St. Augustine Volcano
Augustine Volcano is the most frequently active and the youngest of the Cook Inlet volcanoes. Detterman considered Augustine to be entirely Quaternary and Johnston concluded that volcanism at Augustine began during the late Pleistocene Moosehorn glacial advance 19,000-15,500 IBP. Since its discovery by Captain James Cook in 1778, Augustine Volcano has had seven historical eruptions. 1883, 1935, 1979, 1985, 1986, and 1987. Yount and others, 1987. The activity in 1908, reported by the captain of a steamer enroute to Seward, was minor and probably did not produce any deposits.
The 1883 eruption appears to have been the most violent historical eruption of Augustine, and is thought to have generated a tsunami with wave heights of 7.5 to 9 m at English Bay on the Kenai Peninsula 80 km east of the Davidson Islands, 1884. The tsunami has been attributed to a debris avalanche from the north side of the volcano into Cook Inlet.
The most recent eruption, which began on March 27, 1986, after more than five weeks of increased seismic activity and continued through August 1986 and, is probably typical of most Augustine eruptions. A nearly continuous ash-rich plume rose 3,000 to 4,600 m; periodic explosive bursts reached altitudes of 12,200 m. Numerous pyroclastic flows were generated in the early stages of the eruption and moved down the north flank. Several large flows reached the north shore 5 km away and entered the sea. Much of the snowpack on the upper flanks melted producing lahars. As the eruption evolved, generation of pyroclastic flows diminished and dome building began. A short lava flow also issued down a steep gully on the north flank.
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Military and commercial air traffic was disrupted in upper Cook Inlet during the first week of the eruption when airborne ash was moving northward. Light ashfall occurred over most of the Cook Inlet area, and ash was detected as far north as the Brooks Range several days into the eruption.
Eruptions of Augustine typically consist of multiple phases spanning several months. During each phase, mudflows and pyroclastic flows often accompany explosive ash eruptions. Usually the first phase is the most violently explosive; successive phases often include extrusion of lava, enlarging the central dome and lava flow complex.
Augustine volcanic rocks are of calc-alkaline landsite and deceit composition but of the low-K, rather than medium-K variety. Phenocryst phases include plagioclase the dominant phase, orthopyroxene, clinopyroxene, hornblende, and rare olivine. Much of the lava is rich in glass and highly vesicular. Jurassic and Cretaceous sedimentary strata form a bench on the south side of the island and are overlain by granitoid glacial erratics and hyaloclastites. The volcano consists of a central dome and lava flow complex, surrounded by pyroclastic debris. The irregular coastline of Augustine Island is due to the repeated catastrophic collapse of the summit dome, forming debris avalanches down the flanks and into Cook Inlet. At least 11 avalanches have occurred in the past 2000 years with an average recurrence interval of about 150-200 years. Augustine lies within the area of uplift resulting from the 1964 Alaska earthquake; 30-33 cm of uplift was measured on the northwest Augustine Island, an 8 by 11 km island in lower Cook Inlet is composed almost entirely of the side of the island. A 25-meter-high, south-facing submarine scarp 3 km south of the island, of similar orientation to joint sets in sedimentary rocks of the Kamishak River area on the Alaska Peninsula, is almost certainly of tectonic origin. There are no people that live on the island, it is a island with a volcano on it.
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