Asthenosphere
The asthenosphere is the ductile part of the
earth just below the lithosphere, including the lower mantle. The
asthenosphere is about 180 km thick.
Convection in the Earth's Mantle
The above animation shows how the hot silicate rock of the Earth's mantle is stirred by heat trying to escape. The hot rock (yellow) rises slowly as the denser cold rock (blue) sinks. The layer is at least 700 km thick, and could be as thick as 2900 km. The rock is at temperatures of order 1000 to 2000oC and creeps like a very viscous fluid. Its viscosity is about 20 orders of magnitude greater than that of water so velocity is only centimeters per year. The time interval of this animation is of order 10 million years.
If the Earth is in a rolling boil, how come we don't see this at the surface?
We DO! It is what causes enormous plates of continents and those lying under the oceans to move, sometimes
several inches a year. And when these plates interact, very interesting things occur on Earth.