Mount Mageik: A Compound Stratovolcano in Katmai National Park

Hildreth, Wes; Fierstein, Judy; Lanphere, Marvin; Siems, David

Mount Mageik is an ice-clad 2,165-m andesite-dacite stratovolcano in the Katmai volcanic cluster at the head of the Valley of Ten Thousand Smokes. New K-Ar ages indicate that the volcano is as old as 93+8 ka. It has a present-day volume of 20 km3, implying a long-term average volumetric eruptionrate of about 0.33 km3 per 1,000 years. Mount Mageik consists of four overlapping edifices, each with its own central summit vent, lava-flow apron, and independent eruptive history. Three of them have small fragmental summit cones with ice-filled craters, but the fourth and highest is topped by a dacite dome. Lava flows predominate on each edifice; many flows have levees and ice-contact features, and many thicken downslope into piedmont lava lobes 50-200 m thick. Active lifetimes of two (or three) of the component edifices may have been brief, like that of their morphological and compositional analog just accross Katmai Pass, the South-west (New) Trident edifiece of 1953-74. The North Summit edifice of Mageik may have been constructed very late in the Pleisocene and the East Summit edifice (along with nearby Mount Martin) largely or entirely in the Holocene. Substantial Holocene debirs avalanches have broken loose from three sites on the south side of Mount Mageik, the youngest during the Novarupta fallout of 6 June 1912. The oldest one was especially mobile, being rich in hydrothermal clay, and is preserved for 16 km downvalley, probably having run out to the sea. Mageik's fumarolically active crater, which now contains a hot acid lake, was never a magmatic vent but was reamed by phreatic explosions through the edge of the dacite summit dome. There is no credible evidence of historical eruptions of Mount Mageik, but the historically persistent fumarolic plumes of Mageik and Martin have animated many spurious eruption reports. Lavas and ejecta of all four component edifices of Mageik are plagioclaserich, pyroxene-dacites and andesites (57-68 weight percent SiO2) that form a calcic, medium-K, typically low-Ti arc suite. The Southwest Summit edifice is larger, longer lived, and compositionally more complex than its companions. Compared to other centers in the Katmai cluster, products of Mount Mageik are readily distinguishable chemically from those of Mount Griggs, Falling Mountain, Mount Cerberus, and all prehistoric components of the Trident group, but some are similar to the products of Mount Martin, Southwest Trident, and Novarupta. The crater lake, vigorous superheated fumaroles, persistent seismicity, steep ice blanket, and numerous Holocene dacites warrant monitoring Mount Mageik as a potential source of explosive eruptions and derivative debris flows.

Mount Mageik: A Compound Stratovolcano in Katmai National Park

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