MOUNT ST. HELENS NATIONAL MONUMENT, Wash. -- Mount St. Helens blew off a spectacular cloud of steam and ash on Tuesday, the biggest plume yet in days of rumblings and the latest indication that a larger eruption may be in the works.

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Video: Massive Plume Of Steam And Ash

The volcano has been venting steam and small amounts of ash daily since Friday, but Tuesday morning's burst was the largest, producing a billowing, dark gray cloud that rose thousands of feet above the 8,364-foot-high rim of the crater and streamed to the northeast.

The National Weather Service issued an ash advisory for areas near and northeast of the mountain that lasted until noon.

The steam burst started at 9:03 a.m. and reached an altitude of about 13,000 feet.

The mountain has been emitting steam amid a series of small eruptions and volcanic tremors since Friday. The latest burst posed no danger to humans or property but the town of Randle kept students with asthma inside after getting a light dusting.

MOUNT ST. HELENS Video: Time-Lapse Of Dome’s Growth Video: Time-Lapse Of Fin Growth Video: Thermal Imaging Of Rock Avalanche Slideshow: Hulking Rock Slab Grows In Volcano's Crater Slideshow: Images From 3-8-05 Ash Plume Slideshow: Infrared Shots Of Activity At Mount St. Helens Slideshow: Dome In Mount St. Helens Crumbling Slideshow: Glowing Lava Oozes From Crater Slideshow: Large Plume Drifting North-Northeast Slideshow: Steam Eruption On 10-4 Slideshow: User Submitted Mount St. Helens Images Slideshow: Steam Vent Forms After Eruption Images: Mount St. Helens Gets Steamed Slideshow: Steam Shoots From Crater Webcam: Mount St. Helens VolcanoCam Complete Coverage Of Mount St. Helens

Scientists have said a larger eruption is likely, but there was hardly any chance of a repeat of the mountain's lethal 1980 explosion.

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IMAGES FROM THE MOUNTAIN Slideshow: Large Plume Drifting North-Northeast Slideshow: Steam Eruption On 10-4 Slideshow: User Submitted Mount St. Helens Images Slideshow: Steam Vent Forms After Eruption Images: Mount St. Helens Gets Steamed Slideshow: Steam Shoots From Crater Webcam: Mount St. Helens VolcanoCam Slideshow: Looking Back At 1980 Eruption Slideshow: Images From Mount St. Helens Crater

Scientists said Tuesday's steam burst opened two small new vents in the crater's floor, and that the floor continued to lift up, a sign that magma was still building beneath the volcano. In the last several days, the lava dome within the crater has swelled by about 150 feet.

Since Sept. 23, thousands of tiny earthquakes have shaken the mountain and several steam eruptions have occurred, the most seismic activity at the peak since the months following the 1980 blast.

Earthquakes trailed off after Tuesday's burst, dropping in both magnitude and frequency, said Bill Steele, spokesman for the University of Washington seismology lab in Seattle.

INFRARED IMAGES Infrared images taken by NASA scientists on Thursday, Sept. 30 reveal signs of heat below the mountain's surface.

"We don't really know what it means at this point as far as a prognosis," Steele said. "It could mean the plug (in the volcano's magma channel) is real up there near the surface right now -- that it's not resisting anymore. We're watching very closely."