HR: 1330h
AN: V13A-06    [Abstracts]
TI: Evidence of Subaerial Cryptodome Formation, Phonolite Hill, Utah
AU: * Kapostasy, D D
EM: kapostasy.3@osu.edu
AF: The Ohio State University, Department of Geological Sciences 125 S. Oval Mall, Columbus, OH 43210 United States
AU: Elliot, D H
EM: elliot.1@osu.edu
AF: The Ohio State University, Department of Geological Sciences 125 S. Oval Mall, Columbus, OH 43210 United States
AB: Phonolite Hill is a late-Miocene silicic volcanic center that developed following the initiation of Basin and Range extension in southern Utah. This center shows evidence of an early phreatomagmatic eruptive period followed by the intrusion of a finely crystalline flow-banded rhyolite. Structures and deposits associated with the intrusion of rhyolite suggest that it formed as a cryptodome. Cryptodomes are dome shaped intrusions that form in the shallow subsurface and up-dome overlying deposits (Minakami, 1951). They have been reported to form both subaerially (modern) (Alidibirov et al, 1997) and subaqueously (ancient rock record) (Snyder and Fraser, 1963; Goto and McPhie, 1998; Doyle and McPhie, 2000; Stewart and McPhie, 2003). Phonolite Hill offers the opportunity to describe a Miocene subaerial occurrence of a cryptodome. The dome at Phonolite Hill is approximately 0.4 km³, and is composed of massive, glassy to devitrified rhyolite, with the edge of the dome showing well defined flow banding and columnar jointing. The contact with the surrounding pyroclastic units varies around the dome from thick devitrified lithophysal obsidian to massive obsidian and rhyolite breccias. The brecciated contacts suggest intrusion into weakly to lithified pyroclastic deposits. The base surge deposits associated with the dome at Phonolite Hill suggest that they erupted in the presence of water, but there is no indication that the deposits were erupted subaqueously. Interaction with water either at the surface or below ground probably played an important role in the formation of the dome as well. Intrusion into a wet environment would have allowed for the fracturing of the rhyolite and possible partial fluidization of the sediment upon intrusion, which formed the brecciated rhyolite and brecciated obsidian deposits. These deposits are not found around the whole dome due to poor exposure, but are between 1 and 4 meters thick where visible. Flow-banded rhyolite, columnar jointing, and brecciated rhyolite contacts allow the cryptodome at Phonolite Hill to be directly compared with published examples of other cryptodomes.
DE: 8414 Eruption mechanisms
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly