Science Briefs

The Fatal Attraction of Volcanoes

Volcanic eruptions often exact a large human toll. Yet we usually associate the deaths and other human casualties with only the fiery havoc wreaked around the volcano itself. What quickly springs to mind are the morbid scenes of vast, dead cities, like Pompeii, which was buried under hot rock fragments and glowing nuées ardentes — scorching clouds of hot magma and gas — that were spewed out by the AD 79 eruption of Mount Vesuvius.

Figure 1: Painting

Fig. 1: Eruption of Mount Vesuvius on 24 August AD 79, by Pierre-Henri de Valenciennes (Musée des Augustins, Toulouse).

In 1902, probably the most disastrous nuée ardente in history wiped out almost all of the 28,000 inhabitants of St. Pierre on the island of Martinique after the cloud rolled down the flanks of Mount Pelée. More recently, an enormous mudflow entombed about 22,000 people living in the valleys below the icecap of Nevado del Ruiz, Colombia, which abruptly melted down in the heat of the violent eruption of 1985.

As dreadful as these events were, the distant repercussions of even greater eruptions have inflated the total mortality figures to untold tens of thousands. In 1783, the Laki crater row in Iceland poured out lava and hot gases for seven months, polluting the island's fragile environment and eventually killing over 10,000 people, a quarter of the population, many of them from fluorine poisoning.

The deadliest volcano of all time was Tambora on Sumbawa Island, Indonesia, which killed off more than 88,000 people in 1815. Most died as an indirect result of the heavy ashfall, which blanketed the growing crops and polluted the water supplies over a wide region, including several neighboring islands. Still more bizarre is the disaster that occurred when Indonesia's Krakatau, another island in the same volcanic chain, exploded in 1883. The collapse of its caldera into the water raised giant sea waves that drowned 36,000 victims on nearby and distant shores. Obviously, "volcanic chain smoking" can be hazardous to the health of a local population!

The complete human toll from these and other eruptions must be far greater, however, perhaps in the millions. The reasons are tied, strangely enough, to the upper atmosphere. Powerful eruptions inject sulfur gases into the stratosphere, where the sulfur combines with water vapor to form sulfuric acid aerosols. These tiny particles scatter sunlight back to space and alter atmospheric circulation patterns. The result is that the Earth's surface cools and precipitation increases. Crops then fail to ripen properly, and famine and pestilence follow in scarcity's wake. This sequence of events is known to have happened after the seven greatest volcanic eruptions of the past two millennia.

Mass hysteria has also ensued. People became terrified that the dimmed sun would never recover its brightness in 44 BC (Mount Etna), in AD 536, and in AD 626. The flagellant movement of 1260 began, in part, as a grassroots reaction to famine and disease, whose ultimate cause can be traced to the eruption of an unknown volcano in 1258. In this episode, troops of religious penitents lashed themselves while traveling from town to town across central Europe. Social disruption occurred also after the eruptions of 934 (Eldgja in Iceland), 1783, and 1815.

With the relentless advance of science, however, deeper understanding of these fearful natural events and of their physical consequences began to mitigate the accompanying human tragedies. Open public discussion stimulated the professional scientists' interest, and the two "cultures" of modern society worked together successfully in 1783, and a century later in 1883, to further the cause of volcanic and climatic studies. Today, without a doubt, the role of investigator has devolved almost entirely on the professional scientist. Nonetheless, the impact itself of devastating volcanic eruptions always presses most heavily on the common man. And this in spite of (or rather because of) the fertility and beauty of the volcanic landscapes that attract him in the first place.

Reference

Stothers, R. B. 2000. Climatic and demographic consequences of the massive volcanic eruption of 1258. Climatic Change 45, 361-374.

Francis, P. 1993. Volcanoes. Clarendon Press, Oxford.