Scientists seek source of Mount St. Helens blast
 |  Ash billows from Mount St. Helens on Tuesday. |
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 Mount St. Helens releases a 6-mile-high plume of smoke.
 Mount Saint Helens belches ash and smoke.
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VANCOUVER, Washington (CNN) -- Scientists were flying over the crater of Mount St. Helens on Wednesday to locate the origins of a plume of ash and steam that spewed nearly seven miles high into the air Tuesday night.
They said the "small" explosion is not necessarily a harbinger of a larger or more destructive eruption.
"These are the kinds of events you can expect," said Jon Major, research geologist for the U.S. Geological Survey's Cascade Range Observatory in Vancouver, Washington.
"We just can't say exactly when they're going to happen. By and large, there wasn't that much ash; it wasn't a heavily ash-laden plume."
The U.S. Geological Survey (USGS) in Vancouver began a 24-hour volcano watch after the eruption at 5:20 p.m. (8:20 p.m. ET). Nearly five hours later, the plume remained visible in satellite photos.
Glowing tendrils of lava were spotted inside the mountain's crater following the explosion. Although the plume rose nearly twice as high as one produced by the last eruption in October, the geological agency classified the event as "small," according to an advisory on the Web site of its Cascades Volcano Observatory.
No damage or injuries were reported. Some air traffic had to be routed around the plume as it rose to 36,000 feet.
Ten small earthquakes were measured in the area on Tuesday leading up to the eruption. The largest appeared to be a magnitude 2.5, according to the USGS.
After a relatively quiet night, a small jet of steam could be seen rising from the mountain on Wednesday.
The ash remained in the air over Montana and was visible for more than 50 miles (80 kilometers), prompting some motorists to stop their cars and watch. Falling ash was reported in towns up to 90 miles north and east of Mount St. Helens.
Ongoing activity
Scientists waited for daylight Wednesday to use a surveillance helicopter to fly over the mountain's crater and determine the plume's origins.
Although the geological agency has instruments in place to measure volcanic activity, four global positioning satellite stations and three seismometers stopped transmitting after the blast. USGS scientists say they were likely damaged.
Mount St. Helens, about 45 miles (72 kilometers) east of Vancouver, has been rumbling and spewing steam since September. Scientists said Tuesday's event was to be expected, given the volcano's eruption cycle.
Geologists said activity points to an explosive eruption, though none said they believe it will reach the intensity of the 1980 blast that killed 57 people and knocked more than 1,000 feet off the top of the mountain.
"This is really small potatoes since 1980," Major told a press briefing.
"The main pulse of this eruption lasted 10 minutes," Major said. "In 1980, it lasted nine hours. Short duration, not a lot of ash -- not a big explosion."
The 1980 eruption created the mountain's current crater. A new lava dome has been forming inside that crater for several months, and the geological agency reported Tuesday that the growth will be accompanied by low-level tremors and emissions of steam, volcanic gases and ash.
Penelope Cassidy, a spokeswoman for the Washington State Emergency Operations Center at Camp Murray, said only essential staff was on duty there. The center is outside Tacoma, more than 80 miles (129 kilometers) north of Mount St. Helens.
"We're monitoring the situation as we have since late September," Cassidy said. "It's just part of the ongoing shifting of the mountain."
A five-mile radius around the volcano has been closed to traffic since September.
CNN's Katherine Barrett contributed to this report.
