Mud Flows from Volcanoes

This article on the Popocatepetl volcano discusses an often overlooked threat from large volcanoes--mud floes. From the article:

Mudflows, also called debris flows and lahars (an Indonesian word), occur when water suddenly mixes with volcanic ash near a volcano's summit. The water can come from a multitude of sources — an explosive eruption that melts a mountaintop glacier, a sudden deluge of rain — with equally devastating results.

They are a big ongoing hazard, said Ben Andrews, a research geologist at the Smithsonian's Global Vulcanism Program.

"Thinking about them as mud is technically accurate, but conceptually it's more like a wall of cement flowing," he told OurAmazingPlanet, "and it destroys pretty much everything in its path."

As a flow rushes down a mountainside, it typically picks up large boulders and anything else that lies in its way.

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Both Sheridan and Andrews pointed to a 1985 eruption of Colombia's Nevado del Ruiz volcano to illustrate the insidious danger posed by debris flows.

"That eruption was fairly small," Andrews said. Yet it melted glaciers atop the mountain, producing a moving wall of debris that thundered down. The town of Armero — a full 45 miles (74 km) from Nevado del Ruiz — was essentially wiped away a full two hours after the eruption.

One girl's horrific case came to symbolize the tragedy. Thirteen-year-old Omayra Sanchez was buried up to her neck and hands in muck. For three days, volunteers struggled to free her as water slowly rose, but Sanchez died, held fast by the debris around her. In total, the 1985 mudflow killed more than 23,000 people. Such tragedies are avoidable. "Mudflows can be detected and there can be a warning of up to a half-hour in advance," Sheridan said.

"It's definitely an escapable hazard if there's a warning," Andrews said. "These flows will knock down pretty much everything in their path, but they're restricted to valleys. And they're not moving at hundreds of miles per hour, they're moving at tens of miles per hour."

A good reason to have a packed "bug-out-bag" packed and ready if you live or are traveling near an active or semi-active volcano.