Stratigraphic framework of Holocene volcaniclastic deposits, Akutan Volcano, east-central Aleutian Islands, Alaska

Abstract

Akutan Volcano is one of the most active volcanoes in the Aleutian arc, but until recently little was known about its history and eruptive character. Following a brief but sustained period of intense seismic activity in March 1996, the Alaska Volcano Observatory began investigating the geology of the volcano and evaluating potential volcanic hazards that could affect residents of Akutan Island. During these studies new information was obtained about the Holocene eruptive history of the volcano on the basis of stratigraphic studies of volcaniclastic deposits and radiocarbon dating of associated buried soils and peat. A black, scoria-bearing, lapilli tephra, informally named the "Akutan tephra," is up to 2 m thick and is found over most of the island, primarily east of the volcano summit. Six radiocarbon ages on the humic fraction of soil A-horizons beneath the tephra indicate that the Akutan tephra was erupted approximately 1611 years B.P. At several locations the Akutan tephra is within a conformable stratigraphic sequence of pyroclastic-flow and lahar deposits that are all part of the same eruptive sequence. The thickness, widespread distribution, and conformable stratigraphic association with overlying pyroclastic-flow and lahar deposits indicate that the Akutan tephra likely records a major eruption of Akutan Volcano that may have formed the present summit caldera. Noncohesive lahar and pyroclastic-flow deposits that predate the Akutan tephra occur in the major valleys that head on the volcano and are evidence for six to eight earlier Holocene eruptions. These eruptions were strombolian to subplinian events that generated limited amounts of tephra and small pyroclastic flows that extended only a few kilometers from the vent. The pyroclastic flows melted snow and ice on the volcano flanks and formed lahars that traveled several kilometers down broad, formerly glaciated valleys, reaching the coast as thin, watery, hyperconcentrated flows or water floods. Slightly cohesive lahars in Hot Springs valley and Long valley could have formed from minor flank collapses of hydrothermally altered volcanic bedrock. These lahars may be unrelated to eruptive activity.

Magma‐Fluid Interactions Beneath Akutan Volcano in the Aleutian Arc Based on the Results of Local Earthquake Tomography

“…Scoria and pumice bearing lapilli tephra, called the Akutan tephra, is widely presented as the youngest primary volcaniclastic deposits over the island area forming layers of about 2 m thick. It is interpreted as a result of a major caldera‐forming eruption of Akutan volcano (Waythomas, 1999). Both old and recent volcanic rocks in Akutan demonstrate a broad variety of compositions ranged from magnesian basalts to dacites (45%–62% SiO 2 ) (Romick et al., 1990).…”

“…The volcano is headed by a circular caldera with a diameter of ∼2 km having the highest point of the Akutan Peak at the altitude of 1,303 m. In older literature sources, it was stated that the caldera was formed during an eruption 5,200 years ago (Miller et al., 1998; Reeder, 1983). Later, based on studying tephra stratigraphy, Waythomas (1999) argued that the caldera is significantly younger and might have originated during a large plinian eruption ∼1,600 years before present (BP). In this case, the eruption 5,200 years BP might be responsible for creating another larger caldera, whose traces are identified on the southern flank of the stratovolcano.…”

“…This island hosts Akutan volcano, which is one of the most active in the Aleutian arc, with dozens of documented eruptions starting from the eighteenth century (Siebert & Simkin, 2013). These eruptions were mostly weak or moderate; however, the existence of a Holocene caldera with a diameter ∼2 km and thick scoria‐bearing, lapilli tephra (Akutan tephra) widely spreading over the entire island surface (Waythomas, 1999) indicates that Akutan has a serious potential for explosive caldera‐forming eruptions. During the past century, Akutan demonstrated variable styles of volcanic activity, such as lava flows, gas emissions, and strombolian explosive eruptions ejecting ash plumes (e.g., Miller et al., 1998).…”

Late Quaternary constructional history of the southeastern Ruapehu ring plain, New Zealand

“…The former may be generated by eruption-induced sector collapses, eruptions through crater lakes, melting of debris-laden snow and ice by hot pyroclastic materials, mixing of pyroclastic flows or other large volumes of volcanic debris with water, and evolution by gravity segregation within pyroclastic surges (Janda et al 1981;Boudon et al 1987;Siebert et al 1987;Major & Newhall 1989;Vallance & Scott 1997;Mothes et al 1998;Waythomas 1999).…”

Seismicity and structure of Akutan and Makushin Volcanoes, Alaska, using joint body and surface wave tomography

“…[ Reeder , ], and Makushin's most recent caldera‐forming eruption may have been 1600 years B.P. [ Waythomas , ], although other sources state it as 8000–8800 years B.P. [e.g., McConnell et al ., ].…”