A tsunami of mud

A tsunami of mud


When people talk about floods, the most feared threats are tsunamis from the sea, which rarely exceed a few meters in height. But what about sudden «tsunamis» far from any water, where the «wave» reaches 140 meters in height? We will talk about these today.

How to make a smoothie out of mountains

Imagine a mountainous area, with a sudden powerful downpour. The water penetrates deeply into the ground, reduces friction between the rocks, liquefying the layers on which the rocks close to the surface rest.

Gravity, as always, continues to pull downward. Sooner or later, the water-laden earth fails, the surface layer of rocks slips, and the mud and rock start sliding down the slope.

Consequences of a mudslide near the village of Obchaka in Tajikistan. Photo: N.Ischuk

A chain reaction resembling a snowball occurs. The mass of mud behaves like a liquid—it changes shape and grows in size, collecting more and more rock along the way. It continuously mixes and grinds everything inside, due to friction with the slope.

If a river is encountered along the way, the flow only speeds up, as the extra water reduces the viscosity of the moving mass. And if buildings and cars are encountered, they are easily torn off the ground, and they too are blended into this giant natural mixer. 

The described natural phenomenon is called a mudslide, or a mudflow. If a landslide is a «dry» cascade of rocks, a mudslide can be considered the «wet» analog, resembling an avalanche.

Brands of natural mixers

The speed of mudslides ranges from 3 meters per minute to 5 meters per second. Rain, snowmelt, groundwater breakthroughs, and even careless drilling, can trigger mudslides. Often, mudslides start as landslides that encounter water along the way. 

There are 2 other distinct types of mudslides based on origin—lahars and jökulhlaups.

Lahars are considered to be the most destructive type of mudslides, as they occur after a volcanic eruption, and contain glowing volcanic ash and hot gases. Such mudslides are «fed» by water from glaciers and mountain snow, and can reach up to 140 meters in depth.

A lahar trail after the 1982 eruption of the St. Helens volcano in the United States. Photo: Tom Casadevall, USGS

Jökulhlaups (the word imported from Icelandic) are outbursts of underground lakes and meltwater trapped beneath the Earth’s glaciers and ice caps. They occur after water accumulation under a glacier, when the ice is melted by geothermal springs from below, and the meltwater creates more and more pressure on the ice walls around it. Jökulhlaups can also be caused by melting and collapse of the glacier itself, and hypothetically, by climate change, because glaciers affected by the process of global warming could melt faster.

Where is there a threat of mudslides?

As you have already guessed, the phenomenon should be feared in places where landslides, eruptions and melting of glaciers are also possible. In general, in the mountains and hills, where there is little vegetation, as the roots of plants help to hold the ground in one place. Also, mudslides can occur in earthquake-prone areas, as earthquakes contribute to the occurence of landslides and mudslides.

Given the role of vegetation, wildfires can also be a factor in increasing the threat of mudslides. This is what happened in California after the 2021 wildfires, where increased mudslides are damaging homes on hillsides near Los Angeles.

The aftermath of a mudslide after a rainstorm in Dana Point, Calif. Mudslides are one of the natural phenomena (on par with glaciers) that are changing the planet’s landscape. Photo: Allen J. Schaben / Los Angeles Times / Getty Images

Among the biggest mudslide-related tragedies was the eruption of Colombia’s Nevado del Ruiz volcano in 1985. Back then, the lahar caused the destruction of the town of Armero, and the deaths of at least 20,000 people.

The consequences of mudslides can be made even worse by non-compliance with building codes. In 2013, for example, when a mudslide in the Indian state of Uttarakhand led to the death of six thousand people, the investigation found that the tourist infrastructure was built on obviously unsuitable soil. 

It turns out that it is almost impossible to predict mudslides, but it is possible to avoid casualties. Residents in mudslide-prone areas should have evacuation plans and supplies of essential items ready in advance. And authorities should provide timely warnings if heavy rain or volcanic activity is expected, and ensure that building codes are strictly enforced.

Text: Jason Bright, a journalist, and a traveler

Cover photo: Mike Dierken / Unsplash


Read more:

How a tsunami appears

What is quicksand and how to get out of it

What causes earthquakes and how do they measured

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