Mt. St. Helens yields a trove of valuable data
Denver — According to Klickitat legend, Mt. Helens was once a beautiful Indian maiden, the object of affection of two brothers who waged a fierce war over her, throwing steaming rocks and fire at each other across the Columbia River and devastating the countryside. This so angered the Great Spirit that he turned the three into mountains: the brothers into Hood and Adams, with St. Helens eternally between them.
When Mt. St. Helens rumbled to life last year, she attracted a new horde of suitors: Geologists and volcano specialists from around the world gathered for the spectacular show. The mountain hasn't disappointed them.
Its most recent display occurred April 10, when the volcano sent a plume of steam and ash towering more than 15,000 feet. This eruption, as well as the devastating May 18, 1980, blast and the other lesser eruptions that preceded and followed it, are the most thoroughly documented volcanic events in history.
Its seismic rumblings, swelling and shrinking, smoldering emissions, towering plumes of ash and steam, the rocks and mud ejected, all have been measured with a variety of instruments from ground-based laser ranging devices to a bevy of satellites orbiting overhead.
This has led to a number of impressive statistics. The May 18 explosion was equal to about 10 megatons of TNT --tated Hiroshima. This blast threw into the air a volume of mountain equivalent to a cube 4,600 feet on a side. Some of that material was ejected at speeds up to 500 m.p.h. The temperature in the 220 -square-mile area around the mountain approached 650 degrees F. for a few minutes, subsiding to 250 to 350 degrees for the next several hours. The ash cloud that poured from the mountain traveled 16 miles at 500 m.p.h. and was a searing 1,650 degrees F.
Generally, scientists who have studied the mountain over the last year believe it has become relatively quiescent and is likely to remain so for several months at least. The last two eruptions have been nonexplosive in nature. Gaseous emissions from the carter have dropped subtantially from previous levels. Earthquakes in the immediate vicinity have moderated to low levels.
"We do expect more eruptions. More moderately sized explosive eruptions are quite likely. However, the chance of another the magnitude of the May 18 eruption appears lower that any time since last March," explains Chris Newhall of the US Geological Survey (USGS).
"We have issued an 'extended outlook advisory,'" continues Dr. Newhall. This advisory, issued the last week in March, predicted another nonexplosive eruption in the following few weeks and was based on measurements of swelling within the crater.
This swelling has been a reliable precursor of Mt. St. Helens's previous eruptions, explains Don Swanson, also of USGS. The mountain begins swelling two or more weeks before an eruption. "It's a kind of early warning," Dr. Swanson explains.
The swelling that has preceded explosive eruptions has been substantially greater than that preceeding the nonexplosive ones. There are two basic theories to explain this deformation. One is that it results from the intrusion of stiff molten rock, or magma. The other is that it comes from a buildup of gases released from the magma.
"Prior to the explosive eruptions, it is likely that the swelling was largely due to the high gas pressure. Before the nonexplosive eruptions, the deformation looks more like it was caused by shallow magma movements," the USGS scientist explains.
This fits neatly with measurements of earthquake activity at St. Helens made by seismologists at the University of Washington. "When there is a visible dome in the crater, the eruptions have been preceded by a few hours by shallow earthquakes," explains Steve Malone of the university.
When there is no dome visible, however, the premonitory event has been what seismologists call a "harmonic tremor" -- a somewhat mysterious vibration that can last for hours, even days. Although there are several theories for the origin of these tremors, none is completely satisfactory, Dr. Malone says.
The nonexplosive eruptions have occurred since Mt. St. Helens built a dome in the crater. Therefore, the shallow quakes observed are likely due to magma movements preceeding these events. On the other hand, the harmonic tremors could be associated with the buildup of gases at high pressure within the mountain. The strongest tremor observed here came shortly before the May 18 blast.
The various Indian legends suggest that more than one of the Cascade volcanoes have erupted at the same time. Also, several of these erupted within a short time in the mid-1800s. Yet scientists have considered volcanic eruptions as independent events. This idea may be changing.
"If you'd asked me a year ago, I would have said, 'No, the mountains don't talk to each other,'" says Dr. Malone. Now the seismologist has begun to change his mind. He has found a large area north of St. Helens that has increased significantly in seismic activity. Whether this is a result of the volcanic eruption or because the two have a common geologic cause is not clear. But it raises the possibility of dynamic geological links between Northwest volcanoes, he believes.
Such links have been proposed by advocates of one theory of climate change. Climatologists such as Reid A. Bryson of the University of Wisconsin have argued for some time that volcanic activity can have significant climatic effects. Major volcanoes are known to cool the world climate as a result of the debris they inject into the stratosphere. These particles act somewhat like a sun shade, reflecting an increased fraction of the sunlight back into space. One minority theory holds that a barrage of major eruptions triggers a change from warm interglacial periods to Ice Ages.
Thus, climatologists watched and measured the effects of Mt. St. Helens with great interest. But the mountain has proved a disappointment. For an eruption of its size, Mt. St. Helens has had surprisingly little impact on the world's weather. Yet there is an important lesson in this, says James Pollack of the National Aeronautics and Space Administration, who has coordinated U-2 overflights of the mountain.
"It proved to us that you can't consider volcanic eruptions all alike," he explains. To have major climatic effects, a volcanic eruption must be explosive enough to penetrate the stratosphere and must contain a lot of sulfurous compounds. Mt. St. Helens had the power but not the sulfur.
Most of the material ejected from Mt. St. Helens was silicate ash, which washed out of the stratosphere in a matter of days. It is sulfate, which rapidly converts into sulfuric acid droplets in the stratosphere, that has the predominant climatic effect, scientists now realize. These droplets can float in the stratosphere for years, reflecting sunlight back into space.
Satellite observations showed that Mt. St. Helens about doubled the amount of this aerosol, but this was not enough to cool the climate appreciably. However, these observations also showed that those sulfur particles that reached the stratosphere rapidly converted into highly reflective aerosols. Thus, they began having a cooling effect almost immediately.
"With what we know from Mt. St. Helens, we can now do a better job of understanding the effects that past volcanic eruptions have had on the climate," says Pollack.
Scientists say that many of the insights the mountain has provided remain to be unraveled. Many experts are so busy gathering information that they have not had time yet to analyze. As a result, the fruits of Mt. St. Helens's activity may not become fully apparent until the mountain once again sleeps.