The 2009 eruption of Redoubt Volcano has entered its 10th week. Lava continues to erupt from a vent in the summit crater feeding a blocky, rubbly lava dome
that extends out of the summit crater and more than 500 m (1640 ft) down the north flank of the volcano. Although volcanic seismicity has declined somewhat
in the last few weeks, it remains elevated above pre-eruption background. Volcanic gas emissions also remain elevated. The growing lava dome is potentially
unstable and the possibility of a full or partial collapse remains high. Such a collapse would likely be accompanied by a large ash plume and lahars in
the Drift River Valley. This event could occur with little or no warning. Consequently, AVO continues to monitor the volcano 24 hours per day and the
aviation color code remains ORANGE and the alert level WATCH.
Recent Observations
The current lava dome is estimated to be roughly 40 million cubic meters in volume based on analyses of thermal and satellite images and aerial photographs.
This amount of material would fill approximately 11 Louisiana Superdomes. Using measurements from a Worldview satellite image on May 17, the dome and tongue
of lava that extends to the north is about 950 m (3115 ft) long (north-to-south), 460 m (1510 ft) wide (east-to-west), and 200 m (655 ft) tall. Over
time, the dome is growing in size primarily to the north but to the east and west as well. There is a slight overhang developing at the very front
(northernmost part) of the lava tongue in the upper Drift gorge.
Nighttime images from the AVO webcam continue to show occasional incandescence or glow as rockfalls expose hot interior dome rock. Thermal images obtained
during helicopter overflights indicate portions of the surface of the lava dome exceed 350 degrees C. This value reflects the temperature of the cooling
crust and is not a direct measurement of actual lava temperature as it emerges from the vent. Temperatures of fresh lava of this composition should be
considerably higher (~850 to 950 degrees C).
Seismic activity at Mount Redoubt remains elevated with numerous small volcanic earthquakes and signals from small rock avalanches recorded on the stations
closest to the volcano. Many of these small rock falls generate minor ash plumes that may rise several thousand feet above the crater rim and produce
localized fallout.
Airborne gas measurements over the past several weeks indicate that Redoubt's emissions remain high. For the month of May, preliminary calculations indicate
that average emissions are in excess of 8,000 tonnes/day of SO2 and 15,000 tonnes/day of CO2 . These values are similar to the range of values measured during
the early dome building phase of the 1989-90 Redoubt eruption and are consistent with strong degassing expected during active lava extrusion.
Reviewing data on erupted lava composition, seismicity, and deformation (as measured by high-precision GPS measurements made in the vicinity of the volcano),
AVO scientists infer that magmas feeding the lava dome are rising from at least two main staging locations at 3 to 5 km (2-3 mi) depth below the surface and
8 to more than 15 km (5-9 mi) depth. The processes of magma ascent and eruption have produced a variety of mixed andesitic magmas sampled by AVO since the
start of the eruption in March.
Prognosis and Ongoing Hazards
The last explosive event of the current eruption occurred on the morning of April 4 destroying the lava dome that had grown in the summit crater in late
March and early April. The current lava dome has now been growing for 53 days and is similar in size to the larger domes emplaced during the 1989-90
eruption. During the 1989-90 eruption, the longest period of dome growth between explosions was 36 days. Based on the size of the current lava dome and the
likelihood of increasing instability as it extends down the steep upper Drift gorge, AVO expects it will most likely be destroyed either through gravitational
collapse or an explosion from within the shallow vent system.
The Redoubt eruption is expected to continue for additional weeks to months. During this time, a cycle of relatively quiet periods of lava dome growth
followed by brief explosive episodes of dome destruction will likely take place. Future explosions pose an ongoing threat of lahars in the Drift River
Valley, trace to minor ash fall throughout south-central Alaska, and ash-related impacts to aviation. However, the possibility remains that the lava
dome growth rate may slowly diminish and the dome itself will stabilize and no further explosive activity will occur.