The event was most probably was caused by growth of the new lava dome inside the crater, experts said.
During such eruptions, changes in the level of activity can occur over days to months.
The eruption could intensify suddenly or with little warning and produce explosions that cause hazardous conditions within several miles of the crater and farther downwind.
Under current conditions, small, short-lived explosions may produce ash clouds that exceed 30,000 feet in altitude, meaning commercial aircraft are being warned to avoid the area.
Tuesday's event was a far cry from the active volcano's 1980 eruption.
At 8:32 Sunday morning, May 18, 1980, Mount St. Helens erupted with a force that caused an earthquake of 5.1 on the Richter scale.
The eruption lasted 9 hours.
The north face of the mountain eventually collapsed in a massive rock debris avalanche.
Nearly 230 square miles of forest was blown down or buried beneath volcanic deposits.
A mushroom-shaped column of ash rose thousands of feet skyward and drifted downwind, turning day into night as dark, gray ash fell over eastern Washington and beyond.
In 1982, Congress created the 110,000-acre National Volcanic Monument for research, recreation, and education.
Inside the Monument, the environment is left to respond naturally to the disturbance.