In the past week, he added, blocks of rock have been falling from the tip of the fin at about the same rate as it is growing at the base.
"This is not a lasting feature. It's important to recognize that," said Carolyn Driedger, a hydrologist and outreach coordinator at the Cascades Volcano Observatory.
The USGS scientists say these features typically last for weeks to several months. Nobody knows for sure when the current fin will collapse or be shoved aside.
The fin, which is steadily pushing other parts of the growing dome toward the west, has a smooth surface on the leading edge and is rough and crumbly on its sides.
"The reason for that highly smooth surface is because [magma] comes out at an angle, and the rock is literally getting ground against the walls of the vent," Driedger said.
"It's like a rock in a rock-tumbler being ground, smoothed, and polished."
The magma at Mount St. Helens, Driedger adds, is stiff and viscous, not like the runny lava flows seen in the Hawaiian Islands.
She likened the flow to toothpaste being squeezed out of a tube and making "a splat on the Earth's surface."
Eventually, the growth and crumbling of these rock features might restore the conical peak of Mount St. Helens, which was blown off in the volcano's catastrophic 1980 eruption (wallpaper: Mount St. Helens before and after).
"This is how mountains build themselves," Driedger said.
"It's not necessarily in a phantasmagorical, big, flashy-red-in-the-sky eruption, but by very gentle extrusions that can go on for years at a time."
According to Dzurisin, Mount St. Helens has built very large domes over extended periods of time throughout its geologic history.
Before this bout of dome building, the mountain's north flank grew for years to decades during the first half of the 19th century, he said.
"All indications are we are now in a period of sustained, mostly nonexplosive dome growth," he said.
"Not to say it couldn't change, but all indications are things are pretty stable."
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