Lahars and Mudflows – Hidden Dangers of Volcanoes

What is a Lahar?

A lahar is a volcanic mudflow, which can create a massive flash-flood, which can sweep down river basins and valleys. Lahars typically form when volcanic ash and debris mixes with water, subsequently mobilizing into a cement-like flow that can carry boulders, trees, and sweep away anything in its path.

Lahars can flow extremely long distances at a very fast pace, which is a big reason why they can be so dangerous. Similar to Lahars are Jokulhaups, which are flash-floods caused by glacial water reservoirs suddenly releasing. Jokulhaups are often associated with volcanic activity, most commonly seen in Iceland.

Why Lahars and Mudflows Matter, And Deserve Proper Attention

When people think of volcanoes, they commonly associate the destructive aspect of them with the explosive eruptions and lava flows. While volcanic explosions are decidedly dangerous and can be incredibly destructive, a large portion of life-loss coming from volcanoes in the last 500 years has not come directly from the pyroclastic flows or explosions, but from secondary effects, often involving the interaction of water with the volcanic explosion.

Read more about how water influences volcanic eruptions

The other primary source of death or life-loss around volcanic eruptions has come from famine and crop failure associated with climatic effects (such as those seen during the Laki eruption) or direct crop destruction (such as occurred after the Tambora eruption).

Volcanic Disasters Influenced By Water Interactions

Nevado del Ruiz Eruption – 23,000 deaths in 1985 due to Lahars

Mt. Kelud Eruptions – 15,000+ deaths in multiple eruptions (1586 and 1919) due to Lahars

Krakatoa Eruption – 36,000+ deaths, mostly caused by volcanically influenced Tsunami in 1883

Mt. Unzen Eruption – 15,000+ deaths, mostly caused by volcanically influenced tsunami in 1792

There are a variety of other disasters that have occurred in the past 500 or so years that have also been influenced by volcanic mudflows or tsunamis, but these are the biggest disasters. And while these disasters were enormous and terrible, there are many very realistic scenarios that could be far worse these days due to population growth in regions surrounding certain volcanoes.

The main takeaway here, is that when discussing mitigation or which volcanoes are the most dangerous in the world, we absolutely need to account for the potential of a volcano to produce lahars into populated areas, or the volcano to produce a tsunami.

Lahars and Mudflows, a Recipe for Disaster

In my most recent post on the most dangerous volcanoes in the world, there are a variety of volcanoes that may not fit some of the pre-conceived ideas of what a traditionally dangerous volcano would be.

For example: Nevado Cayambe, a stratovolcano in Ecuador hasn’t had an eruption larger than a VEI-4 in the last 10,000 years. Additionally, there are only 1000 or so people living within 10km, and around 100,000 on the edges of 30km (according to the GVP).

So how is it that this volcano made my list for most dangerous in the world? A more detailed look at the volcano will explain how Lahars could cause major disaster here.

Cayambe Volcano in Ecuador - a Volcanic Lahar Risk
The red highlight tracks riverbeds and drainage from Cayambe, running through highly populated regions of Ecuador.

As we can see in the above graphic, while the area immediately surrounding Cayambe is not highly populated, the major drainage areas, which a lahar would flow into ARE highly populated. While these drainage regions are much further away, even mid-sized lahars can travel incredible distances of over 75+ kilometers. In Ecuador, the glacier capped volcanoes have created many lahars that have traversed well over 100 kilometers, reaching the pacific ocean.

The Two Risks For Lahar and Mudflow Disasters

The risk factors for a lahar are really quite simple. All you need are populated regions in volcanic drainage basins, and a large source of water (summit caldera lake, or summit glacier) on top of the volcano itself.

Now, volcanoes of all sizes with or without glaciers have been known to cause lahars. But the presence of a large glacier on top of a volcano greatly increases the risk of a large mudflow.

As for population within drainage basins, the larger the amount of people living within river basins, and the more constricted that the regions to which a lahar can flow into, the more risk there will be for life-loss.

Glaciers, Lakes, and Lahars

Obviously, glaciers melting can create lahars. But one of the big reasons that Lahars are so dangerous is that when you have a large glacier on top of a volcano, you do not need a large eruption to create a major disaster. In fact, of the disasters mentioned earlier at Kelud and Nevado Del Ruiz, none of the eruptions were larger than a VEI-4.

Normally, VEI-4 eruptions don’t cause much harm, especially when there is any type of mitigation in place. We even saw a VEI-4 eruption at Merapi in 2010, a volcano with a huge population surrounding it of over a million people, which only resulted in a very small amount of life-loss.  This eruption even caused some minor lahars (the video above is from that eruption), but they can’t compare to what could potentially occur at a glacier-clad volcano.

For this reason, volcanoes with summit glaciers or summit caldera lakes near populated regions should be heavily monitored, as even an eruption as small as a VEI-3 can send a very large lahar into a populated region with a timespan of less than an hour. 

Volcanoes That Have a High Lahar Risk

Fortunately, volcanoes that feature the above criteria with summit glaciers in a populated region aren’t many, but they should at least be accounted for. For the most part, these volcanoes are concentrated in a few regions: Northern Andes, and the Cascade Range.  Ecuador is the region of particular concern, with many large stratovolcanoes that feature summit glaciers and holocene activity.

Volcanoes colored red below are especially risky in terms of glacier size, volcanic activity, and populated regions.

Cascade Range:

  • Mt Baker – USA
  • Glacier Peak – USA
  • Mt. Rainier – USA
  • Mt. Hood – USA
  • Garibaldi – Canada

Northern Andes:

  • Nevado Del Ruiz – Colombia
  • Nevado Tolima – Colombia
  • Nevado del Huila – Colombia
  • Cayambe – Ecuador
  • Antisana – Ecuador
  • Cotopaxi – Ecuador
  • Illiniza – Ecuador
  • Chimborazo – Ecuador

Southern Andes:

  • Villarica – Chile
  • Sollipulli – Chile
  • Michinmahuida – Chile

Europe / Western Asia:

  • Mt. Elbrus – Russia
  • Mt. Ararat – Turkey
  • Mt. Damavand – Iran

Japan / Asia

  • Nantai – Japan
  • Numazawa – Japan
  • Towada – Japan


Lahars are extremely dangerous, and can occur very quickly in highly populated regions. They have been responsible for major disasters in the past from volcanoes that are not glacier clad and do not have large population centers nearby. If a glacier-topped volcano were to erupt in a populated region, this could be a major disaster if not properly prepared for.


Leave a Reply