Forget fires and earthquakes, too much traffic and too many people. Californians may have one more reason to worry, a problem called volcanic fury. Long before the Spanish started building missions in the dry, agreeable Mediterranean climate, the mountainous spine of this Pacific rim state was shaped by volcanoes like Lassen Peak and Mt. Shasta.
Long Valley Caldera in eastern California viewed from the southwest rim toward its northeast rim on the horizon 18 miles away.
Courtesy United States Geological Survey.
Volcanologists have been paying particular attention to the huge Long Valley Caldera, a 15- by 30-kilometer volcanic structure adjacent to Mammoth Mountain in the eastern Sierra Nevada. Calderas are ring-shaped volcanic structures that look much like huge craters surrounded by steep cliffs. Calderas can be 50 kilometers in diameter and some are distinct enough to be seen from space. Crater Lake in Oregon is one of the most famous calderas; in New Mexico, Valle Grande, near Los Alamos, is utterly distinct to the eye millions of years after its formation.
Calderas were once thought to have been formed by huge volcanic explosions that spewed a load of geology into space. Today, however, scientists think they form when a magma chamber forms below ground, empties through a series of eruptions, and collapses along a ring fracture (defined). It is this collapse that causes the depression of a caldera. As more magma enters the chamber, new volcanic cones commonly form inside the caldera. (Been to Crater Lake National Park? - think of Wizard Island.)
When it erupted 760,000 years ago, the volcano at Long Valley blasted 500 cubic kilometers of earthly junk into the atmosphere. That's the kind of eruption that can devastate huge areas of landscape, interfere with food production, and alter the weather.
Long Valley and Mammoth Mountain are a geologically active area near the California-Nevada border.
Map source USGS.
Mammoth Mountain, adjacent to Long Valley did erupt in a small way 500 years ago, but the gigantic eruption is three-quarter million years past. That's the good news.
The bad news is that geologists have recently seen some geologic action beneath Long Valley. In 1980, after a series of earthquakes, part of the caldera rose 75 centimeters, indicating that the magma chamber was filling, an event that generally presages eruptions.
(Just to prove we're not discriminating against California, geologists tell us that magma is also filling in under the Yellowstone caldera, where, 600,000 (and by the way 1.2 million and 1.8 million - see a pattern?) years ago, a gonzo eruption blew out 1,000 cubic kilometers of lava.)
More recently, scientists have detected a massive outflow of carbon dioxide around Mammoth Mountain -- about 1,200 tons per day -- another indication of resurgent magma. They blame this carbon dioxide for killing trees in forests near the caldera (in one 30-hectare area of dead trees, the carbon-dioxide concentration in the soil pore space ranged between 30 and 96 percent). The gas could also explain recent cases of dizziness, nausea, and convulsions on the mountain in recent years.
To one prominent volcanologist, the Mammoth Mountain data are cause for concern: "An active magma source is sending out signals from deep beneath Mammoth Mountain, and we must give it much more attention," warned Arizona State University volcanologist Stanley Williams in 1995 (see "Dead Trees Tell Tales" in the bibliography).
Williams notes that seismic images of the subsurface show a "large body of magma below seven or eight kilometers," and thinks a repeat of the cataclysmic eruption is "conceivable." More likely, he says, would be an eruption on the scale of Mt. St. Helens. Indeed, in April, 1996, geologists detected a swarm of earthquakes in the Long Valley caldera. Follow this link for more information and cool pics of the caldera.
Long Valley was a long time ago. When it comes to volcanic fury, Krakatau is the modern gold standard.
