"It is not an exact science," said Thomas Pierson, a research hydrologist at the Cascade Volcano Observatory here. "Nowhere in the world can anyone accurately forecast an eruption. There is a lot of art and subjectivity in this."
The key question is the source of an unusual pattern of the shallow earthquakes that grabbed the scientists attention over the weekend. Those quakes continued yesterday with equal frequency but less intensity.
They could be triggered by recent rains transformed to steam as water percolates down into the crater's lava dome, according to Pierson. That's a frequent end-of-summer event.
Or, the quakes could result from hot magma entering an area beneath the 925-foot high lava dome that sits inside the crater and serves as a kind of volcanic plug. From beneath the dome, the magma could combine with pressurized gases and steam to trigger an eruption, Pierson said.
Scientists say that any eruption could throw up a modest ash plume and also be accompanied by mud flows that would ooze out of the crater's open north end. U.S. Forest Service officials, as a precautionary measure, have closed the popular climbing route to the edge of the crater as well as three other sections of hiking trails.