Where does the melted rock from a volcano come from?

The answer is magma

It is also called volcanic glass

this happens when a volcano explodes and the lava cools down.

The magma moves upward and erupts to form either an oceanic ridge or a hot spot volcano. At subduction zones, volatile compounds (especially water) escape from the subducting plate and lower the melting temperature of the overlying mantle rocks. This triggers melting and magma forms as a result.