Fifteen months after Mount St. Helens reawakened, the volcano is continuing to release massive amounts of lava in an unusual geologic display that in some respects confounds scientists.

Roughly every three seconds, a large dump truck load's worth of lava — 10 cubic yards — oozes into the mountain's crater. And with the sticky molten rock comes a steady drumfire of small earthquakes.

The unremitting, monthslong pace is not common, said U.S. Geological Survey geologist Dave Sherrod. Experts say it is unclear what the activity signifies or how long it will continue.

"One view of this eruption is that we're at the end of the eruption that began in 1980," Sherrod said. "If it hadn't been so cataclysmic ... it might instead have gone through 30 or 40 years of domebuilding and small explosions."

St. Helens' violent May 18, 1980, eruption blasted 3.7 billion cubic yards of ash and debris off the top of the mountain. Fifty-seven people died in the blast, which left a gaping crater in place of the perfect, snowclad cone that had marked the original 9,677-foot peak known as "America's Mount Fuji."

St. Helens — now 8,325 feet — rumbled for another six years, extruding 97 million cubic yards of lava onto the crater floor in a series of 22 eruptions that built a 876-foot dome. The volcano, located about 100 miles south of Seattle, fell silent in 1986.

Then in September 2004, the drumfire of low-level quakes began — occasionally spiking above magnitude 3, but generally ranging between magnitude 1 and 2. Since then the mountain has squeezed out about 102 million cubic yards of lava.

All the recent activity has remained within the crater, though scientists — keenly aware of the potential damage that silica-laced ash can pose to jet engines — monitor St. Helens closely for plumes of smoke and ash that can go as high as 30,000 feet.

"We haven't had that kind of plume since March 8, which is either a blessing or it leads us into complacency," Sherrod said, adding quickly, "We avoid complacency."

It's not entirely clear where the lava is coming from. If it were being generated by the mountain, scientists would expect to see changes in the mountain's shape, its sides compressing as lava is spewed out.

But at the current rate of extrusion, "three or four months would have been enough time to exhaust what was standing in the conduit," Sherrod said.

That suggests resupply from greater depths, which normally would generate certain gases and deep earthquakes. Neither is being detected.

"That's one of the headscratchers, I guess," Sherrod said.