Dangerous volcanoes are everywhere. But, how dangerous is dangerous or deadliest? Recently, I tried to tackle that notion and codify some ideas about what makes one volcano dangerous when it erupts, while another is merely spectacular. It turns out you can condense a lot of the hazard into a few main factors—and many of them depend on how many people make their homes near these volcanoes.
To start building my list of most dangerous volcanoes, I mined the Smithsonian Institute/USGS Global Volcanism Program database. I found 63 volcanoes with over 10 million people living within 100 kilometers (62 miles)—and 14 of those have erupted in the last 200 years. Gede-Pangrango in Indonesia has over 40 million people living within 100 kilometers of the volcano. And 32 of the 63 volcanoes with 10 million people living within 100 kilometers are in Indonesia.
If we expand this and think about places where at least 1 million people live within 30 kilometers (18 miles), Indonesia is far and away the top, with more volcanoes with 1+ million living within 30 kilometers than the next four countries combined: Philippines, Guatemala, El Salvador, and Mexico.
Beyond those population figures, the risk calculation gets trickier: Volcanoes aren’t as binary as “snake venomous, snake not venomous.” To simplify things, I decided to identify the main factors that influence immediate danger from a volcano. One thing many people don’t know: Deaths from volcanic eruptions are low compared to deaths from the consequences of large eruptions (like cooler global temperatures from volcanic aerosols). Here, I’m talking about rapid impacts of eruptions (minutes to days).
I decided that there are four main factors for deciding how dangerous a volcano might be:
Population: We know this one already. I looked at the number of people living within 5, 10, 30 and 100 kilometers of the volcano, then weighted the numbers. As you go out from the volcano, the hazards generally decline. A large population further away might only experience some ash fall or mudflow confined to a river valley—though those can be very deadly if you’re not careful. Populations closer to the volcano bear the full brunt of pyroclastic flows (likely the biggest killer), heavy ash fall, and lava flows (although they aren’t that deadly).
Type of magma erupted: Different types of magma produce different types of eruptions. Sticky, silica-rich magma like rhyolite and dacite can produce explosive eruptions (bad) while silica-poor magma like basalt tends towards lava flows (less bad). So, a volcano that tends to erupt rhyolite and dacite gets a higher score and the converse for basalt.
Type of volcano: Here, I’m getting at the style of eruption as well, but looking at the form of the volcano. Most typically, volcanoes that erupt sticky lavas like andesite and dacite form stratovolcanoes. Sometimes you also get calderas from big explosive eruptions. When you have water involved, you could get more explosive eruptions from silica-poor magma. More points if you have a volcanic form that implies explosivity, fewer points if it suggest more effusive eruptions of lava flows.
How often do eruptions occur: Now, this could be the hardest variable to factor in. You would think that the more a volcano erupts, the more dangerous it is. But this is only kind of true. More eruptions is less good, but they tend to be smaller if there are more. Likewise, fewer eruptions may mean that the eruptions could be larger—like Pinatubo in the Philippines, which hadn’t erupted since 1450 CE until its massive 1991 blast. So, I looked in the Global Volcanism Program database for how many eruptions a volcano has had since 1500 CE (though that record is woefully incomplete for many volcanoes).
The “Big Bang” bonus: This last factor gets at the Pinatubo problem. If I found a caldera-forming eruption in the volcano’s history or a VEI 5+ eruption (i.e., a big bang), then I gave the volcano a few points extra. Helps all those volcanoes that might not erupt often, but have the capability of a very large eruption. However, you never know when a volcano that hasn’t had a big eruption could generate one.
You might have noticed that I ignored the monitoring factor—how carefully groups are keeping an eye on a potentially explosive volcano. It is really hard to quantify, so I decided to ignore it. Hopefully we can assume that none of these volcanoes near large populations would have all their signs of unrest ignored/missed.
Combine these factors in a fancy equation and voila, I generated a list of “dangerous” volcanoes. That list will come in the next post, but just as a teaser, here are some volcanoes that people might be surprised did not make the top 20: Yellowstone (not even close), Hood, Fuji, Rainier, Pinatubo, Nevado del Ruiz, and Cotopaxi. What made it? I’ll reveal that soon.