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Volatiles (H2O, CO2, S, Cl) play a key role in magmatic processes at subduction zones. In this study, the dissolved volatile contents of olivine-hosted melt inclusions from cinder cones in the Lassen segment of the Cascade arc are used to investigate dehydration of subducted oceanic lithosphere, magma formation in the sub-arc mantle wedge, and mafic magma storage and evolution in the crust. Relatively young, hot oceanic lithosphere subducts beneath the Cascade arc. The hydrogen-isotope and trace-element compositions of melt inclusions, when integrated with thermo-petrologic modeling, demonstrate that fluids in Cascade magmas are sourced from hydrated peridotite in the deep slab interior and that the oceanic crustal part of the slab extensively dehydrates beneath the forearc. In contrast to their slab-derived H, the melt inclusions have B concentrations and isotope ratios that are similar to mid-ocean ridge basalt (MORB), requiring little to no slab contribution of B, which is also consistent with extensive dehydration of the downgoing plate before it reaches sub-arc depths. Correlations of volatile and trace element ratios in the melt inclusions (H2O/Ce, Cl/Nb, Sr/Nd) demonstrate that geochemical variability in the magmas is the result of variable amounts of addition of a hydrous subduction component to the mantle wedge. Radiogenic isotope ratios require that the subduction component has less radiogenic Sr and Pb and more radiogenic Nd than the Lassen sub-arc mantle and is therefore likely to be a partial melt of subducted Gorda MORB. These results provide evidence that chlorite-derived fluids from the deep slab interior flux-melt the oceanic crust, producing hydrous slab melts that migrate into the overlying mantle, where they react with peridotite to induce further melting. The basaltic magmas that erupted at Cinder Cone near Mt. Lassen trapped melt inclusions during olivine crystallization at ~7-15 km depth. The melt inclusion compositions require that two different mantle-derived magmas were involved in the eruption, and temporal changes show that arrival of the two batches correlates with two explosive phases of activity. Both magmas experienced rapid crustal contamination before erupting, illustrating the complexities of cinder cone eruptions. This dissertation includes previously published and unpublished co-authored material.