ALBERT

Description: Shimada Seamount is an isolated volcanic feature located between the Clipperton and Clarion Fracture Zones ∼1,150 km west of the East Pacific Rise and ∼600 km west of the inactive spreading center represented by the Mathematician Seamounts. It rises ∼3,900 m above the surrounding sea floor to within 50 m of present-day sea level. The area of Shimada Seamount should be volcanically dormant, because it is far from an active spreading center and is located on oceanic crust of early Miocene age. Nevertheless, evidence was found that Shimada Seamount has formed geologically recently. For example, seismic-reflection profiles-indicate that virtually no sediment has accumulated on the summit or flanks of the seamount; television, still-camera, and dredge-haul data indicate that a platform near the summit at a water depth of ∼180 m is a carbonate build-up formed by coralline red algae attached to fresh pillow basalt. Glassy pillow basalt too young to date by the K/Ar method and showing little or no devitrification and lacking manganese encrustations was dredged from the seamount below the algal reefs (500–750 m). Several cores taken from the adjacent basin (∼3,900 m deep) contain fresh glassy basalt detritus, and one core sampled a thin flow of unaltered basaltic glass at the sediment surface. The origin and history of Shimada Seamount differ importantly from volcanoes generally thought to form at spreading centers, along transform faults, or at hot spots. The existence of Shimada Seamount, therefore, has implications about tectonic processes that occur in interplate regions.