Predicting Eruptions
So, we can help a lot of people if we can do a better job of predicting when and where volcanoes will cause hazards. Various things can be done. For problems such as climate change, the best we can do is to know that every few years or decades some region is likely to experience difficulties with crop production because of eruptions. The solutions are either to maintain a little excess food to feed those endangered people, or to ignore them and figure that some will starve to death. (Many other climate changes, including droughts, give us the same choice. Despite the apparent silliness—either we stockpile food and figure out how to distribute it to the needy, or we let people starve to death—it is surprising how often starving to death is the outcome.)
For tsunamis, an operational warning system now exists for many of the world’s coasts, but much more could be done. One way to avoid volcanic hazards is to stay out of harm’s way. Geologists can map regions where large pyroclastic chunks have fallen, or where landslides have occurred, with great confidence. Using carbon dating of logs caught in debris flows, or tree-ring dating of trees growing on landslides (just hang on; we will explain how ages are learned), scientists can determine the recurrence interval—how often do such disasters happen? Today, whole housing subdivisions are being built around Mt. Rainier National Park in the growing Seattle-Tacoma region that have a danger of destruction by landslide many, many times higher than their danger of destruction by household fires. More than 200,000 people work, and more than 100,000 people live, on debris-flow deposits less than 10,000 years old, with more people coming. (The largest of those flows, the Osceola Mudflow from about 5,700 years ago, came from the top of the mountain, and lowered its peak about 1,600 feet (500 m); Mt. Rainier is now 14,417 feet (4,394 m) in elevation, but the peak once was about 16,000 feet (4,900 m) high.)The homeowners living in danger around a volcano will all carry house-fire insurance, but few if any are insured against the volcano.
(Much argument is attached to sending disaster aid for predictable events even if they are not very common. Should those who wish to live in beautiful but risky areas carry insurance to pay for their gambles? Increasingly, planners are saying “yes,” and much effort is being devoted to quantifying the hazards so that insurance rates can be set wisely. This applies to such things as hurricanes along coasts, earthquakes along faults, and floods along rivers. Geologists have an important role to play in learning hazards and thus setting rates.)
With sufficient care, volcanic eruptions can be predicted with some confidence. Volcanoes usually give off many signals before an eruption: the ground swells as magma moves up; the moving magma and the swelling ground create earthquakes and especially the distinctive harmonic tremors of fluid flowing in a pipe; small eruptions occur; gaseous emissions increase as the magma nears the surface and then cease if the system becomes plugged and builds up pressure for an explosion. A monitoring program of seismographs to detect earthquakes, repeat surveying of laser reflectors set on the mountain together with monitoring using satellites to watch for deformation patterns, gas sampling, and perhaps photographic or other sensors to watch for landslides, can track a volcano’s behavior and allow timely warning. Monitoring of ground shape from space can even see the changes in volcanoes as magma moves under them. The eruption of Mt. St. Helens was predicted well enough to save hundreds of people including the residents of a YMCA camp. The eruption of Mt. Pinatubo in the Philippines in 1991, which heavily damaged the U.S. military bases there, was predicted accurately, allowing timely evacuation and saving tens or hundreds of thousands of lives of residents and military personnel.
The burden of predicting eruptions is very high, though. Imagine telling an Air Force general to abandon his or her assigned duty post, spend perhaps millions of dollars to move tens of thousands of people, and then having nothing happen—the general, and all of those people, would be very unhappy. Imagine instead deciding to wait another day to be sure, and having all of those people (possibly including you) killed. As important as this is, predicting disasters is not for the faint of heart.
The Mt. St. Helen eruption was a small one compared to many others. Each of the major eruptions of Yellowstone moved about 1000 times more material than Mt. St. Helens did, and Yellowstone’s eruptions were not the largest known. Small eruptions are more common than large ones. But, eruptions ten times as big as Mt. St. Helens are perhaps five times as rare, but not ten times as rare. This means that, as for earthquakes, most of the “work” done by volcanoes is achieved by the few big ones, not the many little ones.