Ocean islands form as a consequence of the intermittent eruption of lavas in a localized area as the Earth's lithosphere moves over a hotspot source (Wilson, 1963). Long-lived hotspot volcanism is most succinctly explained by the concept of so-called "plume theory" (Campbell, 2007; Morgan, 1971), which states that upwelling mantle "plumes," originating from the deep Earth, melt as they near the lithosphere, resulting in hotspot volcanism. "Plume theory" predicts linear volcanic chains that age in the direction of plate motion, relative to their fixed plume source. A single island or seamount contains an integrated history of repeated eruptions over a finite period of time (Morgan, 1971), whereas ocean island and seamount chains can collectively record hotspot activity occurring over tens-of-millions of years. As a result, ocean islands Abstract Rarely have small seamounts on the flanks of hotspot derived ocean-island volcanoes been the targets of sampling, due to sparse high-resolution mapping near ocean islands. In the Galápagos Archipelago, for instance, sampling has primarily targeted the subaerial volcanic edifices, with only a few studies focusing on large-volume submarine features. Sampling restricted to these large volcanic features may present a selection bias, potentially resulting in a skewed view of magmatic and source processes because mature magmatic systems support mixing and volcanic accretion that overprints early magmatic stages. We demonstrate how finer-scale sampling of satellite seamounts surrounding the volcanic islands in the Galápagos can be used to lessen this bias and thus, better constrain the evolution of these volcanoes. Seamounts were targeted in the vicinity of Floreana and Fernandina Islands, and between Santiago and Santa Cruz. In all regions, individual seamounts are typically monogenetic, but each seamount field requires multigenerational magmatic episodes to account for their geochemical variability. This study demonstrates that in the southern and eastern regions the seamounts are characterized by greater geochemical variability than the islands they surround but all three regions have (Sr-Nd-He) isotopic signatures that resemble neighboring islands. Variations in seamount chemistry from alkalic to tholeiitic near Fernandina support the concept that islands along the center of the hotspot track undergo greater mean depths of melting, as predicted by plume theory. Patterns of geochemical and isotopic enrichment of seamounts within each region support fine-scale mantle heterogeneities in the mantle plume sourcing the Galápagos hotspot. Plain Language Summary Hotspots are places on Earth where volcanism occurs away from major plate boundaries and are, thus, not explained by the theory of plate tectonics. Instead, "plume theory" predicts that a least a portion of hotspot volcanism is related to the upwelling of hot mantle, which partially melts as it ascends and forms magma that produces ocean-island volcanoes. Chemistry of lavas sourced from these volcanoes are used to test predic...