The 2009 eruption of Redoubt volcano began March 15th, 2009 with a steam explosion. Between March 22nd and April 4th, Redoubt produced multiple significant explosions that sent ash and gas clouds to as high as 65,000 feet (19.8 km) above sea level. After April 4th, the eruption continued with extrusion of a lava dome within the summit crater, eventually producing a blocky lava flow that currently extends ~0.6 miles (1 km) down the north flank of the volcano. Redoubt entered its 14th week of eruptive activity the week of June 22nd.

Observations from overflights, webcam, and satellite imagery have not been able to detect changes caused by dome growth for the past three weeks, thus lava effusion has at least slowed significantly, and may have stopped. Consequently AVO downgraded the aviation color code to YELLOW and the alert level to ADVISORY on Tuesday June 30th, 2009. Volcanic seismicity is still reduced, after a decline in the numbers of earthquakes per day began about May 7th. However, seismicity remains elevated above background. Volcanic gas emissions remain very high, yet have also decreased by about half since early-May. AVO will continue to monitor Redoubt's activity closely, but will begin to transition off 24/7 staffing. Later updates will describe operational changes as they occur.

Despite evidence for the significant slowing, and possible cessation of its growth rate, the lava dome is potentially unstable and the possibility of a full or partial collapse remains high at present. Such a collapse would likely be accompanied by the production of a large ash plume and lahars in the Drift River valley. This event could occur with little or no advance warning.

Recent Observations

As of June 9th, the lava dome is approximately 67.5 million m3 in volume, estimated from analyses of visible and infrared photographs. The lava dome is now larger than any produced during the 1989-90 eruption. The dimensions of the dome have not increased perceptibly over the past 3 weeks, and are estimated to be 1000 m (3280 ft) long (north-to-south) 460 m (1510 ft) wide (east-to-west) and 200 m (655 ft) tall. Rock fall from the slopes of the dome continues, though at a lower rate than the previous month. Recent thermal measurements made during helicopter overflights indicate that most of the lava dome surface is relatively cool (~90°C or less), while smaller, hotter regions reveal much higher interior temperatures (>250°C). The measurements reflect the temperature of the cooling lava and do not represent the actual lava temperature as it emerges from the vent. Temperatures of fresh lava of this composition should be considerably higher (~850 to 950°C).

Seismic activity at Mount Redoubt has declined over the past month, but remains elevated above background levels. Generally, fewer than 6 earthquakes per day have been recorded at Redoubt during the past two weeks. Normal background rates of earthquake occurrence at Redoubt are typically fewer than four located earthquakes each month, which is a significantly lower rate of occurrence than what AVO observes at this time. A M3.1 event did occur ~6 km east to northeast of Redoubt's summit the morning of Tuesday June 30th, also showing that the volcano remains seismically active at this time. Seismicity during the past month comprises small, discrete events and occasional signals from rock falls off the slopes of the unstable dome. Some rock falls have generated diffuse, low-level ash clouds in the summit region and minor ash deposits around the dome.

Geodetic data indicate that deflation of the volcanic edifice has also slowed over the past month. A change in this trend towards renewed inflation could signify unrest leading to renewed eruptive activity.

Airborne gas measurements indicate that Redoubt's emissions remain very high and are currently still at levels typical for an erupting volcano, yet they have declined measurably over the past couple of weeks. These gas emissions are consistent with degassing expected from the lava extrusion that continued through early June. Using past eruptions at Redoubt and other andesitic volcanoes (Augustine and Mount St. Helens) as a reference, we expect the emissions to continue to decline slowly over a period of several months as already-erupted magma degasses.

Prognosis and Ongoing Hazards

The current lava dome has been in place since the last explosive event occurred on the morning of April 4th. For most of this time it grew at a rate that was easily noticed and measured using a combination of satellite- aircraft- and ground-based imaging systems. The growth was accompanied by morphologic changes, frequent rockfall, and cracking of the outer cool skin, exposing incandescent viscous lava. By June 9th effusion of magma had slowed at least to the point that we are unable to detect any further growth, and may have stopped. As of this writing there have been 87 days of uninterrupted growth. This contrasts with a maximum of 36 days of dome growth between significant explosive events in 1990, apart from the final 1990 dome, which was stable for nearly 19 years and was destroyed at the onset of the current eruption.

Declining seismicity and gas emissions also indicate a decline or cessation of eruption. Based on our current observations at Redoubt volcano, and understanding of prior eruptions, AVO considers the following future scenarios as possible:

  1. Continued decline in seismicity and gas emissions with no measureable output of lava leading to an end of the eruption and a return to background.
  2. Renewed explosive activity triggered by sealing of the conduit and the resulting buildup of gas overpressure, or as a result of fresh magma ascending into the shallow conduit beneath Redoubt.
  3. Renewed lava dome growth, caused by increase in magma extrusion rate.
Based on all available monitoring data, AVO considers scenario number 1, waning lava dome growth leading to the end of the eruption, as the most probable outcome at this time. The most likely hazard related to scenario 1 would result from collapse or failure of the lava dome due to gravitational instability. This could happen with little to no warning, and would likely cause an ash cloud to form in association with the resulting hot block and ash flow, as well as lahars that could significantly impact the Drift River valley. Based on the progression of events after cessation of lava dome growth during the 1989-1990 eruption of Redoubt, it is also possible that small steam explosions and continued rock fall events could occur within the immediate vicinity of the summit crater and upper Drift glacier valley.

Although it is considered less likely at this time, renewed explosive activity could also occur (scenario 2), triggered by a build up of gas overpressure as a result of sealing of the conduit. This could be caused either by gas pressurization from continued magma degassing, or as a result of new magma ascending in the conduit beneath Redoubt. Alternatively, the lava dome growth rate could increase in response to an increase in the effusion rate of magma from the shallow system beneath Redoubt (scenario 3). Given the large size of the present lava dome, increased lava dome growth could also lead to failure due to over-steepening of the dome. Either scenario 2 or 3 (or a combination of the two) would likely generate a flow of hot lava blocks and ash down the north slope of the volcano, and a significant ash cloud to elevations of 30,000 feet or more above sea level. Flooding of the Drift River valley would also be a likely outcome. It is likely that renewed activity leading to scenario 2 or 3 would be preceded by a measurable increase in seismic or other unrest, which would bring AVO back to full alert. However, it is also possible that renewed explosions or extrusion could occur with little or no warning.