xviii, 179 p. : ill. (some col.). Includes 3 video files. / Mafic arc volcanoes have eruption styles that range from explosive to effusive. In a broad sense, eruption style is controlled by the rate of magma supply to the vent. In this dissertation I examine relationships between eruption rate and style in two separate studies: (1) an investigation of ongoing activity at NW Rota-1, a submarine volcano in the Mariana arc, and (2) a morphologic study of the Collier Cone lava flow field in the Central Oregon Cascades. The eruptions of NW Rota-1 range from effusive to moderately explosive; eruptions are effusive when mass eruption rate (MER) is low and explosive when MER increases. The explosivity of submarine eruptions is suppressed by seawater because of increased hydrostatic pressure, rapid cooling, and the high viscosity of water relative to air (which limits expansion). The combination of seawater and relatively low MERs limit pyroclast deposition to within meters to tens of meters of the vent. In fact, many pyroclasts fall back into the vent and are recycled. Evidence for recycling includes microcrystalline inclusions within erupted pyroclasts and elevated Cl and Na concentrations in matrix glass. Enrichment of Cl and Na suggests that seawater assimilation provides a geochemical signature of recycling. Recycling is limited to low MER explosive eruptions and is not observed in either effusive lava or deposits from high MER explosions. Direct observations of eruptions allow measurements of eruption rate. However, it is more challenging to estimate MERs of eruptions that were not observed. To address this problem, I develop and test methods of constraining the eruption rate (and duration) of the c. 1600 year old Collier Cone lava flow using the flow morphology. To quantify flow morphology I combine field observations with GIS analysis of Lidar-derived digital topography. Channel dimensions constrain emplacement rates; dominant wavelengths and amplitudes of surface folds constrain spatial and temporal changes in flow rheology. Three videos of eruption activity accompany this dissertation as supplemental files. This dissertation includes both previously published and unpublished co-authored material. / Committee in charge: Dr. Katharine V. Cashman, Chair; Dr. Joshua J. Roering, Member; Dr. Paul J. Wallace, Member; Dr. Patricia F. McDowell, Outside Member; Dr. William W. Chadwick, Outside Member

Two suites of intermediate-felsic plutonic rocks were recovered by dredges RD63 and RD64 (R/V KK81-06-26) from the northern wall of the Mariana trench near Guam, which is located in the southern part of the Izu-Bonin-Mariana (IBM) island arc system. The locations of the dredges are significant as the area contains volcanic rocks (forearc basalts and boninites) that have been pivotal in explaining processes that occur when one lithospheric plate initially begins to subduct beneath another. The plutonic rocks have been classified based on petrologic and geochemical analyses, which provides insight to their origin and evolution in context of the surrounding Mariana trench. Based on whole rock geochemistry, these rocks (SiO2: 49-78 wt%) have island arc trace element signatures (Ba, Sr, Rb enrichment, Nb-Ta negative anomalies, U/Th enrichment), consistent with the adjacent IBM volcanics. Depletion of rare earth elements (REEs) relative to primitive mantle and excess Zr and Hf compared to the middle REEs indicate that the source of the plutonic rocks is similar to boninites and transitional boninites. Early IBM volcanic rocks define isotopic fields (Sr, Pb, Nd and Hf-isotopes) that represent different aspects of the subduction process (e.g., sediment influence, mantle provenance). The southern Mariana plutonic rocks overlap these fields, but show a clear distinction between RD63 and RD64. Modeling of the REEs, Zr and Hf shows that the plutonic suites formed via melting of boninite crust or by crystallization from a boninite-like magma rather than other sources that are found in the IBM system. The data presented support the hypothesis that the plutonic rocks from RD63 and RD64 are products of subduction initiation and are likely pieces of middle crust in the forearc exposed at the surface by faulting and serpentine mudvolcanoes. Their existence shows that intermediate-felsic crust may form very early in the history of an intra-oceanic island arc system. Plutonic rocks with similar formation histories may exist in obducted suprasubduction zone ophiolites and would be evidence that felsic-intermediate forearc plutonics are eventually accreted to the continents.