Most volcanic hazards --- from lava and pyroclastic flows to ash and gas clouds --- are fundamentally multiphase fluid flows. Predicting these fluid flows is of considerable importance in order to inform quantitative hazard assessment, including, for example, potential evacuation and exclusion zones during eruptions. Although recent advances in remote sensing offer some remarkable new insights into the dynamics, these hazardous fluid flows can be difficult to measure directly. Predictive mathematical models therefore play a vital role in allowing forecasters make robust operational decisions. This is the theme of our proposed workshop: the fluid mechanics of volcanic hazards.