Volcanoes: windows on the deep earth
As volcanoes go, it is "an itsy, bitsy, nincompoop of a little thing," says volcanologist Ian Carmichael of the University of California at Berkeley. In a book about the history of volcanoes, it would probably show up as a footnote, at least in terms of explosiveness and violent activity. Compared to the rage of a Krakatoa, the volcano that nearly removed an Indonesian island from the map in 1883, the volcano on Mt. St. Helens ranks as an irritated sigh.Skip to next paragraph
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Nonetheless, the eruption of Mt. St. Helens in Washington state this spring has given scientists a rare opportunity to study some of the earth's intricate and unseen plumbing, "a window on the deep earth and how it works," says Dr. Carmichael. It also surprised many Americans into the knowledge that they have a number of active volcanoes in their own back yard, the Pacific Northwest.
Hawaiian volcanoes ooze molten lava frequently, but Mt. St. Helens is the first eruption on the continental US since 1917, when California's Mt. Lassen began to fizzle out after three years of activity. There are, in fact, three potentially active volcanoes in northern California and six in the Cascade Mountains which stretch across Oregon and Washington.
"This type of volcano erupts infrequently," says Brian Baker, a volcanologist with the University of Oregon, "perhaps once every two centuries. When they do blow they tend to blow in remote places where scientists can't get at them very easily."
St. Helens, though, is much more convenient, and it "blew" at a leisurely pace, giving scientists several days of advance warning with a series of earthquakes. A sort of Who's Who in volcanology convened at the base of the mountain, over it, and earlier, on it.
"This one has come to life where we can get at it," comments Dr. Baker. "We will get an enormous quantity of information from it, from 10 to 50 times more than we usually get. This provides a terrific opportunity to study the physics of the thing."
Volcanoes similar to those in the Cascades erupt quite frequently in the Aleutian Islands off Alaska, "but no one can ever get there to pay much attention to them," says Robert Mallis, deputy coordinator for geothermal research at the United States Geological Survey (USGS) in Menlo Park, California.
The keen interest in Mt. St. Helens and other volcanoes stems from both scientific and practical motives. Earthquakes and volcanoes provide scientists with the most visible evidence of what goes on underground -- what Dr. Carmichael calls "the deep earth." An eruption, though, also adds to the body of knowledge that ultimately will help predict future volcanoes.
The most violent and dangerous volcanoes give little warning of their impending explosions. The magma and the pressure building under it lie too deep for the swelling and seismic activity to show on surface instruments.
Mt. Soufriere, on the Caribbean island of St. Vincent, sounded the alarm only five hours before it burst into life last April. Short as it may have been, such lead time looks luxurious compared to the suddenness with which volcanoes engulfed the countryside in the days before modern scientific measurements.
In AD 79, Mt. Vesuvius literally dumped 20 feet of ash on Pompeii, Italy, before people knew what hit them. Another Caribbean volcano exploded so quickly and with such ferocity in 1902 that the entire island population of 30,000 was killed. The eruption of Krakatoa, heard hundreds of miles away, had resulted in a similar tragedy in 1883. The ash it spewed into the stratosphere was detectable, worldwide, for three years.
Volcanic ash -- pulverized rock, basically -- can actually alter the climate of both the local area and broad stretches of the world. The fine ash that is blown high into the air reflects the sun's rays back out into space, countering the blanketing effect of the carbon dioxide in the atmosphere. A 19th-century eruption in the Philippines shot so much ash into the stratosphere (6 to 15 miles up), says Dr. Baker, "that 1816 was known as 'the year without summer.' Crops failed; snow stayed on the ground longer than usual. The climatic effects can be significant, although they are always short-lived."
The difference between a highly explosive volcano, such as Mt. Soufriere, and the less violent Mt. St. Helens amounts, essentially, to age, he says. Magma -- the molten material -- in an older volcano, such as Soufriere, is thicker and more resistant to the pressure buildup. When it finally gives, the magma and whatever debris rests on top of it, come out of the mountain like a shot. Within the more middle-aged Mt. St. Helens, the magma is thinner and does not require as much pressure to begin its movement up through the inside of the mountain.