The mantle is a layer of the Earth between the crust and the core. The upper mantle starts below the crust and goes down to a depth of around 200 miles to the boundary with the lower mantle. The temperature of the mantle ranges from 900 degrees Fahrenheit near the crust to as high as 7000 degrees near the core.he Upper Mantle: Temperature and Depth. The upper mantle begins just beneath the crust and ends at the lower mantle. The thickness of the upper mantle is between 200 and 250 miles.