Conduit implosion during Vulcanian eruptions

Fragmentation experiments and detailed textural studies were performed on Vulcanian pyroclasts of variable porosity from Soufrière Hills volcano, Montserrat. We use textural differences to identify two types of pyroclasts with distinct eruption histories. Our experiments show that high-porosity Type 1 pumice fragments rapidly, and that low-porosity Type 2 fragments relatively slowly. Both types of pyroclast contain fractured crystals containing burst melt inclusions. The orientations of the fractures relative to the crystal alignment indicate that Type 1 decompressed vertically and Type 2 decompressed laterally. We interpret that Type 1 clasts were derived from the conduit center, whereas Type 2 clasts were derived from the implosion of the conduit lining. We propose a conceptual model of rapid fragmentation of pumice in the conduit center, and a slower fragmentation of a pressurized conduit wall, vulnerable to implosion. Conduit implosion can explain observed widening of volcanic vents during eruptions. We also hypothesize that the fragmentation front is concave; this has important implications for modeling of explosive eruptions.