Augustine volcano erupts quietly.

Augustine volcano erupts quietly

Avalanches, glowing lava and 10,000-foot-high trails of ash and steam signaled on Aug. 20 that the Augustine Island volcano in Alaska entered a new phase of activity, the third outburst since it erupted last March 27 (SN:5/17/86, p.309). Located at the entrance to Cook Inlet, 175 miles southwest of Anchorage, the 4,025-foot volcano is also sending dense flows of ash and steam down its slopes and building a new dome of molten rock to replace the one it partially blew off in March.

This activity is characteristic of volcanoes having magma rich in silica. According to Charlotte Rowe, a graduate student at the University of Alaska in Fairbanks who is conducting research on the volcano, the magma flowing up through the volcano is thick and slow-moving, unlike the runny magma of the silica-poor Hawaiian volcanoes. "It oozes out like toothpaste coming out of a tube,' Rowe says. When it reaches the surface, the magma tends to cool and plug up the opening, forming a dome.

Silica-rich magma also holds a higher proportion of gases, which bubble out of the upwelling molten rock. If a dome seals the opening of the volcano, the pressure builds up inside until the gas explodes in a new eruption.

The March explosion, says Rowe, was mainly the release of gases. The magma is continuing to ooze upward, cracking open the remains of the old dome and daily sending three to four avalanches of debris down the slopes. Rowe says the absence of significant earthquakes at this stage indicates that the magma now flowing is part of the eruption that began violently in March. "If there were new magma from deeper levels . . . we would expect to see seismic activity similar to that just prior to the March eruption,' she says. While the recent activity could continue for months or years, the likelihood of another explosion soon is "probably less than half.' Rowe notes, however, that sparse information about the volcano makes predictions difficult.

Photo: In a calmer continuation of its March eruption (left), Alaska's Augustine volcano spews ash and steam (right).