Volcanic ash originating from the fragmentation of magma damages infrastructure and the environment. Bubble expansion is crucial in magma fragmentation, but low-intensity eruptions frequently emit ashes with fewer bubbles. We here conducted tensional experiments on silicate melt at a high temperature, at which the melt elongates or fractures depending on the strain rate. A fracture occurs by appearing of a crack on the melted silicate rod, followed by a generation of small fragments. The fracture surface shows a smooth and rough region dichotomy, similar to those observed on glass fracture surfaces at room temperature. The rough surface region generates small fragments. Interestingly, the measured stress-strain curves indicate fragmentation occurs under viscous deformation. These results suggest that silicate melts under viscous deformation fragment, as glass does at room temperature. The ductility around the crack tip promotes void nucleation and coalescence, causing the crack to branch to generate dense, fine volcanic ashes.