After a week of steadily increasing seismic rumbling, Mount St. Helens on Friday burped a tall column of steam and ash that officials said was probably not a precursor to a large, sustained eruption from the most active volcano on the West Coast.
The volcano is being closely monitored, in part because of its cataclysmic explosion in 1980. That explosion killed 57 people, triggered the largest landslide in recorded history and blew 520 million tons of ash eastward across the United States.
The plume from Friday's 24-minute noontime eruption rose 10,000 feet before drifting away in light winds on a spectacularly clear autumn day. No injuries or damage to property were reported.
When the eruption ended, government monitoring instruments showed a sharp decrease in the seismic activity that in recent days had been registering as many as four small earthquakes per minute on the mountain, which is in a sparsely populated area of southwest Washington.
"This was a relatively small eruption," said John Major, a hydrologist with the U.S. Geological Survey. He said USGS observers near the volcano reported that no molten rock flowed from the point in the mountain's lava dome where the steam vented.
At a news conference, Major said the eruption appeared to have punched a hole about 100 feet in diameter in the glacial ice that covers part of the mountain's horseshoe-shaped dome. He said there was a small increase in melting ice and snow off the volcano, but no reported mudflows, or lahars, which historically have been damaging consequences of volcanic eruptions in the Cascade Range.
After the 1980 eruption, a lahar traveled 50 miles from Mount St. Helens, ripping out bridges and destroying homes.
In comparison, Friday's eruption was a "hiccup," Tom Pierson, a USGS geologist, told reporters.
"This is exactly the kind of event we have been talking about and anticipating," Major said. He noted that there is no evidence that a major flow of gas-rich molten rock, which could trigger a sustained eruption, has ascended from its known location seven miles underground to the top of the mountain.
Absent such a flow of molten rock, USGS scientists have said it is unlikely that recent increases in seismic activity on Mount St. Helens will result in a significant eruption that could endanger people or property outside the immediate vicinity of the mountain.
Major compared Friday's "small explosive eruption" to the activity that came to be regarded as normal in the six years after the 1980 eruption.
In those years after Mount St. Helens literally blew its head off, more than a dozen lava eruptions built a rock dome inside the mountain's crater. That dome is now partially covered by a glacier, which is growing because the edges of the mountain crater shade snow and ice from summertime sun.
Major said that USGS scientists will be watching their instruments closely, and that if seismic activity levels off for several consecutive days, they may be able to conclude that the threat of more eruptions has passed.