What is the thick middle layer of the geosphere?

The thick middle layer of the geosphere is called the mantle. It lies between the Earth's crust and the outer core, extending to a depth of about 2,900 kilometers (1,800 miles). The mantle is composed mainly of silicate rocks and behaves as a viscous solid, allowing for convection currents that drive plate tectonics. Its temperature increases with depth, reaching up to 4,000 degrees Celsius (7,200 degrees Fahrenheit) near the outer core.