Lahars Triggered by Melting Snow and Ice

Dark lahar

Mount St. Helens, Washington

Dark pathways created by lahars streak the sides of Mount St. Helens during its catastrophic eruption on May 18, 1980. The lahars were triggered by the sudden melting of snow and ice from hot volcanic rocks ejected by the initial explosive activity and subsequent pyroclastic flows.

Many of the largest and most destructive historical lahars accompanied eruptions from volcanoes mantled by a substantial cover of snow and ice. Pyroclastic flows are the most common volcanic events that generate lahars--even relatively small pyroclastic flows can quickly melt large quantities of snow and ice. The hot flowing rock debris erodes and mixes with snow and ice to form water and trigger snow avalanches on steep slopes. Lava flows moving slowly across snow usually do not melt snow and ice rapidly enough to form large lahars but the eruption of lava beneath a glacier can result in substantial ponding of water, which may lead to enormous outpourings of water. The largest historic lahars in terms of discharge (volume of material per second) have occurred in Iceland, where these glacial outburst floods are called Jökulhlaups.

Histogram of lahars generated by melting snow and ice

Number and type of volcanic events known to have generated lahars by melting snow and ice during historical eruptions. This 1989 study was based on 108 historical eruptions from around the world (see references below).

Snowmelt triggered by pyroclastic flows, surges, and directed blasts

Snowmelt triggered by surficial and subglacial lava flows


Major, J.J, and Newhall, G.C., 1989, Snow and ice perturbation during historical volcanic eruptions and the formation of lahars and floods, Bulletin of Volcanology, v. 52, p. 1-27.