Mt. St. Helens is resting again after its third major eruption in less than a month, but it is far from dormant. Having established a tenuous pattern, scientists now say, the volcano can be expected to continue this kind of intermittent activity over many months if not years.
The June 12 eruption that spread ash and pumice over 4,500 square miles of northwest Oregon and part of southwest Washington did not entirely surprise knowledgeable observers. But it did reverse earlier official pronouncements that St. Helens had entered its "rebuilding" stage in which lava mounds up inside the new crater to form a dome.
Instead, the "magma" (molten material within the Earth's crust) remains gas-rich and thus likely to erupt violently from time to time.
"I would say that we could probably look forward to some more explosive activity, but of declining intensity," said Prof. Stephen Harris of California State University at Sacramento. Dr. Harris, who teaches volcanology, is the author of "Fire and Ice," an authoritative book on the Cascade Mountain Range volcanoes.
"St. Helens is an unusual volcano in that it erupts a wide variety of lava types, all the way from a very low silica basalt which forms the kind of flows seen in Hawaii right up to the very gas-rich dacite eruptions," Dr. Harris said. "This makes it more unpredictable."
The most recent ash and steam eruption was similar to those of May 25 and the afternoon of May 18 (which followed by several hours the initial blasting of St. Helens's top).
Once the magma becomes less gaseous, scientists expect lava (magma that surfaces) to begin flowing from the volcano's open vent. Based on past eruptions of St. helens, this flow probably will not be more than two to five miles long.
"It would be almost certain to remain within the immediate vicinity of the volcano," said Dr. Robert Christiansen, who has been coordinating monitoring and scientific investigation of St. Helens for the US Geological Survey (USGS).
Scientists stress, however, that the region close to st. Helens still is very hazardous. Particularly to the north and west (the direction of the initial lateral blast), flowing avalanches still are possible.
Last week's eruption covered Portland, Ore., and surrounding communities with a coat of ash greater than the previous May 25 eruption. But the effects were mitigated by a welcome rainfall that allowed the city's annual rose festival to be held as scheduled. Some smaller communities nearer the mountain were temporarily evacuated, but no deaths or injuries were reported.
The most controversial aspect of the latest eruption was its coincidence with a strong earthtide, thhe result of the moon being close to Earth. This was also the case just before the first major eruption May 18.
University of Wisconsin geophysicist Robert Meyer calls any connection between tides and volcanic activity "very purely speculation of the worst kind." Other scientists feel that, while a strong gravitational pull from the moon would not of itself cause an eruption, it could trigger such activity that was building up and inevitable in any case.
French scientists on the island of Guadeloupe in the West Indies in 1975 detected a series of volcanic actions that closely mathced tidal patterns.
"There's nothing at all wrong with that hypothesis," said Dr. Christiansen of the USGS. "It has been demonstrated in some cases that indeed it works."