Pumice fall and Fisher ignimbrite (ca. 9100 yBP) overlapping Cape Lapin debris avalanche deposits on the north shore of Unimak Island, Field Station 18jrssh011.

Pumice fall and Fisher ignimbrite (ca. 9100 yBP) overlapping Cape Lapin debris avalanche deposits on the north shore of Unimak Island, Field Station 18jrssh011.

Date: Aug 6th, 2018
Volcano(es): Fisher Shishaldin
Photographer: Schaefer, Janet
URL: avo.alaska.edu/image/view/123471

Fisher Tuff/Funk Ash/CFE

From Miller and Smith (1987): "Fisher caldera, near the west end of Unimak Island (Fig. 1 [in original text]), is one of the largest calderas in the entire Aleutian arc-diameter greater than 11 km. The caldera is surrounded by a thick ash-flow sheet deposited by highly mobile flows (Miller and Smith, 1977)."
"The ash-flow tuffs fill glaciated valleys and are therefore Holocene in age. An organic layer directly beneath the ash-flow tuff on the north side of the island yielded a 14C age of 9120+/-200 yr (sample 2, Table 1 [in original text]), which would be a maximum age for the caldera-forming eruption. Support for this date as the approximate age of the eruption comes from 14C dates (Funk, 1973) determined for a 10- 50-cm-thick pumice-rich tephra unit in the Cold Bay area 120 km east of Fisher (Fig. 1 [in original text]). Organic material directly beneath this tephra unit yielded 14C ages of 9660+/-615 and 10,625+/-550 yr. Granulometric, statistical, and isopach analyses of the tephra deposits by Funk indicate that they were deposited during one pulse of activity from a source on Unimak Island within 180 km of Cold Bay. The climactic eruption of Fisher, the closest caldera west of Cold Bay, is a logical but not unequivocal candidate for the source of the tephra."
From Stelling and others (2005): "Fisher Caldera formed during a single, three-phased eruption 9372+/-198 14C years ago. Each phase produced a distinct deposit, which we collectively call the Fisher Tuff: a Plinian dacitic pumiceous fall deposit dispersed to the northeast, followed by a voluminous pyroclastic flow containing banded pumice of mingled dacite and basalt distributed predominantly to the north, followed by a dacitic pyroclastic flow distributed to the south and west (Fig. 3B [in original text]). At its most proximal localities, the northeastern fall deposit is up to 10 m thick and abruptly changes ~2/3 up-section from a lower tan pumice to an upper slightly coarser, gray pumice. The tan and gray pumice layers are compositionally identical (67 wt.% SiO2), and the change in color may reflect differences in the degree of oxidation. The gray (darker) CFE pumices have greater vesicularity than the pink, opposite to the observations of Klug and Cashman (1994) who noted correlations between color changes and pumice vesicularity. Although a thorough analysis of the caldera-forming deposits is still in progress, the abrupt coarsening of pumice clasts at the color break may indicate an increase in mass flux during the production of the gray pumice."
The Global database on large magnitude explosive volcanic eruptions (LaMEVE; 2017) reports a magnitude of 6.7, bulk eruptive volume of 56 cubic km, dense rock equivalent eruptive volume of 23.3 cubic km and airfall tephra volume of 50 cubic km for the eruption.


Latitude: 54.956986111111

Longitude: -164.16403333333

Image courtesy of the AVO/ADGGS.
Please cite the photographer and the Alaska Volcano Observatory / Alaska Division of Geological & Geophysical Surveys when using this image.
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