Mount Augustine is a relatively new volcanic vent in lower Cook Inlet, Alaska. It sits about 70 miles WSW from Homer, Alaska, in the inlet itself, and 85 miles north of the Katmai – Novarupta complex. It sits in an island measuring some 8 x 11 km with a single cone to 4,100 feet (1260 m). It is surrounded by four webcams and two webicorders, all output available via the Alaska Volcano Observatory (AVO) at the following link.
The cone is relatively new, with rocks no older than 40,000 years present. It has exhibited most of the expected activity out of a stratovolcano including multiple flank collapses, dome building, dome collapses, debris flow caused local tsunamis, and pyroclastic flows.
The most recent eruption was 2006. The precursory phase with increasing earthquakes started May 2005 through January 2006. It was marked with increasing frequency of earthquakes and visible steaming and some light ash dusting in December. An explosion in December disabled two seismic stations located closest to the summit.
The first explosive activity took place on January 11, 2006, with a pair of afternoon explosions that produced plumes to nine kilometers high. Ejecta was primarily old, weathered fragments indicating no new magma at first – a throat clearing. Subsequent explosions two and three days later dusted the Kenai Peninsula with ash. Analysis of the ash showed new magma. Heights of the plume exceeded 14 km. The explosions also produced pyroclastic flows, lahars and debris avalanches. Dome building was observed between explosive episodes.
By Jan. 28, activity was near continuous and this phase lasted until Feb. 2. Plume heights during this period were measured at 9 km. There were also pyroclastic flows, debris avalanches that destroyed yet another set of seismographs on the flanks of the volcano.
The final phase of the eruption was effusive from Feb. 2 through late March. In addition to dome building, there were blocky, highly viscous lava flows down one of the flanks. This phase is estimated to have produced 30 million cubic meters of material.
Following the main eruption, steam and anomalous seismic activity were observed through 2007. AVO lists this as a VEI 3 eruption.
Earlier eruptions and activity were similar in size with six of them between 1935 – 1986. Additionally, there were several debris flows and avalanches observed down the steep, unstable flank.
The largest observed eruption of Augustine was in 1883 (apparently a good year for volcanoes), with a VEI 4 eruption that included a flank collapse of the north part of the island that put around a half a cubic km of debris into Cook Inlet, creating a localized tsunami. Runup of the tsunami was 6 – 8 feet (2 – 2.5 m) in Homer – Seldovia. Analysis of older tsunami deposits in the Inlet find runups from 20 – 26 feet (6 – 8 m) in areas closer to the island, indicating the 1883 event was not an isolated incident, and this had happened in earlier eruptions.
Runup of tsunamis here in Cook Inlet is highly dependent on high tide or low tide, as there are many parts of the Inlet where the difference in high and low tide exceeds 30 feet (10 m). A well timed collapse and nobody will know the difference. Do the same thing at high tide and a lot of things get wrecked and wet.
Augustine was one of the locations where the Krafts studied pyroclastic flows before their unfortunate demise at Unzen in 1991. They were on the mountain during the eruption of 1986 and photographed and filmed at least one pyroclastic flow. I believe I saw a video taken by them in one of the Discovery Network’s endless All Disaster, All the Time shows but have not been able to find a YouTube of the film they took. Did find a photo attributed to their visit, which is rather closer than I would want to get to one, but they were resident experts and I a mere strap-hanger.
Augustine will continue to be one of the more active volcanoes in Alaska. It does not yet appear to have built to the point where a caldera forming eruption is in its near future. But if our experience with stratovolcanoes here in Alaska is any indication, somewhere along the line, it will do that sort of eruption.
Its location in Cook Inlet is a bit to the east of the volcanoes that populate the Alaska Range / Alaska Peninsula, which may or may not mean anything.
The Photovolcanica web site notes that it is a prolific volcano, with output between 10 – 100 times that of similar subduction volcanoes. They point out that it has two historic flank collapses – 1883 and one 450 years ago, and that its voluminous output is what builds the mountain to the point where it regularly collapses. Photovolcanica believes its summit may be at or near a maximum height before a future collapse, and finish with speculation that a flank collapse is in the not so distant future, something that we do not look forward to seeing.