Subduction zones play an important part in shaping our planet. Yet, we cannot presently observe the mechanisms by which they initiate or their subsequent evolution. It is for this reason that we have chosen the Aleutian and Izu-Bonin-Mariana (IBM) Arcs to investigate the initiation of subduction around the Pacific Rim at ~50 Ma. These arcs initiated at approximately the same time, have older rocks exposed for study, and exhibit a variety of subduction characteristics along their lengths, enabling us to investigate not only their initiation, but their evolution through time. The Aleutian Arc in particular changes convergence angle and chemistry rather significantly along arc, allowing us to investigate the effect of the oblique subduction in the western Aleutians on the geochemistry. New geochemical and geochronologic data on Attu and Kiska have enabled us to gain a more complete understanding of the processes occurring in this part of the arc. Through these investigations, the following conclusions have been reached: 1) Initial volcanism on Attu was tholeiitic; it became more incompatible element-depleted in the north due to incipient back-arc rifting, with a more extensional signature in the north and a stronger subduction signature in the south; rifting magmatism was characterized by fractional crystallization and assimilation at 16 Ma; and Attu abruptly switched to calc-alkaline subduction volcanism at ~6 Ma. 2) Kiska’s geochemistry is intermediate between that of Attu (western Aleutians) and Adak (central Aleutians), which coincides with its placement on the ridge. Volcanism shifted northward and transitioned to smaller volume volcanism and a more compressive environment, eventually leading to the present calc-alkaline regime. 3) Based on their geochemistry Rat Island is part of the Aleutian Arc and is not geochemically related to Bowers Ridge. Rat Island transitioned from tholeiitic to calc-alkaline at ~15 Ma due to deeper melting, a more water-rich source, and a shift from a transtensional to a transpressional tectonic environment due to block rotation and changes in plate motion. Rat Island experienced the same northward shift in volcanism as the rest of the arc, and its geochemistry reflects its placement on the Aleutian Ridge. 4) By comparing the IBM and Aleutian arcs, we investigated variations in the types of subduction initiation occurring around the Pacific at ~50 Ma and how this affected the evolution of these arcs. The IBM Arc experienced a decreasing flux of sediments with time. The Bonin had the largest contribution of sediment, followed by the older Mariana samples, the modern Mariana samples, and the Izu and Aleutian samples. The older IBM samples experienced more variable degrees of partial melting. The Mariana fore-arc basalts and the mid-ocean ridge (MORB-like) Attu samples represent the depleted end-members of the two arcs. The Mariana segment experienced the largest degree of partial melting, followed by the Bonin segment, and the Izu segment. The fluid flux remained relatively stable with time. The fluid flux is greatest in the Aleutians, followed by the Mariana segment, the Bonin segment, and the Izu segment. Most of the fluid was derived from oceanic crust rather than sediment.