Stratigraphy, distribution, and geochemistry of the Newberry Volcano tephras (Oregon)

Newberry volcano has a long history of silicic pyroclastic volcanism and has produced many extensive tephra deposits. During the last approximately 550,000 years, the volcano has erupted at least 60 rhyolitic and dacitic tephras. These record a range of eruptive styles including plinian, sub-plinian, pyroclastic flows, and pyroclastic surges and both dry and phreatomagmatic activity. Tephra dispersal from individual eruptions ranges from very limited (i.e. small pumice rings a few hundred meters to a kilometer in extent) to extensive (i.e. plinian airfall deposits which have been recognized several hundred kilometers downwind). The most recent plinian eruption, the approximately 1250–1450 year old Newberry Pumice, produced an eruption column which reached a height of ∼25 km and was dispersed by unusually high velocity winds. Some Pleistocene eruptions, including tephra 9822B, may have sent pyroclastic material to heights of 40 km or more. Several deposits including Evans Well, 9822B, Qat/Qbt, 995B, and Qtae are compositionally heterogeneous and probably record the mixing of magmas before or during eruption.; The similarity coefficient and discriminant function analysis have been used to compare electron microprobe determined glass compositions for proximal Newberry tephra deposits and distal ash layers. Newberry Pumice correlates to six distal locations in Oregon and Idaho. Paulina Creek, Ice Quarry, 9912D, 9920C, 984F, 984G5, 9917C, and 9881C correlate to ash layers at Summer Lake, Oregon. Paulina Creek tephra also correlates to several locations in northern California and is the source of the Olema ash bed. Tephra 9917C is highly similar to Pringle Falls tephras K and D. Lava Pass tephra correlates to a distal ash preserved at a depth of 142 m in the Knolls core in the Bonneville basin, Utah. A coarse, 0.6 m thick deposit of Wono tephra has been located on the northwest flank of Newberry volcano.; The large number of eruptions and established tephra correlations suggest that additional correlations of Newberry tephra are likely to be made to distal sites, both studied and yet to be studied. The volcano's extensive eruptive history and relatively recent activity also indicate that future large ash and pumice producing eruptions are likely.*; *This dissertation is a multimedia document (contains text and other applications not available in printed format). The CD requires the following system requirements: Adobe Acrobat.