A recent PBS documentary identified Kilauea, on the island of Hawaii, as “The Most Dangerous Volcano in the World.” A curious choice, in my opinion, for any rating of a volcano's danger must take into account both the intrinsic hazard and the number of lives at risk.
Eruptions of Kilauea are certainly spectacular. Its cherry-red lava flows and roaring lava fountains are impressive, especially when seen at night. But relatively few people have been killed by Kilauea's lava, because it is usually possible for people to get out of its path. The one major fatality at Kilauea was caused by a steam explosion. In 1794, lava-heated groundwater killed a troop of soldiers marching past the caldera. But such explosions at Kilauea are rare. Therefore, Hawaiian volcanism is a fairly gentle sort of thing.
All volcanoes pose a danger to volcanologists, and I personally rate Grimsvatn in Iceland as most hazardous. Why? Because I fell into its caldera barely escaping with my life (see sidebar, “Bill & The Volcano”.) The volcano lies buried under an ice sheet 500 meters thick. Its eruptions can melt enormous volumes of ice. In 1996, gigantic floods were caused by an eruption that liquefied six cubic kilometers of solid water. Fortunate for Icelanders, the floods make their way from the volcano to the sea through an uninhabited region.

The Three Big Ones
The last three volcanic eruptions to cause major loss of life were Krakatoa, Indonesia, where 32,000 were killed in 1883; Mt. Pelee, Martinique, where 29,000 were killed in 1902; and Nevada del Ruiz, Colombia, where 23,000 were killed in 1985. Fiery lava was not the culprit in any of these disasters.
Krakatoa, a small island, exploded catastrophically. The resulting sea wave washed whole villages on nearby Java and Sumatra away. A cloud of super-heated rock particles and poisonous gases known as a pyroclastic eruption rushed down the volcano's slope destroying the town of Saint-Pierre on Martinique. Unlike lava, it travels so fast that it cannot be outrun.
Like Grimsvatn, Nevada del Ruiz in the Andes Mountains is also covered by a glacier. A small eruption melted part of this ice, and the melt water produced a mud flow that inundated the town of Armero, located in a valley below the volcano. Interestingly, none of these three volcanoes had been active in the century prior to their eruption, although all had some historic activity.
The eruptions of Ruiz and Pelee were moderate-to-large in size, not gigantic. Their effects were confined to a ten-to-twenty kilometer radius around the volcano. The large death toll was the result of a moderate hazard combined with a moderately sized town.

Recipe for Risk
The first criterion for identifying the Most Dangerous Volcano in the World is to decide whether an eruption would endanger a nearby population center. To threaten a population, the geography must be right for the potential hazard; the mudflow, pyroclastic cloud, or lava, must be able to travel from volcano to town.
The Most Dangerous Volcano need not have been recently active. With the mean time between major eruptions at a given volcano hundreds to thousands of years, even geological evidence of eruptions of the last few tens of thousands of years is not enough to classify it as hazardous. If we were to consider only volcanoes that endanger small towns of a few thousands or tens of thousands of people, there would be literally hundreds of candidates. The Most Dangerous Volcano in the World has to be chosen from amongst the ones that neighbor major cities.

City Clastics
Four such cities come to mind: Seattle, which is endangered by Mt. Rainier; Tokyo by Fuji; Mexico City by Popocatepetl and Naples by Vesuvius. All of these cities are fortunately far (50-100 km) from their respective volcanoes, so only a large eruption would cause major damage. But as these cities grow, their suburbs crowd ever closer to the volcano.
We know so little about the cause of volcanic eruptions that it is difficult to rank their relative danger. (That's right, volcanologists have been studying volcanoes for maybe two hundred years and still they don't understand the cause for eruptions.) Are Rainier and Fuji, which have erupted only minimally in historic times, less dangerous than Popocatepetl and Vesuvius, which “burp” more often? Has Vesuvius proven itself the Most Dangerous, because of a 79 A.D. eruption that totaled Pompei and Herculaneum? We simply don't know. The simplistic sort of volcanic monitoring performed by most “volcano observatories” isn't going to answer these questions. But, since most major eruptions are preceded by a “warm-up” period of weeks to months long, monitoring of this signal will probably save lives.
Any successful forewarning presumes a city of millions of people can be evacuated. No one had the guts to try with Saint-Pierre or Armero even though the possibility was discussed.
The rarity of gigantic explosions like Krakatoa presents us with an even greater problem. Although they occur perhaps once per century, they are most dangerous because they have the potential to cause massive destruction over a wide area. The great Santorini Island eruption of 1627 B.C generated a sea wave in the Mediterranean that has been implicated in the fall of the Minoan civilization. The 1815 eruption of Tambora in Indonesia killed 50,000 Sumbawa Islanders and threw so much dust into the upper atmosphere that it caused the so-called Year Without a Summer.
The Most Dangerous Volcano in the World is the next to have a gigantic explosion. We just don't know which one it will be. But don't hold your breath, it may not be for a while.


William Menke is a professor in the
Department of Earth & Environmental Sciences.
He can be contacted at: 203 Seismology,
Lamont-Doherty Earth Observatory,
Rte. 9W, Palisades, NY 10964.
Phone 914 365 8438; Fax 914 365 8150.