Lahar Mudflows

In 1985, a volcano in Colombia called Nevado del Ruiz erupted — but the eruption itself wasn't what caused most of the destruction. It melted the mountain's icy snowcap, and the meltwater mixed with volcanic ash and loose rock to create a churning wall of mud and debris that raced down the mountain at highway speeds. That river of mud, called a lahar, buried the town of Armero and took more than 20,000 lives in one night. A lahar is one of the most dangerous things a volcano can produce, and it doesn't even need lava to do it.

- What a lahar actually is and what it's made of - Why lahars can be more deadly than lava - How fast lahars move and how far they can travel - What scientists do to warn people before one arrives

The word 'lahar' comes from Indonesian, a country with many active volcanoes. A lahar is a mixture of volcanic rock, ash, and water that flows like wet concrete down a volcano's slopes and valleys. It can form in several ways: an eruption melting snow or ice on top of the volcano (like at Nevado del Ruiz), heavy rainfall washing loose volcanic ash off a mountainside even years after an eruption, or a crater lake breaking through its walls. The mixture is often thick enough to carry huge boulders and even uproot trees, but fluid enough to flow for many miles.

Lahars can travel at speeds ranging from about 20 to over 60 miles per hour (30-100 km/h) depending on how steep the slope is and how much water is involved, and they can travel over 50 miles (80 km) from the volcano, growing bigger as they pick up more mud, rock, and debris along riverbeds — this is why a town doesn't have to be right next to a volcano to be in danger. As a lahar moves further from the volcano, it typically slows down but can also spread out and grow in volume, since it keeps scouring material out of the valley it travels through.

After the Armero disaster, volcanologists built lahar-specific warning systems. Acoustic flow monitors (sensors that detect the ground vibration and rumbling sound of an approaching lahar) are placed in valleys near dangerous volcanoes like Mount Rainier in Washington State, which has glaciers that could melt during an eruption. These sensors can give downstream communities minutes to tens of minutes of warning — not a lot of time, but often enough to reach higher ground if evacuation routes are practiced in advance.