The visitor center near the top of Mount St. Helens is named for David Johnston, the geologist who predicted that the volcano would explode not upward but sideways. He was six miles away when the volcano erupted on May 18, 1980. Johnston saw the eruption, radioed it in and was killed by the pyroclastic blast of gas and rock.
Mount St. Helens, like most of the peaks in the Cascade Range, is part of a “ring of fire“ around the edge of the Pacific Ocean. The oceanic plates are burrowing under continental plates and causing earthquakes and volcanoes, even about 100 miles inland from the coast. From the Johnston Observatory, you can see a line of volcanoes—all quiet for now—stretching to the north and south.
The eruption was the first in the continental United States since Mount Lassen, in Northern California, erupted in 1915 (also well worth a visit). The Mount St. Helens eruption killed 57 people, destroyed 230 square miles of forest and rained ash as far east as Wisconsin.
Almost 30 years later, you can still see the dead zone as you approach the mountaintop: toppled trees, charred stumps, ash and mud flows. But the zone is coming back to life, and now the mountain is the site of an important ecological study of how species return to land that has been sterilized.
4. Meteor Crater, Arizona
If it weren't for Earth's water, our planet would look a lot like the moon—pockmarked and blasted by impacts from comets, asteroids and meteorites. Our thick atmosphere burns up most space detritus before it hits Earth's surface, but some big chunks still get through. Most impact sites are impossible to see because they're covered by water or vegetation. (There's a big impact crater half-submerged in the Chesapeake Bay, and of course the remnants of the dinosaur-killing asteroid off the Yucatán Peninsula.)
The best place to see the remnants of an impact is Meteor Crater, east of Flagstaff, a privately owned tourist attraction. The crater is 4,000 feet wide, almost 600 feet deep and will put the fear of Near Earth Objects into you.
3. Niagara Falls, New York
The border town is great kitschy fun, but it's also fascinating geologically. The falls may not be the highest in the world, but their width and the volume of water spilling over them (about six million cubic feet per second) make them stunning (and deafening).
Niagara Falls is where one great lake (Erie) drains into another (Ontario). The lakes were carved by glaciers at the end of the last ice age. A hard capstone (the top of the falls) eroded more slowly than the soft shale below, creating the falls.