After two months of restlessness and a day of heightened earthquake activity, Alaska’s Mount Redoubt Volcano erupted explosively on Sunday, March 22, 2009, at 10:38 p.m. Alaska Daylight Time, according to the Alaska Volcano Observatory. Overnight, four additional large eruptions occurred, the fifth one larger than the previous ones. Scientists estimated the plume reached a height of 50,000 feet above sea level. As the ash spread north and northeast, ashfall advisories were issued for communities north of Mt. Redoubt. Geologists warned of the possibility of mudflows and debris avalanches from melting ice on the mountain’s summit.

Photograph taken during observation / gas collection flight to Redoubt Volcano late in the afternoon on March 26, 2009. At this time, no ash was being emitted from the volcano, however a vigorous water vapor and volcanic gas plume was rising to about 15 - 20,000 feet and drifting to the east. Iliamna Volcano is just visible a left. The discoloration in the atmosphere around the volcano can be thought of as volcanic smog.

Picture Date: March 26, 2009
Image Creator: Neal, Tina;

Image courtesy of AVO/USGS.
Please cite the photographer and the Alaska Volcano Observatory / U.S. Geological Survey when using this image.

In early May 2009, the Alaska Volcano Observatory (AVO) warned that Mount Redoubt could erupt explosively at any time with little or no warning. The AVO cited the volcano’s seismic activity, gaseous output, and lava-dome growth as reasons for concern. The AVO warned that a collapse of the lava dome could send significant amounts of ash and meltwater down nearby Drift Glacier.

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite acquired this image of Mount Redoubt and its surroundings on May 5, 2009, when the volcano exhibited little visible activity besides a plume of vapor. In this false-color image made from a combination of visible and infrared light, the bright white steam plume hovers over the volcano’s summit. Immediately southwest of this plume, clouds appear fairly thin and dull. North of the volcano, pristine snow rests on the land surface, but southeast of the volcano, ashfall from earlier eruptions has stained the icy surface. (In the large image, the vegetation in the coastal areas and in river valeys is red.)

Redoubt is a stratolvolcano—a steep-sloped, conical volcano composed of layers of hardened lava, solidified ash, and rocks ejected by previous eruptions. Starting on March 22, 2009, after weeks of unrest, the volcano suddenly erupted five times in one night. Redoubt remained intermittently active afterward.

These eruptions all occurred in darkness, which means that photo-like images from satellites weren’t possible. But the plumes were detectable in thermal infrared imagery captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra (top) and Aqua (bottom) satellites. Temperatures range from warmer (black) to colder (white).

The Terra MODIS image was captured at 12:30 a.m. March 23, just 16 minutes after the third large eruption. Two plumes of ash are visible: a long white plume reaching north, and a smaller one just northeast the volcano. Communities along the trajectory of the ash plume included Skwenta and Talkeenta.

It might seem counterintuitive that the plumes of volcanic ash are the coldest things in these images—after all, aren’t volcanic eruptions hot? The coldness of the plume is a sign of how high it reached in the atmosphere. Terra and Aqua can’t see through clouds or thick smoke or ash. When no clouds are present, Terra and Aqua see the thermal infrared energy (heat) radiated by the Earth’s surface. (This ability frequently makes it possible for the sensors to detect volcanic eruptions.) But when clouds or thick ash block the satellites’ views of lower altitudes, then the heat the satellites see is the heat from the top of the cloud or ash plume. The stronger the eruption, the higher into the atmosphere the plume climbs, and the colder its top becomes.

After weeks of heightened seismic activity, Alaska’s Redoubt Volcano began erupting explosively on the night of March 22, 2009. On March 26, 2009, the volcano erupted again, sending a column of ash some 19,800 meters (65,000 feet) above sea level, and sending a lahar—a slurry of water and volcanic debris—through the Drift River Valley.

NASA’s Landsat 5 satellite captured this true-color image of Redoubt Volcano and its immediate surroundings on March 26, 2009. Against the backdrop of a snowy landscape, the volcano’s steam plume must be found by its texture rather than its color. Above the volcano summit, the vapor billows like giant balls of cotton. On either side of the steam plume, brown volcanic ash appears. Although some of the ash is probably airborne (a large swath of airborne ash appears in a wider view of the area acquired on the same day), some of the ash has settled on the ground. Settled ash is especially apparent east of the plume, where small ridges appear surrounded by darker ash. North of the volcano summit, the lahar appears nearly black, narrowing into the Drift River Valley.

Redoubt is a stratovolcano composed of alternating layers of hardened lava, solidified volcanic ash, and debris produced by earlier eruptions. Sitting on the western side of Cook Inlet, the volcano rises to a height of 3,106 meters (10,197 feet). It is part of the Pacific “Ring of Fire”—a horseshoe–shaped zone of heightened seismic activity around the Pacific Ocean Basin.