In the spring of 2010, Iceland’s Eyjafjallajökull volcano kaboomed, sending a cloud of ash into European airspace. The resulting disruption in air travel (ash + engines = bad) was the largest on the continent since World War II, costing an estimated $5 billion.

And yet, Eyjafjallajökull’s meltdown was moderate, as volcanologists classify it. On the “volcanic explosivity index”—which is based on the volume of ejecta like ash and rock—it was a 4. Compare that to the 1815 eruption of Mount Tambora in Indonesia, which gets a 7: It blasted so much material into the atmosphere that it cooled the planet, leading to widespread crop failures. In the Philippines, Mount Pinatubo’s 1991 eruption...