A rapidly flowing, high-concentration, poorly sorted sediment-laden mixture of rock debris and water (other than normal streamflow) from a volcano (Lavigne et al. (2007) Forum Geografi 21, 1); ‘transient sediment-water flows whose properties are unsteady so that the sediment load fluctuates during the flow’ (Lavigne and Thouret (2003) Geomorph. 49, 1). Lahars most commonly occur when a crater lake or an ice-dammed lake suddenly overflows; perhaps because of an eruption, the collapse of a dam, heavy rain, snow melt, or the mixing of a nuée ardente with lake water. See (28 Sept. 2007) GeoNet on the lahar from Mt Ruapehu, New Zealand. Berti and Simoni (2007) Geomorph. 90, 1–2 propose a method for delineating lahar-hazard zones in volcanic valleys.