Volcanic debris avalanches (VDAs) start as rapid gravitational collapses (landslides) of weakened volcanic edifices, which can vary from limited-volume flank failures that leave relatively shallow scars on volcano sides to enormous edifice-collapse failures involving the volcano summit and core, leaving deep-seated, bowl-shaped, open-sided depressions. VDA movement begins as sliding but quickly progresses to thorough fracturing of edifice rock, crushing disaggregation of some of that rock, and subsequent highly inertial, particle–fluid flow of rock fragment mixtures and other debris that are typically wet but unsaturated. Intergranular spaces are occupied by air and volcanic gases as well as minor amounts of liquid water. VDAs differ from nonvolcanic rock avalanches; they tend to involve larger volumes of material, produce higher proportions of fine-grained material, and have proportionally longer runout distances. Moving VDAs typically transport numerous debris-avalanche blocks—fractured but still-intact pieces of the source edifice, which can vary up to hundreds of meters in diameter and which form prominent positive-relief features in VDA deposits (hummocks). VDAs either directly deposit their debris masses, typically within a few tens of kilometers of their source, or transform to volcanic debris flows (VDFs) that can flow much farther downstream.