A magnitude 3.0 earthquake rattled Washington's Mount St. Helens Friday morning, triggering a rockfall and sending an ash plume above the rim.
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"This is the largest quake that has occurred in some time, and its exact cause is under debate," according to a statement from the U.S. Geological Survey.
The moderate temblor occurred at 5:22 a.m. local time. While such a quake would not likely be felt by most people, sensitive instruments on the mountain have no trouble detecting it.
Earthquake activity sometimes precedes volcanic eruptions, but scientists can't yet make firm predictions from the seismic activity.
While the volcano has been relatively quiet lately, a lava dome inside the main crater continues to grow. Minor eruptions of ash and volcanic gas have been common.
An eruption "could intensify suddenly or with little warning and produce explosions that cause hazardous conditions within several miles of the crater and farther downwind," says the USGS.
Mount St. Helens sprang back to life last fall with a series of strong eruptions that spewed ash miles into the sky.
The volcano's last major upheaval was in 1980, when 57 people were killed.
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