What Causes Avalanches and Mudslides?

This rockfall followed a mudslide after heavy rainfall on Gran Canaria, Spain. (Photo: Thomas Dekier/Shutterstock)

Watching an avalanche from afar or via video, it's hard not to see the tremendous release of snow as beautiful in its own way. But avalanches, and their seasonal cousins, landslides — can be incredibly deadly. Even when they don't kill, they can cause massive damage to infrastructure like roads, as well as buildings and cars that get caught up in the flow.

And in the coming years, we're likely to see more of both of them.

The winter of 2019 saw greater-than-normal avalanche dangers throughout the Western U.S., especially in Colorado. Avalanches were "running bigger than the last 20 to 30 years," Spencer Logan, an avalanche forecaster with the Colorado Avalanche Information Center, told CNN. Some of those avalanches were larger than any in the last 50 years.

And the past years have also been challenging for Californians due to mudslides and landslides, which have also become more intense. That's likely due to a combination of wildfire-ravaged landscapes that have less vegetation to hold everything in place, as well as heavier rains from bigger storms. Logging trees (and their supportive roots) contributed to the deadly landslide that killed 44 people in Washington state in 2014.

How they happen

In both cases, it's all about physics, as well as local conditions.

Snow slides off mountains when certain parameters are met. First, you need a solid snowfall. Then if temperatures are warm enough, the surface of the snow melts and then freezes again, creating an icy layer when the temperatures drop again. When it snows again on top of this fragile base layer — also known as a "plane of weakness" — it can slide off when the weight of the snow becomes too heavy for the steepness of the slope. Than an avalanche happens.

Avalanches and landslides are natural phenomena; they occurred long before humans came along, as fossil evidence shows. And of course there are landslides on Mars, a planet where we have yet to set foot.

Climate change plays a part

But climate change and the extreme weather it has brought, is also set to increase both size and frequency of landslides and avalanches.

In the case of landslides, it's because longer periods of drought, followed by heavy rains (both linked to climate change), are a recipe for a slide. When the ground is very dry, it doesn't absorb water as well, and heavy rains that happen quickly in storm events don't have time to absorb into the soil. So there's surface-soil liquification, which can then undermine larger rocks or earth that sit on top of it, creating a landslide.

Avalanches are more likely to occur when temperatures in snowy areas rise above freezing. That icy layer forms, allowing snow to slide off the sides of mountains. Colorado just experienced its third-warmest winter ever, and that's following other recent warming winters — meaning more avalanche potential in a place where they occurred naturally already.