Lahars, Part 1: The Long-Distance Killer

This isn’t a mountain stream overflowing with snow melt. It might not even be cold. Meet the lahar, one of a volcano’s favorite long-distance murder tools.

It’s a mix of water and volcanic ash — basically, a mudflow. This one, photographed last week at La Soufriere St. Vincent by volcanologist Dr. Richard Robertson, occurred when rain fell onto the volcano’s steep, ash-covered slopes.

Mud isn’t scary. But you don’t want to step into this kind of mudflow. If just six inches of running water can knock down an adult (National Weather Service), imagine what will happen when that water is loaded with powdered rock.

Lahars, like this one in Peru, are perfectly capable of carrying whole rocks and boulders, too.

It’s an awful way to go, especially if fresh ash has transferred its heat to the water. And lahars are responsible for almost a third of the historical deaths from the direct effects of an eruption. (Brown et al.)

In a way, these people are lucky because the lahar is so watered down: this was uploaded in 2015 and the volcano (Mount Pinatubo, in the Philippines) erupted in 1991. Rain has recycled that ash through many secondary lahars, like this one. But this is still sad to watch because they cannot accept that it’s impossible to save their homes in this lahar-prone drainage, far from Pinatubo. (See note at YouTube, which includes translation of interview.)

Volcanic hazards

Let’s assume, for the sake of a general example, that you don’t live near an active volcano and haven’t faced this in the real world yet.

An eruption happens, say, at Vesuvius and you’re in the neighborhood.

Once that initial moment of sublime awe (Perret, page 46) passes, what direct hazards from the volcano make you fear for your life? (Let’s overlook other very real but indirect threats like regional panic, etc.)

Everybody who isn’t close enough to be dodging projectiles and explosive blasts from the vent worries about pyroclastic flows and rightly so. These are basically unsurvivable and can run out for ten miles or more from the vent.

Crunching the numbers, as Brown et al. did, shows a recorded death toll of almost 60,000 people from pyroclastic flows. Things like lava, ash fall, and volcanic gas don’t begin to approach that terrible number.

Pyroclastic flows are the leading cause of death within six miles or so of an erupting volcano. However, the high-impact events are few and far between.

Almost half of these (28,000) are from a single tragedy (the destruction of Saint-Pierre, Martinique on May 8, 1902, during Mount Pelee’s eruption).

That one eruption is Number Two on the list of top seven highest death tolls in Brown et al. (Table 3).

At the top of the list is the 1883 eruption of Krakatoa (Krakatau is today’s name), sitting out in the Sunda Strait (36,000 recorded deaths from tsunami).

The 1815 Tambora blast had deadly indirect effects by changing global climate, but “only” 12,000 deaths are recorded from its pyroclastic flows, ashfall, and tsunami (Brown et al. note that the actual totals for these two colonial-era disasters were probably much higher).

Tambora has the fourth highest documented toll, and the fifth is Japan’s worst volcanic disaster — a collapse at Unzen in 1792 that caused a tsunami in which 10,139 people died.

Overall, Brown et al. counted 56,822 deaths from tsunami, most of them fairly close to the volcano (though farther than the typical pyroclastic flow would reach).

Surprisingly, lahars are the leading cause of death nine miles or more from a volcano. Per Brown et al., there are reports of a total of 56,315 people dying in lahars — almost as many recorded victims as in tsunami.

Yet few laypeople without a fire mountain in the neighborhood have ever heard of a lahar! Those who have tend to underestimate the impact: it’s “just” a mudflow.

Those who do live near volcanoes know better.

At least eighteen Indonesians died when a tropical cyclone in early April 2021 made landfall near erupting Ile Lewolotok on Lembata Island.

One of the top-seven death tolls is very modern: the loss of 24,000 lives from a lahar at Colombia’s Nevado del Ruiz Volcano in 1985.

This is Number Three, even worse than the recorded direct losses from Tambora in 1815.

Next week, we’ll look at that and the other two lahar tragedies, both of them at Kelut Volcano in Indonesia (it has a crater lake). Then we will start to check out some of the many ways that humanity is fighting this long-distance volcanic killer.

Featured image: Dr. Richard Robinson, lahar on La Soufriere St. Vincent, April 29, 2021.


Brown, S. K.; Jenkins, S. F.; Sparks, R. S. J.; Odbert, H.; and Auker, M. R. 2017. Volcanic fatalities database: analysis of volcanic threat with distance and victim classification. Journal of Applied Volcanology, 6(1): 1-20.

National Weather Service. 2021. Turn around, don’t drown. Last accessed April 30, 2021.

Perret, F. A. 1924. The Vesuvius eruption of 1906: study of a volcanic cycle (No. 339). Carnegie institution of Washington. Retrieved from

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