St. Helens was an awesome reminder that Earth is not a boring, static hunk of rock, but a living planet wrapped around a giant ball of ancient heat. It reminds us that the planet coalesced from a hot cloud of gas and dust about 4.5 billion years ago. Heat from the surface radiated to space, cooling the skin. But the high-temperature rock inside was insulated by the crust, and actually has gained heat from radioactive decay and gravitational energy.
Here's the long and the short of it: We're sitting on a ball of fiery rock wrapped by a couple dozen kilometers of cool rock.
That's unstable. Heat rises -- because hot substances are less dense than colder ones -- so magma is always looking for an escape hatch through the crust. And since heat is a form of energy, rising magma brings up oodles of energy -- enough to power cataclysmic explosions. Enough to reshape the face of the planet, but we'll save that for later.
Volcanoes don't just appear anywhere, fortunately. They require a place where the crust is too weak to contain the magma. A few volcanoes -- in Hawaii and elsewhere -- are located above mid-oceanic "hot spots" -- magma pipelines.
Most volcanoes, including Mount St. Helens, are in the "Ring of Fire" around the edge of the Pacific. At the junction between tectonic plates, the heavier ocean floor sinks beneath the lighter continental crust in a process called subduction.
If you were to drill 100 kilometers into a subduction zone, you'd see the sinking crust carrying seawater under fantastic pressure (about 30,000 times atmospheric pressure) and temperature (1,000 degrees Celsius). When this water mixes with hot rock, the rock becomes soft enough to flow. Just as salt melts ice by lowering its melting point, water lowers the melting point of rock.
The softened rock is squeezed by all the rock above it, and it eventually reaches a magma chamber a few kilometers below the volcanic vent. As the magma enters cooler surroundings, it gets a boost from buoyancy. As the magma continues rising, falling pressure liberates high-pressure gas that was trapped inside it, and the volcano explodes.
Bingo -- you get a mess of hot rock all over the place.
(Hawaiian volcanoes are a different story. Due to the chemistry of the lava, it traps less gas and also flows faster. Although Hawaiian volcanoes can produce impressive lava fountains, they don't do that St. Helens "explosion is the thing" thing.)
Mount St. Helens in 1980 was impressive, but by volcanic standards it was just a close call. In 1985, about 25,000 people died in Colombia, when lahars (mudflows) from Nevado del Ruiz buried the city of Armero, as you'll see in a later photo.
That's the kind of thing that keeps volcanologists up at night. And plenty of other cities face grave volcanic threats:
Naples, Italy: Three million live in the shadow of Vesuvius, made infamous for burying Pompeii back in 79 AD.
Mexico City: The world's largest metropolis is 55 kilometers from giant PopocatÈpetl, a 5,465-meter volcano that's erupted about 17 times since Columbus "discovered" the New World.
Japan and Indonesia: These densely crowded nations are studded with active volcanoes.
Calif.: 760,000 years ago, a massive eruption spewed 600 cubic kilometers of rock -- about 600 times as much as St. Helens. (Remember this geological guideline: If it happened before, it can happen again.)
Seattle-Tacoma, Wash.: On a good day in either city, you can see the massive icecap of Mount Rainier, the mother of Cascade volcanoes. If you're in the Sea-Tac real-estate biz, you don't want to read this USGS comment on Rainier: "Some debris flows have traveled as far as the present margin of Puget Sound, and much of the lowland to the east of Tacoma and the south of Seattle is formed of prehistoric debris from Mount Rainier."
The debris flows that reached Puget Sound -- technically lahars -- resemble a slurry of wet concrete, and they can move at 50 miles an hour. A deep lahar can obliterate anything in its path. The U.S. Geological Survey says at least 60 lahars of various sizes have moved down valleys from Mount Rainier during the past 10,000 years.
Add it up. Whether it's Seattle, Naples, Mexico City, Java or Japan, as a soaring population runs out of safe places to live, sooner or later a volcano will write headlines of havoc.
Some perspective. Mount St. Helens emitted one cubic kilometer of ash. That size eruption happens every decade or so. Krakatau put out about 20 times as much stuff. That can happen roughly once every century. But 75,000 years ago, Mount Toba, also in Indonesia, blew out 3,000 cubic kilometers. That eruption could have changed the global climate.
Later, we'll get to even larger lava flows, but for now, here's the moral of the story:
Gargantuan eruptions are rare. Gargantuan eruptions happen.
What's what with eruption prediction?