San Francisco — The recent "swarm" of earthquakes around Oregon's Mt. Hood is a not-so-subtle reminder that the whole Cascade Range continues to be geologically alive and kicking.
Because of important subsurface differences, scientists say Mt. Hood is not likely to explode as violently as its notorious cousin 60 miles to the northwest , Mr. St. Helens. But with the knowledge that Mt. Hood does have a history of massive glacial avalanches and mud slides, officials are taking no chances.A "volcano hazard watch" has been issued; Oregon officials are preparing evacuation plans and taking other precautionary steps.
Beyond the recent tremors and watchfulness, there is renewed interest in the US Northwest's link in the volcanic "ring of fire" that encircles the Pacific Ocean.
Scientist acknowledge that there could be a connection between recent activity at Mt. Hood and the earlier eruptions at Mr. St. Helens. Early in the 19th century, four mountains in the range -- Baker, Ranier, St. Helens, and Hood -- were all active at the same time.
"The range is probably as active as it has ever been," says Stephen Harris, author of "Fire and Ice" and an expert on the Cascade volcanoes.
The public is most familiar with the dozen or so larger mountains that stretch from Lassen Peak in northern California to Mt. Garibaldi in British Columbia. But Dr. Harris points out that there are in fact some 2,800 volcanoes in the Cascade Range. Most are cinder cones and "shield volcanoes" with relatively low profiles. From time to time, many emit steam and showers of pumice.
"Each one of these volcanoes is quite different," says University of California geologist William Wise, who has written numerous scholarly articles on Mr. Hood. "Mt. St. Helens is a very young one and still growing. Mt. Hood quit its big activity at least 10,000 years ago. That doesn't mean that it's dead, but it is winding down."
The US Department of Energy is probing several thousand feet into Mt. Hood, searching for hot water and magma (molten rock). These are relatively near the surface at Mt. St. Helens, and if found at similar depths in Mt. Hood would indicate the possibility of violent eruptions. So far, such material has not been detected.
The top of Mt. Hood has numerous steam jets, gas vents, and other "hot spots" that continually reach the boiling point of water. Clouds of steam and sulfurous fumes sometimes can be seen in Portland, 50 miles away. About one-half of the other major Cascade volcanoes have been active within the past 150 years, and only two or three are considered extinct.
A recent unpublished US Geological Survey report does not rule out potentially dangerous volcanic activity at Mt. Hood. If the volcano erupts, it would not be expected to throw off as much ash as St. Helens has, but could result in much greater mud and ice flows since it has the largest glacier in Oregon. this could sweep into the Hood and Williamette rivers and in turn cause flooding of the Columbia River around Portland.
"It's certainly very possible that we could have an eruption," Dr. Harris says of Mt. Hood. "If these earthquakes persist, it could mean that magma is moving under the mountain." Extra seismographs and temperature-sensing devices now are in place around the mountain.
Scientists say it is too soon to tell whether the recent volcanic activities at Mt. St. Helens and Mt. Hood are related or merely coincidental. "Events at St. Helens are sending some pretty good shock waves over the whole range," said Dr. Wise.
Meanwhile, Oregon officials are relieved that Mt. Hood has remained relatively quiet. They were chagrined to have the recent earth tremors occur just as they launched a media blitz to boost the summer's sagging tourism.
At the same time, they are completing a plan for emergency warning and evacuation of people within a 50-mile radius of the mountain. They are mindful that, as Dr. Harris says, Mt. Hood's fires are not extinguished.