Lahar

A lahar is a flow of water mixed with volcanic debris and sediment that rushes down a volcano’s slopes and drainage channels. It can travel far beyond the volcano and bury communities under mud and rock.

Volcanic eruption: The release of magma, ash, and gases from a volcano onto the surface or into the atmosphere. Eruptions can melt snow and ice or eject debris that triggers lahars.
Snowmelt: The melting of accumulated snow as temperatures rise or heat is added. Eruption heat can melt snow rapidly and supply water for a lahar.
Nevado del Ruiz: A glacier-covered stratovolcano in Colombia whose 1985 eruption generated destructive lahars. Its eruption-triggered flows buried Armero and demonstrated how far lahars can reach.
Volcanic hazard: A potentially harmful process or event associated with volcanic activity. Lahars are a major volcanic hazard because they can occur during or after eruptions.
Volcanic ash: Fragments of volcanic rock and glass smaller than two millimeters, produced during explosive eruptions. Ash supplies fine sediment that mixes with water and helps lahars flow.
Glacier: A persistent mass of ice that moves slowly over land under its own weight. Volcanoes capped by glaciers can generate lahars when eruptions melt or destabilize ice.
Armero tragedy: The 1985 disaster in which lahars from Nevado del Ruiz destroyed the Colombian town of Armero. The deaths at Armero exposed the consequences of inadequate warning and evacuation.
Volcanic hazard map: A map showing areas exposed to volcanic processes such as lava flows, ashfall, or lahars. Lahar hazard maps identify valleys and settlements that may need warnings or evacuation.
Pyroclastic density current: A ground-hugging current of hot gas, ash, and volcanic fragments that moves rapidly away from an eruption. Unlike a lahar, it is driven by hot gas and does not require water.
Rainfall: Water that falls from clouds as liquid precipitation. Heavy rain can mobilize loose volcanic ash and debris long after an eruption.

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