SISTERS, Ore. – The swelling bulge on the west flank of the South Sister volcano is slowing, and geologists say there are no signs that the uplifted region will erupt in the near future.
The latest statistics from instruments monitoring the bulge indicate the uplift has slowed to about half its former rate of about an inch a year.
Geologists believe the bulge is being created by magma pushed into a chamber four or five miles underground, and its slowing means less the liquid rock is flowing upward. They don’t know why the rate has seemed to ease.
“It’s possible it’s slacking off, but over the coming years, the rate will change,” said geologist Willie Scott at the Cascades Volcano Observatory in Vancouver, Wash. “That’s why we want to continue to monitor it as closely as we can, because we’re seeing a phenomenon that we haven’t been able to study before, and we want to see where it goes.”
Scientists have been monitoring the central Oregon volcano since 2001, when a comparison of newly acquired satellite measurements showed a 10-mile-wide portion of the mountain had risen more than 4 inches over four years.
Since then, instruments have been set up around the mountain, and geologists are using three different techniques to measure ground movement.
A swarm of more than 300 small earthquakes struck the area in March 2004, but the bulge has since been quiet. Only five quakes were recorded this year, none with a magnitude greater than 1.5.
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