A plume of ash, steam and gas rises from the summit crater of Redoubt volcano, Alaska, on March 15, 2009. The plume was observed during an observation and gas-measurement flight by scientists of the Alaska Volcano Observatory, probably the result of a steam explosion in the volcano’s active shallow hydrothermal system. (Photo courtesy of Alaska Volcano Observatory)

 

 

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

When strong volcanic tremor (continuous shaking of the ground) was recorded beneath Redoubt volcano Jan. 23-25, scientists of the Alaska Volcano Observatory issued a notice that the volcano could erupt with very little additional warning. They began around-the-clock surveillance in case it did. 

During the previous months, there were other signs of activity that the volcano was restless. As early as July and August 2008, there were reports of sulfur smells on, and downwind of, Redoubt. 

There were both shallow and deep earthquakes beneath the volcano, increased emission of volcanic gases (including odorless carbon dioxide gas), and visible melting of ice and snow around the summit crater, Drift glacier, and the lava dome that erupted 19 years ago. 

These are common warning signs that typically occur as molten rock rises to shallower levels beneath a volcano. They are signs well-known to all volcano watchers, and the Redoubt activity is very well-documented on the observatory’s Web site, www.avo.alaska.edu/activity/Redoubt.php. 

At the time of this writing (March 19), the most recent new activity occurred March 15, when the first known small explosion and short-lived ash and steam plume occurred in the summit crater. The plume reached only about 5,000 feet above the volcano, and ashfall was limited to within a few miles of the summit crater. 

The source of explosion and plume was a new hole in the ice about 450 feet in diameter near the 1990 lava dome. 

Since late January, seismic activity at Redoubt has waxed and waned, and scientists continue to look for evidence that new magma has risen to within 3-5 miles of the surface. They want to know whether this magma is continuing to move upward or stagnating at a shallow level. 

Everyone wants to know the outcome, but in the meantime, constant vigilance is required from scientists, residents of Alaska, the airline industry, and the Cook Inlet oil facilities in case the new magma rises quickly to the surface as it did during the most recent eruption in 1989-1990. 

Nineteen years ago, Redoubt went from a state of “quiet” to significant explosive eruption in only 23 hours. Hundreds of small earthquakes, beginning Dec. 13, 1989, were the tip-off that magma was probably moving to the surface. 

Scientists of the new Alaska Volcano Observatory, established the year before, issued notice that the volcano could erupt at any time. The eruption began Dec. 14. 

Then, Dec. 15, an even stronger explosive eruption generated a cloud of volcanic ash and gas that was blown quickly by strong winter winds north of Anchorage. A 747 passenger jet descending to land at Anchorage International Airport unknowingly flew into the ash cloud and quickly lost power in all four engines. 

After a few harrowing minutes, the pilots were able to restart the engines and land safely at Anchorage. The jet suffered $80 million in damages, and this encounter set in motion an aggressive, forceful effort to prevent such encounters in the future. 

During the next several months, the eruption of a series of lava domes in the summit crater and their “collapse” down the volcano’s steep, ice-covered north flank generated pyroclastic flows and many more ash clouds and lahars (volcanic debris flows) that swept down Drift River Valley 22 miles into Cook Inlet. 

The largest lahar on Jan. 2, 1990, swept through part of an oil terminal facility located at the mouth of the river, causing minor flooding of some buildings. No damage was done to the terminal’s oil tanks. 

Since 1990, jet traffic over the North Pacific has increased, and the oil terminal is still operating (safety improvements were made in 1990 in preparation for future lahars from Redoubt). In historical time (since 1760 in Alaska), 28 volcanoes have erupted along Alaska’s 1,500-mile long chain of volcanoes, some erupting dozens of  times. 

Scientists of the Alaska Volcano Observatory have greatly expanded the monitoring of volcanoes with continuously recording instruments from 4 in 1990 to 31 today. Many more are monitored with only satellite imagery as a way to detect new thermal “spots” on a volcano or clouds of ash and gas dispersing downwind.