2009 Mount Redoubt Eruptive Activity - Aftermath of 2009 Eruptions and Prognosis For Future Activity

Aftermath of 2009 Eruptions and Prognosis For Future Activity

On April 11, 2009, the Alaska Volcano Observatory released the following statement into the public domain on their USGS affiliated website regarding the aftermath of the 2009 eruptions of Mount Redoubt, and a prognosis for future activity.

Summary

The 2009 eruption of Redoubt Volcano continues. A lava dome is currently growing in the summit crater, accompanied by intermittent emissions of volcanic gases and minor amounts of ash. Additional explosive events are likely and could send ash to greater than 30,000 feet above sea level. Such a cycle of dome growth and explosive dome destruction may continue for many months. The potential for lahars (volcanic mudflows) and other flooding down the Drift River Valley remains, as does the potential for trace to minor ash fall on communities near Redoubt (e.g., the Kenai Peninsula Borough, The Municipality of Anchorage, and the Matanuska-Susitna Borough, and possibly more distant areas).

Observations

The 2009 eruption of Redoubt Volcano began with a minor explosion of steam and ash just after 13:00 AKDT on March 15. Major explosive events began at approximately 22:40 March 22 AKDT, and since then AVO has recorded more than 19 separate explosions. Plume heights, as measured by radar and confirmed by pilot reports, have exceeded 50,000 feet (15 km) above sea level on multiple occasions. To date, the largest explosion occurred at 05:55 AKDT April 4, lasted more than 30 minutes, and is comparable in size to the largest event of the 1989/1990 Redoubt eruption.

Three major lahars (volcanic mudflows), on March 23, March 26, and April 4, have inundated the Drift River Valley and its downstream coastal fan. All of these reached the Cook Inlet and affected the Drift River Oil Terminal (DROT). The peak discharge rates of these lahars remains under investigation, but all are considered significant, and the April 4 event may have exceeded the size of any lahar observed during the 1989/1990 eruption. Smaller lahars that did not impact DROT have also occurred.

Several of Redoubt's recent explosions have resulted in measurable ash fall over populated areas as distant as Delta Junction (340 miles northeast of Redoubt), with more significant ash fall in more proximate areas, including the Susitna Valley, the Kenai Peninsula and the Anchorage bowl. On the afternoon of March 28, ash fall in Anchorage closed the airport from 17:00 until 07:00 the next morning (March 29). The maximum ash fall measured so far in a populated area is about 1/16 inch (1.5 mm) near Seldovia following the April 4 explosion. A measurement of about 1/8 inch (3 mm) following the explosion on March 26, was recorded near Silver Salmon Creek Lodge, about 30 miles (48 km) south of the volcano. Trace ash fall has also been observed between explosive events during times when the volcano is emitting a continuous low altitude (< 15,000 feet ASL) gas and ash plume.

A rich variety of seismic signals have been recorded at Redoubt throughout the eruption and in the preceding months of unrest. These seismic events include: (1) typical "rock-breaking" earthquakes; (2) volcanic "tremor", indicative of steam and other volcanic gases or fluids vigorously propagating through cracks; (3) large, far-reaching, "cigar-shaped" signals resulting from volcanic explosions; (4) ground shaking from lahars as they pass nearby seismometers; and (5) small, repetitive, self-similar events associated with the slow extrusion of lava (this kind of extrusion is often referred to as "dome building"). More than 1700 earthquakes have been located since mid-January, and many more occurred that were either too small or of such a character as to make location impossible. Typically, the magnitudes of these events are small, the average being around M0.5. However, on April 9 a magnitude ML 3.3 earthquake occurred about 2.5 miles (4 km) E/NE of Redoubt's summit, possibly resulting from a crustal adjustment to the ongoing withdrawal from Redoubt's subterranean magma reservoir.

Since the fall of 2008, AVO has flown 13 gas measurement flights, and of these, 5 have occurred since the eruption began on March 15, 2009. Elevated carbon dioxide (CO₂) emission rates (up to 2000 tonnes/day) were measured in October–November, 2008. Starting in late January 2009, and coincident with a strong increase in seismicity, gas emission rates rose to a level (> 5000 tonnes/d) suggesting significant unrest at the volcano, and emissions stayed at this level until the eruption began. Since that time, emissions of both CO₂ and sulfur dioxide (SO₂) have been very elevated, sometimes reaching levels in excess of 10,000 tonnes/d. These volcanic gas emission rates are among the largest ever measured in Alaska, though such high values are consistent with an openly degassing volcanic system that is actively extruding lava. Based on measurements and observations from Redoubt's previously observed eruptions, these gas emission rates are likely to drop substantially when the eruption wanes.

The combination of gas and ash emission from Redoubt since the beginning of the eruption has on occasion resulted in a brownish-yellow volcanic haze in the Cook Inlet region. The volcanic haze contains small quantities of ash, water vapor, sulfur aerosols, and liquid droplets suspended in the air. The main concerns for human health in volcanic haze consist of ash, sulfur dioxide gas (SO₂), and sulfuric acid droplets (H₂SO₄), which forms when volcanic SO₂ oxidizes in the atmosphere. Volcanic haze can be both an eye and respiratory irritant. The State of Alaska Epidemiology Bulletin No. 5 summarizes the health effects associated with volcanic emissions and is available on the web at http://www.epi.hss.state.ak.us/bulletins/docs/b2006_05.pdf.

Visual and satellite observations show that by March 27, a deep crater about a half mile (800 m) across formed in the middle of the summit amphitheater, likely from one or more of the explosions during the early stage of the eruption. By April 4, a north/south elongated lava dome, about a half mile (800 m) long had grown out of the new crater, at an elevation of around 8,000 feet (2,400 m) above sea level. Most if not all of this dome was destroyed during the large explosion of April 4. A large amount of ice and snow has been removed from the upper Drift Glacier. The Drift River Valley has also been mostly denuded of snow and is covered with lahar deposits. The latest observations, as of about April 9, show a growing lava dome in about the same place as the previous dome. This new dome is about a quarter mile (400 meters) in diameter, and roughly circular as viewed from above.

Prognosis and Ongoing Hazards

The 2009 Redoubt eruption began with a series of large explosions on March 22–23, followed by less-energetic dome growth in the summit crater. On April 4, another large explosion occurred, which was in turn followed by growth of the current dome. There will likely be additional explosive events during the coming days to months, but their exact nature, as well as timing, remains uncertain. Because the dome sits at the lip of a steep slope leading out of the crater it can become unstable, and (as in 1990) collapse into the Drift River valley, producing avalanches of hot lava blocks, pyroclastic flows, lahars, and ash columns tens of thousands of feet tall, which can carry ash to Alaska communities. During periods between explosions, residents and pilots may also notice occasional sulfur smell and hazy conditions due to low-level ash and volcanic gas emissions.