The mantle is the mostly solid bulk of Earths interior, lying between Earths dense, super-heated core and its thin outer layer, the crust. The mantle is mostly solid rock, but behaves like a viscous fluid over long periods of time, and forms convection currents due to the heat coming from the core. The special feature of the upper mantle is its ability to flow like a fluid. The upper mantle has a soft, weak layer called the asthenosphere, which is capable of flowing. This property facilitates the movement of lithospheric plates, which contributes to plate tectonics, volcanoes, seafloor spreading, earthquakes, and mountain-building. The heat generated by the radioactive decay of uranium within the inner core elevates the temperature of Earths interior, including the mantle. This then results in the melting of rock material in the upper mantle and causes it to flow like liquid.