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Wallman, P.C., 1991

Structure and subsurface vent geometry of the Novarupta basin, Valley of Ten Thousand Smokes, Katmai National Park, Alaska

Bibliographic Reference

Wallman, P.C., 1991, Structure and subsurface vent geometry of the Novarupta basin, Valley of Ten Thousand Smokes, Katmai National Park, Alaska: Palo Alto, California, Stanford University, Ph.D. dissertation, 211 p.


The 1912 eruption in Novarupta basin on the Alaska Peninsula, in what is now Katmai National Park, was the largest eruption of the century and formed the Valley of Ten Thousand Smokes. A rhyolitic tephra fall and compositionally zoned ash-flow sheet were erupted during the first day from the Stage I vent. Dacite fall units from days two and three vented from a smaller Stage II vent, which is nested in the Stage I vent. The summit of Mt. Katmai began to collapse during Stage I of the eruption and experienced periods of episodic collapse through Stage II. Four sets of structures define a roughly elliptical Stage I vent 2.5 x 3 km in diameter. Topographic cross sections, the inferred depth to a possible magma chamber, and structural interpretations of the surficial fractures, combined with numerical models for compaction of the vent-filling tephra, suggest that the vent system at Novarupta is a flared vent with steeply dipping walls to the north, south, and east, and more gently dipping walls to the west. Two sets of mapped structures are not directly related to formation of the Stage I or II vents. These are (1) 'radial' structures that trend across the circumferential structures which outline the basin, and (2) normal faults, grabens, and slides on a topographic high (the 'Turtle'). The subparallel orientations of the 'radial' structures, the dominant joint set in the bedrock, and inferred regional stress trajectories suggest that magma was not transported directly to the vent from beneath Mt. Katmai, which collapsed, but from beneath Trident Volcano. Magma withdrawn from underneath Mt. Katmai is inferred to have been transported laterally along the volcanic front to Trident. Two conceptual models are proposed for the origin of the graben systems on the Turtle: (1) doming of the tephra due to a shallow igneous intrusion and (2) compaction of the tephra pile. Numerical modeling of these processes combined with field observations and comparison to known cryptodomes suggest that the Turtle and its grabens most likely formed due to inflation of a shallow igneous intrusion.

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