic Ridge, the entire oceanic crust is recycled approximately every 100 million years. By comparison, portions of continental crust are billions of years old.
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