The near-invisibility of ice-slurry flows in the geological record belies their signifi-cant hazard at snow-capped volcanoes. These four-phase flows exhibit extreme rates of volumetric bulking and unusually high mobility. Mechanisms of their motion are clarified through two examples generated on 25 September 2007 at Mount Ruapehu, New Zealand. Brief explosions through Crater Lake ejected 5700 m3 of acidic water that entrained 60 times this volume of snow as it traveled over a snow-covered glacier. The resulting ice-slurry traveled up to 7.7 km (height/length [H/L] ratio of 0.16). A cogenerated second flow took a more tortuous initial path before riding over the already frozen deposits of the first unit and beyond (H/L 0.13). For the first time, downstream evolution of the kinematic properties of propagating ice-slurry fronts could be characterized as well as the longitudinal variation of the physical properties of their resulting deposits. The chemistry and composition of the deposits show that during flow, vertical percolation of water through the porous ice–particle–water–air mixture generated a basal zone of high internal pore pressure. This effect was particularly strong when a thick, high-density flow front formed, which raced ahead of the tail to control runout and consequent hazard.