Earth's Surface Layers and Convection
LithosphereThe lithosphere is the outer solid part of the earth, including the crust and uppermost mantle. The lithosphere is about 100 km thick, although its thickness is age dependent (older lithosphere is thicker).The lithosphere below the crust is brittle enough at some locations to produce earthquakes by faulting, such as within a subducted oceanic plate.
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 second law of thermodynamics says heat will move from hot to cold until equilibrium is achieved. But the deepest parts of the mantle are hot, while the top is cool. The fundamental energy source for this heat within the mantle is primarily radioactive decay of natural elements like uranium. This heat, in its effort to 'get out' drives convection of the mantle, a sort of boiling effect.
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.