Twenty years ago, Mt. St. Helens, in Washington State, "blew." It was the volcano's first eruption in recorded time, although as early as 1978 a team of scientists from the US Geological Survey had labeled it "the most dangerous volcano in the Cascade Range." In June 1991, Mt. Pinatubo in the Philippines spewed forth its own mix of ash, gases, mud, lava, and all the other debris that had been building within the mountain for centuries. Between those...