Why does the earths crust not melt into the mantle?
Crust sitting on top of the mantle does not melt b/c the mantle itself is not molten. Shallow mantle is brittle rock but as you get very deep it starts to be more elastic (behaving more like a liquid). But only in certain places (ex. mantle plumes) does it actually become molten in the sense that the rock actually becomes magma.
