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GEOREF RECORD

Eruptive history of Mount Katmai, Alaska

Wes Hildreth and Judy Fierstein
Eruptive history of Mount Katmai, Alaska
Geosphere (Boulder, CO) (November 2012) 8 (6): 1527-1567

Abstract

Mount Katmai has long been recognized for its caldera collapse during the great pyroclastic eruption of 1912 (which vented 10 km away at Novarupta in the Valley of Ten Thousand Smokes), but little has previously been reported about the geology of the remote ice-clad stratovolcano itself. Over several seasons, we reconnoitered all parts of the edifice and sampled most of the lava flows exposed on its flanks and caldera rim. The precipitous inner walls of the 1912 caldera remain too unstable for systematic sampling; so we provide instead a photographic and interpretive record of the wall sequences exposed. In contrast to the several andesite-dacite stratovolcanoes nearby, products of Mount Katmai range from basalt to rhyolite. Before collapse in 1912, there were two overlapping cones with separate vent complexes and craters; their products are here divided into eight sequences of lava flows, agglutinates, and phreatomagmatic ejecta. Latest Pleistocene and Holocene eruptive units include rhyodacite and rhyolite lava flows along the south rim; a major 22.8-ka rhyolitic plinian fall and ignimbrite deposit; a dacite- andesite zoned scoria fall; a thick sheet of dacite agglutinate that filled a paleocrater and draped the west side of the edifice; unglaciated leveed dacite lava flows on the southeast slope; and the Horseshoe Island dacite dome that extruded on the caldera floor after collapse. Pre-collapse volume of the glaciated Katmai edifice was approximately 30 km (super 3) , and eruptive volume is estimated to have been 57 + or - 13 km (super 3) . The latter figure includes approximately 40 + or - 6 km (super 3) for the edifice, 5 + or - 2 km (super 3) for off-edifice dacite pyroclastic deposits, and 12 + or - 5 km (super 3) for the 22.8-ka rhyolitic pyroclastic deposits. To these can be added 13.5 km (super 3) of magma that erupted at Novarupta in 1912, all or much of which is inferred to have been withdrawn from beneath Mount Katmai. The oldest part of the edifice exposed is a basaltic cone, which gave a (super 40) Ar/ (super 39) Ar plateau age of 89 + or - 25 ka.The seismic record of caldera collapse includes 14 earthquakes of magnitude 6.0-7.0. By combining the times of earthquakes, the hours of downwind plinian-fall episodes from Novarupta, and the stratigraphic record of hydrothermal explosion breccia and phreatic mud layers ejected around the caldera rim and intercalated within the Novarupta pumice-fall sequence, it can be inferred that collapse began in the 11th hour of the 60-h-long eruption and continued fitfully for 3.5 days. Several big landslides and pumiceous debris flows shaken loose by the collapse-related seismicity are bracketed in time by their levels of intercalation within the Novarupta pumice-fall sequence. An intracaldera lake was approximately 10 m deep by 1916, drained away in 1923, and has since deepened progressively to approximately 250 m today.Compositionally, products of Mount Katmai represent an ordinary medium-K arc array, both tholeiitic and calcalkaline, that extends from 51.6% to 72.3% SiO (sub 2) . Values of (super 87) Sr/ (super 86) Sr range from 0.70335 to 0.70372, correlating loosely with fractionation indices. The 5-6 km (super 3) of continuously zoned andesite-dacite magma (58%-68% SiO (sub 2) ) that erupted at Novarupta in 1912 was withdrawn from beneath Mount Katmai and bears close compositional affinity with products of that edifice, not with pre-1912 products of the adjacent Trident cluster. Evidence is presented that the 7-8 km (super 3) of high-silica rhyolite (77% SiO (sub 2) ) released in 1912 is unlikely to have been stored under Novarupta or Trident. Pre-eruptive contiguity with the andesite-dacite reservoir is suggested by (1) eruption of approximately 3 km (super 3) of rhyolite magma first, followed by mutual mingling in fluctuating proportions; (2) thermal and redox continuity of the whole zoned sequence despite the wide compositional gap; (3) Nd, Sr, O isotopic, and rare earth element (REE) affinities of the whole array; (4) compositional continuity of the nearly aphyric rhyolite with the glass (melt) phase of the phenocryst-rich dacite; and (5) phase-equilibrium experiments that indicate similar shallow pre-eruptive storage depths (3-6 km) for rhyolite, dacite, and andesite.


EISSN: 1553-040X
Serial Title: Geosphere (Boulder, CO)
Serial Volume: 8
Serial Issue: 6
Title: Eruptive history of Mount Katmai, Alaska
Affiliation: U. S. Geological Survey, Volcano Science Center, Menlo Park, CA, United States
Pages: 1527-1567
Published: 20121116
Text Language: English
Publisher: Geological Society of America, Boulder, CO, United States
References: 67
Accession Number: 2013-006123
Categories: Quaternary geologyIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. geol. sketch maps
N58°04'60" - N58°25'00", W155°19'60" - W155°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201304
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