VANCOUVER, Wash. - Scientists have discovered what has caused more than a million small earthquakes on Mount St. Helens over the past two years.

Comparing it to a loose wine cork, they said a huge rock plug is moving up a fraction of an inch, sticking and moving again. It grinds away and falls apart at the top, forming the spine visible from the surface.



The rock plug is about as long as a football field.

Scientists said the physics, the mechanics of what is going on miles deep inside the mountain, are really pretty simple.

"If you turn a crank on a piece of machinery, and every time you turn it one revolution, it squeaks a little, these earthquakes are kind of similar," said Dick Iverson, a geoscientist.

When the mountain erupted in 1980, it was plugged similarly but back then the cork was in pretty tight.

"What's happening now is more analogous to having a loose cork, and gas that's ... coming up through the magma and so forth is able to sneak out around the edges of this loose cork," Iverson said.

That means the mountain that has been quietly rumbling for two years should keep doing just that.

"There's nothing in the behavior we observe or in the calculations that would suggest that we're in for any sort of big explosion any time soon," he said.