Explosive eruptive record in the Katmai region,Alaska Peninsula: an overview |
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Authors: | Judy Fierstein |
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Institution: | (1) Volcano Hazards Team, US Geological Survey, MS-910, 345 Middlefield Road, Menlo Park, CA 94025, USA |
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Abstract: | At least 15 explosive eruptions from the Katmai cluster of volcanoes and another nine from other volcanoes on the Alaska Peninsula
are preserved as tephra layers in syn- and post-glacial (Last Glacial Maximum) loess and soil sections in Katmai National
Park, AK. About 400 tephra samples from 150 measured sections have been collected between Kaguyak volcano and Mount Martin
and from Shelikof Strait to Bristol Bay (∼8,500 km2). Five tephra layers are distinctive and widespread enough to be used as marker horizons in the Valley of Ten Thousand Smokes
area, and 140 radiocarbon dates on enclosing soils have established a time framework for entire soil–tephra sections to 10 ka;
the white rhyolitic ash from the 1912 plinian eruption of Novarupta caps almost all sections. Stratigraphy, distribution and
tephra characteristics have been combined with microprobe analyses of glass and Fe–Ti oxide minerals to correlate ash layers
with their source vents. Microprobe analyses (typically 20–50 analyses per glass or oxide sample) commonly show oxide compositions
to be more definitive than glass in distinguishing one tephra from another; oxides from the Kaguyak caldera-forming event
are so compositionally coherent that they have been used as internal standards throughout this study. Other than the Novarupta
and Trident eruptions of the last century, the youngest locally derived tephra is associated with emplacement of the Snowy
Mountain summit dome (<250 14C years B.P.). East Mageik has erupted most frequently during Holocene time with seven explosive events (9,400 to 2,400 14C years B.P.) preserved as tephra layers. Mount Martin erupted entirely during the Holocene, with lava coulees (>6 ka), two
tephras (∼3,700 and ∼2,700 14C years B.P.), and a summit scoria cone with a crater still steaming today. Mount Katmai has three times produced very large
explosive plinian to sub-plinian events (in 1912; 12–16 ka; and 23 ka) and many smaller pyroclastic deposits show that explosive
activity has long been common there. Mount Griggs, fumarolically active and moderately productive during postglacial time
(mostly andesitic lavas), has three nested summit craters, two of which are on top of a Holocene central cone. Only one ash
has been found that is (tentatively) correlated with the most recent eruptive activity on Griggs (<3,460 14C years B.P.). Eruptions from other volcanoes NE and SW beyond the Katmai cluster represented in this area include: (1) coignimbrite
ash from Kaguyak’s caldera-forming event (5,800 14C years B.P.); (2) the climactic event from Fisher caldera (∼9,100 14C years B.P.—tentatively correlated); (3) at least three eruptions most likely from Mount Peulik (∼700, ∼7,700 and ∼8,500
14C years B.P.); and (4) a phreatic fallout most likely from the Gas Rocks (∼2,300 14C years B.P.). Most of the radiocarbon dating has been done on loess, soil and peat enclosing this tephra. Ash correlations
supported by stratigraphy and microprobe data are combined with radiocarbon dating to show that variably organics-bearing
substrates can provide reliable limiting ages for ash layers, especially when data for several sites is available. |
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Keywords: | Ash layer Tephra Radiocarbon dates Holocene eruptions Microprobe data Katmai Alaska |
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