Sister develops tell-tale bulge
After 1500 years of quiet an Oregon volcano threatens to blow.
22 May 2002 TOM CLARKE The South Sister volcano, near the town of Bend, Oregon. © USGS
An ominous bulge on a dormant volcano in Oregon, accompanied by the faint whiff of magma from deep within the Earth, suggests that the mountain is rousing itself from a 1,500-year slumber.
It is unlikely that the peak, called South Sister, will blow in the immediate future, say vulcanologists on the ground. All the same, they are stepping up efforts to monitor the volcano and to predict how an eruption might affect the town of Bend in central Oregon, just 22 miles away.
"About one in ten such cases might culminates in an eruption," says Clive Oppenheimer, a volcano researcher at the University of Cambridge, UK. The groundswell may be the beginning of an awakening that takes decades or centuries, he says.
The bulge was first spotted last year in satellite images of the Three Sisters chain of peaks. Returning to archive images, scientists working for the US Geological Survey (USGS) determined that the bulge first arose in 1996 and has grown about three centimetres a year since. It is now 13 centimetres high and covers about 100 square kilometres on the western flank of the volcano. Compared to the 100-metre-high bulge that preceded the devastating eruption of neighbouring Mount St Helens, South Sister's hump is modest to say the least. But, says Oppenheimer, it covers a vast area, so the magma movement that is causing it "must be very big".
South Sister's groundswell ringed by contour lines. "We're now tracking its growth," says USGS volcanologist Charles Wicks, based in Menlo Park, California. He and his colleagues have installed global-positioning system (GPS) and seismic monitoring instruments at the centre of the bulge. The GPS device measures changes in height and position to within millimetres, confirming that the bulge is continuing to rise.
What looks like volcanic activity smells like it too. Sensitive molecular detectors are picking up changes in the levels of chlorine and sulphur ions in spring water coming from the mountain and the type of helium gas seeping from rocks. The signatures are characteristic of new magma, suggesting that it is rising from deep within the Earth beneath South Sister. But geologists believe that an eruption is a way off, because both carbon dioxide in the air and seismic rumblings are noticeably absent.
Volcanoes typically start to release CO2 when magma is close to the surface. Also, the rumblings and small earthquakes that always precede eruptions can only occur when the existing rock cracks to let magma through.
The only way to establish if and when South Sister will erupt is to keep watching, says Wicks. The magma is still about 4 miles deep and not yet breaking through rock, so "anything could happen anywhere", he says. If it blew, South Sister could put on quite a show. The Three Sisters are composite volcanoes. Other composites include Vesuvius in Italy, Mount Fuji in Japan and Mount St Helens, just north of the Sisters in Washington state, which have all erupted with devastating consequences.
The wilderness around Bend is becoming increasingly settled. Some 110,000 people live in areas that were criss-crossed by rivers of hot ash and rubble in previous Three Sister eruptions. Based on the early evidence, the USGS is drawing up an emergency-response plan. It details which areas may be most at risk and how best to evacuate the area if South Sister were to wake from her slumber.