Tsunami generation by pyroclastic flow during the 3500-year B.P. caldera-forming eruption of Aniakchak Volcano, Alaska

Abstract

A discontinuous pumiceous sand, a few centimeters to tens of centimeters thick, is located up to 15 m above mean high tide within Holocene peat along the northern Bristol Bay coastline of Alaska. The bed consists of fine-to-coarse, poorly to moderately wellsorted, pumice-bearing sand near the top of a 2-m-thick peat sequence. The sand bed contains rip-up clasts of peat and tephra and is unique in the peat sequence. Major element compositions of juvenile glass from the deposit and radiocarbon dating of enclosing peat support correlation of the pumiceous sand with the caldera-forming eruption of Aniakchak Volcano. The distribution of the sand and its sedimentary characteristics are consistent with emplacement by tsunami. The pumiceous sand most likely represents redeposition by tsunami of climactic fallout tephra and beach sand during the approximately 3.5 ka Aniakchak caldera-forming eruption on the Alaska Peninsula. We propose that a tsunami was generated by the sudden entrance of a rapidly moving, voluminous pyroclastic flow from Aniakchak into Bristol Bay. A seismic trigger for the tsunami is unlikely, because tectonic structures suitable for tsunami generation are present only south of the Alaska Peninsula. The pumiceous sand in coastal peat of northern Bristol Bay is the first documented geologic evidence of a tsunami initiated by a volcanic eruption in Alaska.

Numerical simulation of tsunamis generated by caldera collapse during the 7.3 ka Kikai eruption, Kyushu, Japan

“…Nomanbhoy and Satake, 1995;Watts and Waythomas, 2003). Additionally, tsunami deposits, tsunami boulders, or erosional signatures (Bryant, 2001) can also be important clues for revealing the characteristics of tsunamis (Waythomas and Neal, 1998;Minoura et al, 2000;Carey et al, 1996Carey et al, , 2001Freundt et al, 2006). The 7.3 ka eruption of Kikai volcano is the most recent and notable caldera-forming eruption in Japan during the Holocene Arai, 1978, 2003).…”

Late Glacial and Holocene Glacier and Vegetation Fluctuations at Little Swift Lake, Southwestern Alaska, U.S.A

“…A thick pumiceous tephra at 208 to 232 cm depth is most likely the Aniakchak tephra, which is prominent in lake records throughout the Ahklun Mountains (Kaufman et al, 2003) and has been described elsewhere in western Alaska (Riehle, 1985;Beget et al, 1992). Its age is well constrained at ;3600 6 100 cal yr B.P., based upon over a dozen 14 C ages (Beget et al, 1992;Waythomas and Neal, 1998). A minimum limiting age of 4250 6 100 cal yr B.P.…”

Theoretical analysis of tsunami generation by pyroclastic flows

“…[3] Geologic descriptions, mapping of pyroclastic flow deposits and observations of historical eruptions show that during explosive eruptions at coastal volcanoes, pyroclastic flows often reached the sea and either did or were inferred to have initiated water waves ( Figure 1) [Latter, 1981;Cas and Wright, 1991;Carey et al, 1996Carey et al, , 2000Waythomas and Neal, 1998;Legros and Druitt, 2000]. The behavior and characteristics of subaqueous pyroclastic flows are not well known, although it is clear that pyroclastic flows are capable of passing into and under water [Sparks et al, 1980;Cas and Wright, 1991;McLeod et al, 1999].…”