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.
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.
Putting age aside, volcanologists divide volcanoes into three categories.These categories are based on plate tectonics, the generally-accepted theory that the earth's surface consists of seven, huge, constantly-moving plates. One type of eruption, such as the one that blasted Iceland in 1973, occurs when two of these plates separate and allow molten material to rise to the surface.
A second type, encompassing Mt. St. Helens and the more violent volcanoes, happens when two of these plates crunch together and squeeze the magma up towards the top -- like squeezing out toothpaste.
The third type, though, baffles scientists. They occur in the middle of a plate, on so-called hot spots. The lava is very thin and just bubbles out like pancake batter. The Hawaiian volcanoes, which open up every two or three years, fit this category, and their lava flow is tame enough to permit scientists to walk right in and take samples, gingerly. Volcanologists are not quite sure what creates the hot spots. For that matter, they can only theorize about what goes on within the other two types, since the bump and grind of plate tectonics goes on way out of sight, over 400 miles beneath the surface.
Mt. St. Helens's origins lie in what scientists term "subduction," the movement of the Pacific Plate under the North American Plate. The same process created California's famous San Andreas fault and, according to Dr. Carmichael, makes an earthquake there "inevitable."
Two theories exist, says Dr. Baker, about how subduction creates a volcano. According to one, the grinding pushes rocks with a relatively low melting point deeper into the earth where the temperatures run in excess of 1,000 degrees C. ( 2,025 degrees F.), hotter than molten steel. The rocks melt, becoming magma, and rise towards the surface.
The second theory, to which Dr. Baker subscribes, is more complicated but boils down to this: the subduction lets water from the Pacific Ocean seep down into the deep earth. The water turns to steam which permeates the rock around it and reduces the local melting point. The rock melts into magma and joins with the steam heading for the top. "It is the most logical of the two theories ," says Dr. Baker."Don't you think?" Uh, sure.
The connection between plate tectonics and an eruption "is tenuous, though," adds Dr. Carmichael. "The relationship is there but on a geologic time scale. The whole process for something like Mt. St. Helens may have begun 100,000 years ago."
What triggers the eruption? "The key to a volcanic eruption is gas," explains Dr. Baker. A volcano is "like a soda pop bottle. If you leave it alone it will sit there very quietly, but if you put your thumb over the top and shake it then everything wants to squirt out the top. Inside a volcano there is liquid (the magma) with gas in solution. When it gets agitated it starts looking for a way out.
"Mt. St. Helens has been doing that for a long time. Several hundred years ago there was a mudslide 50 miles long. There was another eruption just over 2, 000 years ago."
The blasts from these volcanoes in the Pacific Northwest are seldom large, and rarely spew out ash as much as 10 feet thick (compared to 20 feet thick for Mt. Vesuvius). There have been exceptions, though. Some 6,600 years ago -- a blink of the eye in geologic time -- Mt. Mazama in California erupted so violently that in a few hours the mountain collapsed, creating Crater Lake. The ash from Mazama was, in spots, 200 feet thick.
Despite the attention given Mt. St. Helens, the eruption of a volcano is by no means rare. "Volcanic eruptions are taking place all over the world all the time," says Mr. Mallis of USGS. "This one will aid volcanology by adding one more piece of data."
As spectacle, though, a volcano is hard to beat, as the TV views of Mt. St. Helens have amply shown. "A volcano is the most awe-inspiring thing you can imagine," says Dr. Carmichael. "You get the sense that man can do very little to change the earth. The power in a volcano is many orders of magnitude greater than that in a hydrogen bomb."
The power of a volcano can also be harnessed for human use, though infrequently.A project in Hawaii will eventually convert some of the power in Kilauea Volcano near Hilo into more electricity than the island now uses. A similar project in El Salvador currently generates 20 times more electricity there than oil does. Officials of the island of Heimaey, Iceland have tapped the still-warm lava flows from a 1973 eruption to heat one fifth of the town's homes. When water is sprayed over the masses of lava, which is 130 meters thick in places, it percolates down and exits as steam. The system will soon be enlarged, says Thorbjorn Sigurgeirsson of the University of Iceland, to include all the town's homes, saving hundreds of thousands of dollars a year in fuel bills for the island's 5,000 residents.