Molten rock is rock heated to a temperature that is sufficient to turn it into a thick liquid. Molten rock often is associated with lava from a volcano. Additionally, molten rock exists below the surface of the Earth.
Magma spewed from a volcano includes molten and semi-molten rock. The temperature at which a rock melts and becomes molten depends on the mineral or minerals within the rock itself. Different minerals possess different melting points. Carbonatite minerals have lower melting points while komatite minerals melt at higher temperatures. Many rocks are comprised of different minerals, resulting in disparate melting points for an individual rock.