Strictly speaking, it's only the heat generated within the Earth that melts various silicate minerals and forms the molten magma... When the magma is cooled very quickly, as during the eruption of a volcano, it doesn't form crystals, but rather obsidian type rocks, which fracture easily in the violent eruption, into smaller, glass particles from the size of dust up to much larger rocks. We've found (here in the western U.S.) boulders of obsidian the size of a small car. These weather and break into much smaller pieces quickly (relatively speaking) unless buried in the intial pyroclastic outflow...