Lahars: Rivers of Volcanic Mud and Debris

U.S. Geological Survey Fact Sheet 2018–3024, Reducing the Risk from Volcano Hazards. By Jon J. Major, Thomas C. Pierson and James W. Vallance.

Lahar, an Indonesian word for a volcanic mudflow, is a mix of water, mud and volcanic rock rushing down a channel from a volcano. Lahars can form during or after eruptions — or when a volcano is quiet — often with little or no warning, and can travel far and fast, destroying or burying everything in their path. They are among the greatest threats volcanoes pose, so USGS scientists study their deposits and reach, model how they flow, map lahar hazards, and help communities understand and reduce the risk.

The remains of Armero, Colombia, after a lahar from the November 1985 eruption of Nevado del Ruiz travelled more than 45 miles and killed more than 20,000 residents. USGS photo by R.J. Janda.

Why they are so destructive

Lahars can run many tens of miles down valleys at tens of miles per hour, spilling far beyond river channels. Unlike a flood, which usually leaves buildings standing under a thin layer of mud, a lahar is a slurry carrying rocks and wood that can demolish buildings and bury land under muddy sand and gravel many feet — sometimes tens of feet — deep, making it hard to reoccupy for a long time. From 1600 to 2010, lahars killed more than 44,000 people worldwide — nearly 20 percent of all recorded deaths from eruptions.

• Primary lahars are directly tied to eruptions.
• Secondary lahars come later or between eruptions — weeks, months or even centuries afterward — when heavy rain, landslides, lake breaches or glacier releases remobilise volcanic sediment.

Two kinds of flow. A debris-flow lahar is roughly half sediment, half water and looks like wet concrete, carrying anything from clay to boulders as big as cars. A more dilute hyperconcentrated-flow lahar has more water than sediment and looks oily or creamy; it carries mostly sand but can roll big rocks along and badly damage banks and bridge piers. A lahar can change from one to the other as it erodes, drops sediment and mixes with river water.

Mud on these trees shows how deep a lahar ran in the Toutle River valley after the 1980 eruption of Mount St. Helens. USGS photo by L.J. Topinka.

How lahars start

1. Eruptions melting snow and ice. On snow- and ice-covered volcanoes, hot pyroclastic currents — fast flows of hot rock debris and gas — scour and mix with snow and ice, making floods and slurries that pick up more sediment as they race downhill.

• At Mount St. Helens in 1980, this made lahars of about 20 million cubic yards on the west and east flanks that travelled many tens of miles at tens of miles per hour.
• At Nevado del Ruiz in 1985, a modest eruption melted summit snow and ice; the floods turned into lahars that dropped more than 16,000 feet and travelled more than 60 miles. One burst from a canyon 45 miles from the summit and destroyed Armero, killing more than 20,000 people.
• At Redoubt, Alaska, in 1989–90 and 2009, lahars ran more than 25 miles to Cook Inlet and threatened or damaged an oil storage and transfer terminal, which its owner closed in 2017 to remove the risk.

2. Landslides that turn to flows. When a big, wet slice of a volcano collapses, it can become a flow. The Osceola Mudflow at Mount Rainier, 5,600 years ago, turned from slide to concrete-like flow within a mile, then — nearly a cubic mile in volume — filled valleys 250–500 feet deep, ran more than 60 miles to Puget Sound, filled one of its bays and flowed up to 10 miles underwater. Most big volcanic landslides do not transform at once: Mount St. Helens' 0.6-cubic-mile debris avalanche of 1980 stopped, then over hours its waterlogged parts liquefied into the North Fork Toutle River lahar — 180 million cubic yards, the eruption's largest — which flowed about 60 miles to the Columbia River, filling the channel and disrupting international shipping for months. Smaller, rain- or snowmelt-triggered landslides often do become lahars, as at San Vicente, El Salvador (2009), and Izu Oshima, Japan (2013).

3. Floods that pick up sediment. Some lahars begin as sudden floods — from crater lakes, lakes dammed by volcanic debris, eruption meltwater or glaciers — that swallow enough sediment to become slurries.

• The largest known lahar at Mount St. Helens, about 2,500 years ago, came from a breached debris-dammed lake: about 1 billion cubic yards. After 1980, when the debris avalanche dammed another lake, the U.S. Army Corps of Engineers built a drainage tunnel to keep it from happening again.
• In 1982, an explosion in St. Helens' crater melted snow into a temporary lake whose release became a 20-million-cubic-yard lahar that ran at least 50 miles, damaging a small sediment dam.
• Kelut, Indonesia, blasts water out of its crater lake in eruptions: lahars killed more than 5,000 people in 1919 and 211 in 1966. Tunnels now keep the lake small.
• At Ruapehu, New Zealand, the crater lake has breached its ash dam between eruptions; in 1953 the lahar destroyed a railway bridge minutes before a passenger train plunged into the valley, killing 151. An automatic warning system now stops trains.
• Glacier outburst floods can make lahars large, as in Iceland, or small but damaging, as often at Mount Rainier (typically tens of thousands of cubic yards, travelling a few miles).

A lahar at Mount St. Helens made by crater snow melting in a small explosion, March 1982. USGS photo by T. Casadevall.

4. Rain on fresh ash. A blanket of fine ash stops the ground soaking up rain, so runoff erodes ash and loose debris and can become lahars, especially where rain is heavy. Each is modest, but they can recur for years and choke channels, sending rivers off course. After Mount Pinatubo (1991) they destroyed several villages over many years; Irazú, Costa Rica, produced scores in the 1960s; and at Chaitén, Chile, less than an inch of rain after days of ashfall filled a river channel with 16 feet of sediment within 24 hours, forcing the river through a town 6 miles downstream.

Reducing the risk

Communities, transport, power and communications, water supplies and other infrastructure downstream of volcanoes are at risk, and even small lahars are dangerous because they are fast, carry rocks and wood, and can come without warning. The best defences are to:

• know which areas are most vulnerable and what conditions can trigger lahars;
• have plans and preparations in place beforehand, and activate them when a volcano becomes restless;
• know what to do if a lahar comes — above all, get to high ground quickly.

Many people living in valleys below volcanoes are unaware of the danger. The USGS monitors the volcanic regions of the Pacific Northwest, Alaska, California, Hawaii and Yellowstone, prepares lahar-hazard maps, and works with community leaders and officials — and its monitoring aims to catch early signs of unrest and warn the public of eruptions and lahars.

Sources

• Major, J.J., Pierson, T.C., and Vallance, J.W., 2018, Lahar—River of volcanic mud and debris: U.S. Geological Survey Fact Sheet 2018–3024. https://doi.org/10.3133/fs20183024 · https://pubs.usgs.gov/publication/fs20183024
• The photographs come from the fact sheet's PDF; others there are credited to researchers and agencies outside the USGS and are not reproduced.
• Rewritten in hubnx's own words.