Lahar

Synonyms

Volcanic mudflow; Debris flow

Definition

A lahar is a flowing slurry of rock debris and water originating on the slopes of a volcano. The term may also mean the deposit of such a flow.

Discussion

Characteristics: Lahars contain grains from clay to large boulders. The flowing material is water-lubricated sand or mud, but the whole mixture maybe more than half of incorporated cobbles and boulders. Volumes can reach 10⁹ m³ and peak discharge may exceed 10⁷ m³/s. Depending on proportions of freshly erupted volcanic debris and snow or water, temperatures range from nearly 100°C to 0°C but are typically below 50°C. Flowing lahars peak swiftly and wane more slowly, passing any one place within minutes to a couple hours but sometimes lasting several hours.

Origin: Lahars can originate by eruption of hot fragmental debris onto snow or ice. A melting mass then flows swiftly downslope, incorporating more and more ash and rock from the volcano’s slopes. Some large lahars initiate from...