planet

Asthenosphere

The layer of rock immediately beneath the lithosphere, hot enough to creep like a ductile solid on geological timescales. One thing is worth stating plainly because it is so often got wrong: the asthenosphere is not liquid and not magma — its melt fraction is typically under one percent. It is solid rock that happens to flow, like a block of asphalt if you are patient enough to watch. Plates do not float on a sea of magma; they rest on this slowly creeping rock.