Report on Augustine (United States) — 23 November-29 November 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 23 November-29 November 2005
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2005. Report on Augustine (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 23 November-29 November 2005. Smithsonian Institution and US Geological Survey.
59.363°N, 153.43°W; summit elev. 1252 m
All times are local (unless otherwise noted)
On 29 November AVO raised the Concern Color Code at Augustine from Green to Yellow after recording long-term important changes in seismicity and ground deformation consistent with renewed volcanic unrest. There were no indications that an eruption was imminent or certain.
Beginning in May 2005, there was a slow increase in the number of earthquakes under Augustine. The earthquakes were generally small (less than M 1) and concentrated roughly 1 km below the volcano's summit. These earthquakes slowly increased from 4-8 earthquakes per day to 20-35 earthquakes per day. Additionally, data from a Global Positioning System (GPS) network on Augustine indicated that a slow, steady inflation of the volcano started in mid-summer 2005, continuing until the present. The GPS benchmark located nearest the summit moved a total of 2.5 cm. This motion is consistent with a source of inflation or pressure change centered under the volcano. This is the first such deformation detected at Augustine since measurements began just prior to the 1986 eruption. No reports of increased steaming were received by AVO, nor have satellite data shown increased thermal activity.
Geologic Background. Augustine volcano, rising above Kamishak Bay in the southern Cook Inlet about 290 km SW of Anchorage, is the most active volcano of the eastern Aleutian arc. It consists of a complex of overlapping summit lava domes surrounded by an apron of volcaniclastic debris that descends to the sea on all sides. Few lava flows are exposed; the flanks consist mainly of debris-avalanche and pyroclastic-flow deposits formed by repeated collapse and regrowth of the volcano's summit. The latest episode of edifice collapse occurred during Augustine's largest historical eruption in 1883; subsequent dome growth has restored the volcano to a height comparable to that prior to 1883. The oldest dated volcanic rocks on Augustine are more than 40,000 years old. At least 11 large debris avalanches have reached the sea during the past 1800-2000 years, and five major pumiceous tephras have been erupted during this interval. Historical eruptions have typically consisted of explosive activity with emplacement of pumiceous pyroclastic-flow deposits followed by lava dome extrusion with associated block-and-ash flows.