"The sand is going to subside, and individual grains of sand are going to move inward a little bit toward the place directly over the balloon."
Scientists observed this phenomenon when Mount St. Helens began erupting in 2004. Instruments tracked by satellites have been found to be sinking downward and toward the volcano.
But within the past two years, the instruments have become nearly motionless, even though magma continues to ooze out onto the surface.
"That's a piece of evidence that would be consistent with this idea that we've got an open system—that is, what's coming out the top is being replaced from the bottom," Dzurisin said.
"But the volcano isn't that clear-cut," he added.
Scientists are also contemplating a closed system in which the magma oozes out so slowly that the land deformation is undetectable.
Specialized photographers fly over Mount St. Helens about once a month and take pictures of the lava dome growing inside the crater with a high-resolution camera.
Scientists then use these images to create digital elevation models with computer software.
By comparing one model to another, the scientists are able to keep tabs on how fast the lava dome is growing.
In 2004, when the eruption began, the rate of growth was 8 cubic yards (6 cubic meters) a second. Within a year the rate dropped to between about 1.5 and 2.5 cubic yards (1 and 2 cubic meters) a second.
But since last April the rate has held fairly constant around 0.6 cubic yard (0.4 cubic meter) a second.
A steady, consistent flow would support the open-system theory, Dzurisin noted.
"We all agree those are the numbers, and we all agree they are lower than they were earlier in the eruption, but we're just going to have to wait and see whether it continues to decline or whether it has in fact leveled off," he said.
Scientists are also studying the chemical signature of the lava that comes out at the surface.
If different lava suddenly appeared, Dzurisin said, that would be strong evidence for an open system.
But scientists examining the lava say it's all been pretty much the same since the eruption began in 2004.
"So now you've got a piece of evidence that maybe it's really a closed system," Dzurisin said.
Of course, he added, perhaps the lava from the greater depth has yet to reach the volcano surface.
"You can imagine how that dialogue goes on," he quipped.
Katharine Cashman is a geologist at the University of Oregon in Eugene who has studied Mount St. Helens. She considers the volcano an open system because lava has been spewing for several years.
Modeling evidence also suggests that so much material has come out that it must have a deeper, more plentiful source than a shallow volcanic magma reservoir, she said.
But the problem, Cashman added, is that scientific instruments are unable to see deep into Earth's crust, making the question difficult to answer.
"You can only look at other volcanoes around the world, and there are some that just keep erupting for decades," she said.
"So it's possible that once it gets going, it can keep going. But it could also stop."
"It's a little humbling. It makes me realize how imprecise the science is," she said.
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