What is the Earth's Mantle?
The Earth's mantle is a ~2,900 km (1,800 mi) thick shell of compressed and heated rock, beginning below the Earth's crust (lithosphere), which extends 5 km (3.1 mi) below the ocean floor and 30 to 50 km (19 - 31 mi) below the continents. The Earth's mantle makes up 70% of Earth's volume, in comparison 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 shielding the mantle from outer space. The crust and mantle are separated by a transition area called the Mohorovičić discontinuity (the "Moho") where a certain type of seismic wave quickly speeds up when transiting through.
Like the crust, the mantle is largely composed of oxide compounds such as olivine, pyroxenes, spinel, garnet, peridotite, and eclogite. The mantle differs in its chemical ratios from the crust, however. It is composed of roughly 45% oxygen, 23% magnesium, 22% silicon, 6% iron, 2% aluminum, 2% calcium, with trace amounts of sodium, potassium, and other elements. Like the crust, the mantle can broadly be regarded as silicate. Below the mantle are the outer core and inner core of the Earth, making up about 29% of the Earth's volume, and composed primarily of molten (outer core) or solid (inner core) iron and nickel.
The upper mantle (aesthenosphere) has a low density relative to the rest of the mantle, and flows fluidly, like a plastic. Conditions gets hotter and denser as one descends, until the rock melts entirely where the lower mantle ends and the inner core begins. Convection in the upper mantle causes continental drift. The main driver of mantle convection is the overlying lithosphere sinking back into the mantle through subduction zones of the edges of the oceans. By subducting crust through the ocean's edges and regenerating it in divergent boundary areas (where plates move apart) such as the Mid-Atlantic Ridge, the entire oceanic crust is recycled every 100 million years or so. By comparison, portions of the continental crust are billions of years old.
Written by Michael Anissimov
