Dear Matt,

When I think about volcanoes, I picture molten magma deep inside the Earth. Or burning hot lava pouring down the side of a mountain. But you’re right that there have been news reports that mention cold lava.

I talked about what that could be with my friend Katie Cooper. She’s a geologist at Washington State University.

She told me the news may be using “cold lava” to describe a lahar. That’s a mix of water and rocky debris that sometimes whooshes down the side of a volcano. It’s also called a debris flow or a volcanic mud flow.

“A lahar is a slurry of water and a bunch of debris,” Cooper said. “And what makes it dangerous is that, when you mix debris with water, it becomes a lot more mobile. Then it can move really, really fast. It can be incredibly devastating.”

In fact, a lahar rushing down a steep slope can go faster than 125 miles per hour. Experts say it looks like a bunch of wet concrete pouring down the side of a volcano. As it moves, it picks up more debris. That could be rocks, soil, plant matter or even pieces of buildings. The lahar can grow to be more than 10 times bigger than its original size. Once it moves from a steep area to a flatter area, it slows down and becomes smaller again.

A damaged bridge is in the background. In the foreground are large rocks and debris.
A lahar destroyed this bridge in Washington during the eruption of Mount St. Helens on May 18, 1980. Image: Richard Waitt, USGS

Lahars mostly happen on, or near, stratovolcanoes. Those are cone-shaped volcanoes which form when layers of lava and ash erupt and then harden. That happens over and over for thousands of years until a volcano forms.

A lahar can happen during an eruption. Maybe the eruption melts snow or ice on the volcano. Or it could throw out water from a lake on the volcano. If that water mixes with super-hot debris flowing from the volcano, it could form a lahar.

A lahar can also happen without an eruption. Sometimes rain mixes with debris left over from an earlier eruption. Or a bunch of debris slides into a stream and makes a dam. Any extra water can mix with debris and go sliding down the volcano’s slope.

Cooper reminded me that there are stratovolcanoes where Cooper and I live in the Pacific Northwest. They’re part of the Cascade Range. It stretches from the bottom of Canada through Washington, Oregon and part of California. The range includes thousands of volcano vents and 20 major volcanoes. There’s evidence of past lahars here.

“We should keep in mind that we live with active volcanoes,” Cooper said. “It’s good to remember that we’re on a tectonic plate boundary here in the Pacific Northwest.”

A plate boundary is where two pieces of the Earth’s crust slide together. Most volcanoes are on plate boundaries.

But don’t worry. Scientists monitor the volcanoes in the Cascades and all over the world. Their goal is to keep us safe and learn more about the Earth we call home.

You could say their work totally rocks.


Dr. Universe