Avalanches, Landslides, and Mudslides: Definitions and Causes

The noise and chaos evident in videos of avalanches, landslides, and mudslides make the lethality of all three types of events clear.

Mudslides are a type of landslide, but there are many subtle differences between avalanches and landslides. Common to all three is the inexorable pull of gravity. Learn what these natural disasters are and what causes them.

Avalanche Definition

An avalanche is a rapid and violent movement down a steep slope of a large mass of ice and snow.

When the falling matter comes to a stop, it quickly becomes solid, depriving people and animals stuck underneath the surface of the debris of air and leaving them with little way to escape. The Federal Emergency Management Agency (FEMA) has called asphyxia the major cause of avalanche deaths.

Guinness World Records has pegged the avalanche resulting from the May 18, 1980, Mount St. Helens volcanic eruption as the fastest on record, with debris moving at 250 miles per hour.

According to the National Snow and Ice Data Center (NSIDC) at the University of Wisconsin Eau Claire, “a large avalanche in North America might release 300,000 cubic yards of snow. That's the equivalent of 20 football fields filled 10 feet deep with snow.”

NSIDC reports that, while most avalanche fatalities are in the winter, they also happen in the spring as temperatures warm. 

Landslide Definition

The term landslide encompasses the term mudslide. Land in the form of boulders, rocks, dirt, mud, and accompanying vegetation either collapses with a slope or slides or tumbles down it.

Landslides can progress quickly like avalanches or move fairly slowly. Not all landslides involve water, and they don’t always move through channels in the land. Mudslides do require water (mud is by definition sodden) and they usually move through some sort of channel like a stream bed or a canyon.

The USGS says that landslides occur in all 50 states. Every year they count for 25-50 deaths domestically and thousands worldwide. 

What Causes an Avalanche?

Avalanches can be initiated by earthquakes or by the failure of the mountain coated with thick ice and snow. By and large, however, avalanches are caused by characteristics of the snow itself. And, according to a 2004 study by researchers in Iceland and Canada published in the peer-reviewed journal Natural Hazards, back-country skiers who decide badly what to do about the snow inadvertently initiate many avalanches. 

The NSIDC has listed several causes of avalanches:

• New, heavy snow can cause a slab of ice and snow to break away and tumble down a mountain. Indeed, avalanches are most likely to occur in the 24 hours after a heavy snowfall.
• Snow that has been partially melted by unseasonably warm weather can become unstable. 
• Snow that has warmed on a sunny day might turn icy when temperatures go below freezing at night. If a new, thick layer of snow falls on top of ice, the entire layer might slide down the mountain.
• Rain turning to ice can also provide nothing to which the next layer of snow can reliably bind. 
• Very wet snow is by nature lubricated with water and can easily slide. 
• Snow that has been blown uphill or over a mountain peak from windward to leeward side can create an unstable mass that is not well supported by the snow that is lower on the gradient. 
• Snow masses resting on ice that overhangs mountain cornices are vulnerable to collapse.

What Causes a Landslide?

Like avalanches, landslides can be sparked by geological events such as volcanoes and earthquakes. They are also easily triggered by floods and by heavy rains that follow droughts.

These are some other causes of landslides and mudslides:

• The USGS has noted that, in Alaska, global warming and the melting of permafrost can destabilize mountainsides and contribute to landslides.
• Deforestation contributes to landslides when no roots are left to hold mountain soil in place.
• Wildfires make slopes more susceptible to landslides in at least three ways. First, depending on how intense the fire was, the soil can bake into a hard crust that repels water. When rain falls, debris and ash might form a sludge that slides down a hillside. Second, wildfires leave the ground barren of vegetation that would normally absorb rainfall and reduce runoff. Without that vegetation, slopes are more susceptible to buckling and sliding. Third, trees and brush hold soil in place. Once they are burnt away, steep slopes become vulnerable to sliding. 
• According to a February 2021 publication of the USGS, post-wildfire landslides are becoming more common in California, averaging at about one per year.

The Deadliest Avalanche in Recorded History

Sometimes real avalanches and real landslides happen at the same time. This seems to be what occurred in 1970 on the north coast of Peru on Mount Huascaran, which is the highest peak in that country.

As described in its 1970 Preliminary Report on the Geologic Events Associated with the May 31, 1970, Peru Earthquake, the United States Geological Survey (USGS) reported that an earthquake measuring 7.7 on the Richter scale caused a slab of ice and rock that was about 2,600 feet wide and 18,000-21,000 feet high to break away from a mountain wall. More than 82 million cubic feet of snow, ice, and rock came roaring down.

At the mountain’s base were a few populated areas including the provincial capital Yungay with a population of about 18,500 and the smaller town of Ranrahirca. Both disappeared under about 165 feet of debris.

In Yungay, only 320 bodies were found, while 15,000 inhabitants and several thousand Sunday visitors were declared missing and presumed dead. In Ranrahirca, all but about 50 of the roughly 1,850 inhabitants died. 

Normally, when an avalanche comes to a stop, the ice and snow quickly “set” into something as solid as concrete within two or three seconds. This combined avalanche and landslide at Mount Huascaran, however, moved a lot of tiny rock particles with it.

As ice melted from friction and other dynamic forces, the dirt became liquified. By the time it hit the bottom of the mountain it was thick mud. The debris was squishy sludge for about eight days. In Yungay, all that peeked out of it were the tops of a few palm trees and part of a cathedral. Among the debris at the mountain’s bottom were two large rocks. One weighed 14,000-tons. The other weighed 7,000 tons.

In 2009, a team of scientists from Canada, Florida, and Peru -re-examined the human impact of the Mount Huascaran avalanche and landslide. Publishing in the peer-reviewed journal Engineering Geology, they claimed that, because of name confusion between the city of Yungay and the province of Yungay, the 1970 USGS report had counted some deaths twice. The new team estimated the total number of casualties at closer to 6,000.

Given that most of the bodies were never retrieved, the correct casualty count will probably forever be beyond reach. Fifty years after the calamity, however, Yungay is again a thriving provincial capital. Ranrahirca, on the other hand, is only described in the past tense.