Date of Award


Document Type

Union College Only

Degree Name

Bachelor of Science



First Advisor

Holli Frey


Crystallization, Geographic Location, Mineral Composition, Temperature


Multiplekm³ explosive deposits are exposed east of the South Sister volcanic range in central Oregon, centered around Bend. A comprehensive chemical and petrologic study of the crystal-poor (vol%) andesites (57-63 wt% SiO₂) to rhyolites (>70 wt% SiO₂) of the 5-6 named tuffs has not been completed. The physically distinct units of this study range in bulk chemistry: the Tumalo Tuff is 70-74.1 wt% SiO₂, the overlying Shevlin Park Tuff is 63.8-65.1 wt% SiO₂, and the Bend Pumice is 72.5-74 wt% SiO₂. The Tumalo Tuff and Bend Pumice share similar rare earth element trends with a deep europium anomaly, while the Shevlin Park Tuff’s is much shallower. Comparison of trace element trends and FeO*/MgO ratios also suggest that the Tumalo Tuff and Bend Pumice are distinct from other units in the Central Cascade Arc. SEM quantitative analyses of ilmenites and magnetites were used in conjunction with the thermometer of Ghiorso and Evans (2008) to calculate the temperature and oxygen fugacity of the samples. The Shevlin Park Tuff (TT-12 and TT-13) give average temperatures of 959 ±33°C and 934 ±33°C, and fO₂ of 0.25 ±0.10 and 0.27 ±0.13 ∆NNO, respectively. Tumalo Tuff and Bend Pumice samples show multiple compositional populations that give various temperatures and fO₂. One particular subset in most of the Tumalo and Bend samples yields temperatures ~700°C and fO₂ of ≤-0.22 ±0.62 ∆NNO. These Fe-Ti oxides are in equilibrium with each other. The nearby 2-2.3 ka rhyolitic domes of the South Sister volcanic field record Fe-Ti oxide temperatures ~850˚C and fO₂ ~0.7 ∆NNO (Stelten and Cooper, 2012). The stark differences between the Tumalo Tuff and Bend Pumice when compared to South Sister rhyolites suggest that the magmas don’t share the same source despite geographic proximity. Unlike the South Sister complex and ii the Shevlin Park Tuff, which have discrete mineral compositions and display the chemistry and fugacity typical of a classic subduction zone, the Tumalo Tuff and Bend Pumice contain oxides that span a broad compositional range. The different oxide subsets which yield lower temperatures and fugacities indicate reducing conditions, which suggests an extensional environment. However, the populations with higher temperatures and fugacities give an arc-like signature. It is possible that multiple mechanisms including toroidal flow and reheating events during a period of transition from extension to subduction-driven magmatism are responsible for these younger explosive deposits in the Central Cascade Arc. A better understanding of this transition phase may give insight into the possibility of future hazards associated with the Tumalo Volcanic Center.