The eruption of Iceland's Eyjafjallajokull caused billions of dollars in economic damage, and left millions of travelers stranded. But on the Volcanic Explosivity Index — volcanologists' tool for rating the severity of an eruption — the event rated only a two out of a possible eight. More severe eruptions cause "death and destruction" on a planetary basis, points out Simon Winchester in The Guardian. "They can darken skies and cause devastating changes in the weather. They can and do bring about the abrupt end to the existence of entire populations of animals and people" — not to mention economic damages that could run into the trillions. Here are six volcanoes worth fearing:
1. KATLA (Iceland)
Last erupted: 1918
Effects of a major eruption: If Katla goes off, its eruption will be 10 times stronger than Eyjafjallajokull's. Katla's larger ash plume would shoot higher in the air and spread over larger areas of Europe for a longer period, with much more devastating effects on air travel and economic trade. An eruption could tip Europe's economy — perhaps even the world's — back into severe recession or a depression.
Likelihood: Fairly high. The two volcanoes, only 12 miles apart, tend to erupt in tandem, and Katla is slightly overdue in its 80-year cycle.
2. CUMBRE VIEJA (La Palma, Canary Islands)
Last erupted: 1971
Effects of a major eruption: In 2001, U.S. and British scientists warned that a major eruption of Cumbre Vieja could cause the enire western flank of the volcano to fall into the sea, creating a "mega-tsunami" in the Atlantic. Traveling at 500 miles per hour, it would wipe out Florida, coastal Brazil, and parts of Europe with waves up to 160-feet high.
Likelihood: The scientists say the "year to year probability" of a major eruption is low, but preparations should be taken anyway given the potentially cataclysmic damages.
3. MT. VESUVIUS (Italy)
Last erupted: 1944
Effects of major eruption: Famous for wiping out Pompeii and Herculaneum in 79 A.D., Vesuvius would do much greater damage today. About 3 million people live near the volcano, 600,000 of them in the "red zone." An eruption would kill at least 8,000 people and cause more than $24 billion worth of damage, according to Willis Research Network, which just named Vesuvius the most dangerous volcano in Europe. The ash would change weather patterns in Europe and leave the Naples area a "lifeless desert."
Likelihood: Scientists say Vesuvius is overdue for an explosion.
4. POPOCATÉPETL (Mexico)
Last erupted: 2000
Effects of a major eruption: The third-tallest active volcano in the Northern Hemisphere, Popocatépetl is only 40 miles west of Mexico City and its 18 million inhabitants, and 30 miles east of Puebla, a city of two million. A large eruption could send deadly mudslides into the populous valleys below, creating "catastrophic" loss of life.
Likelihood: After an 80-year dormant period, Popocatépetl is showing signs of activity.
5. MT. TAMBORA (Sumbawa, Indonesia)
Last erupted: 1967
Effects of a major eruption: Tambora erupted in spectacular fashion in 1815 and changed weather patterns around the globe, causing "frosts in Italy in June and snows in Virginia in July, and the failure of crops in immense swaths across Europe and the America." The blow-up killed more than 71,000 people directly, and many more through famine and sickness.
Likelihood: Tambora is still active and, given its history and Indonesia's 222 million inhabitants, closely monitored.
6. YELLOWSTONE "SUPERVOLCANO" (U.S.)
Last erupted: 640,000 years ago
Effects of a major eruption: When the Yellowstone Caldera, or "supervolcano," in Yellowstone National erupts again, it will render a huge swath of North America, from Vancouver to Oklahoma City, uninhabitable. It would have incalculable human and economic consequences. The last eruption of similar magnitude — 73,000 years ago in Sumatra — plunged the entire planet into a decade-long volcanic winter and nearly wiped out the human race.
Likelihood: Geologists see signs that it could be preparing for another major blowout soon, although "soon" could mean thousands of years.