The new research on the tiny quakes, which have magnitudes of less than 2.0, will be published in tomorrow's issue of the journal Nature.
Not all geologists are convinced. One of them is Bernard Chouet of the U.S. Geological Survey office in Menlo Park, California.
Chouet doubts that the plug's tiny motions can cause quakes within Mount St. Helens.
In an effort to pinpoint the location of the quakes, Chouet and colleagues examined data from 19 seismometers installed on the mountain in 2005. Seismometers measure ground movement.
Chouet's team concluded that the drumbeats were not being created by rock sliding against rock in the magma channel.
Rather, they say, vibrations in a steam-filled horizontal fracture about 330 feet (100 meters) beneath the crater floor are causing the drumbeats.
"This mechanism has nothing to do with stick/slip," he said.
In his scenario, the pressure probably comes from steam, as water inside the volcano is heated by the rising magma.
"There's plenty of water in the ground, and there's plenty of heat," Chouet said.
"It looks like Mount St. Helens is huffing and puffing and basically behaving like a steam engine," Chouet said.
Steam engine or not, none of the scientists are predicting that Mount St. Helens is about to explode like it did in 1980.
Study leader Iverson said in an email, "Large explosions at Mount St. Helens are certainly possible, as they are at any volcano.
"But with the current style and level of volcanic activity at Mount St. Helens, we believe the probability of large explosions is very low."
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