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Fisher reported activity



Event Name : Fisher Turquoise Cone Collapse

Start: 5100 Years BP C-14 (raw)
Stop: 3500 Years BP C-14 (raw)

Tephrafall: BibCard BibCard BibCard
Cone: BibCard
Eruption Type:Explosive
MaxVEI: 5 BibCard
Eruption Product: other BibCard

Description: From Stelling and others (2005): "Post-caldera eruptive activity has been largely concentrated at two intracaldera cones, as well as one cone outside the caldera to the northwest. The western intracaldera center has been informally termed "Turquoise Cone" after its turquoise-colored crater lake (Fig. 3C [in original text]). A bench low on the southern flank of Turquoise Cone connects with the wave-cut terrace on the southern caldera wall, indicating that the Turquoise Cone edifice extended above lake level. The growth of Turquoise Cone was largely responsible for separating the large caldera lake and ultimately damming the northern lakes before the southern lake drained. The erupted products of Turquoise Cone are compositionally diverse, ranging from pl+ol+cpx high-alumina basalt to sparsely plagioclase-phyric dacite. The oldest deposits from Turquoise Cone are the most mafic (47 wt.% SiO2), whereas the last eruptions from this vent are the most silicic (66 wt.% SiO2)."

"The present-day amphitheater-like form of Turquoise Cone resulted from a significant collapse event. Through geometric extrapolation of existing deposits and their inclinations, the pre-collapse volume of Turquoise Cone is estimated to have been ~3 km cubed, and slightly more than half of that volume has been removed. No deposits from the collapse event have been recognized, and we believe that they have been buried by subsequent eruptions from Turquoise Cone and other intracaldera vents. We are thus unable to constrain the trigger or structure of the collapse."

"Several pumice fall layers exposed inside and outside the caldera, and a dacitic lava flow on the northern flanks of the cone probably represent some of the last eruptions of Turquoise Cone (Table 1 in original text). We believe that these eruptions may have resulted in the collapse of Turquoise Cone. The pumice fall layers correlate compositionally with a tephra layer near Cold Bay, Alaska (120 km east), not only in major-element chemistry but delta 18 O signature (Bindeman and others, 2001). Exposures of these pumice deposits in the southeastern caldera wall are ~1 m thick ~12 km from Turquoise Cone, suggesting a significant Plinian eruption. Collapse from this eruption may not have been sufficiently large to remove the entire missing portion of Turquoise Cone, and removal of additional portions of the edifice may have occurred through subsequent phreatomagmatic eruptions. Soil directly below Turquoise Cone pumice deposits outside the eastern caldera wall has been dated at 5120 (+/-70) 14C years (Table 3 [in original text]), which is consistent with bracketing ages for the Cold Bay deposit of 6070 and 3600 years old (Carson, 1998; Dochat, 1997). If the collapse of Turquoise Cone resulted entirely or in part from this eruption, these data suggest an approximate date of collapse. Lacustrine deposits exposed on the northern shore of Turquoise Lake, and thus younger than the collapse event, have been dated to be 3490 (+/-50) 14C years old (Table 3 [in original text]), bracketing the collapse between~3500 and ~5100 years ago."

The Global database on large magnitude explosive volcanic eruptions (LaMEVE; 2017) reports a magnitude of 6.2, bulk eruptive volume of 14.3 cubic km and a dense rock equivalent eruptive volume of 5.5 cubic km for the eruption.

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Page modified: March 30, 2017 14:36
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