DETAILED GEOLOGIC MAPPING EXPOSES COMPLEX ERUPTIVE HISTORY AND FUNDAMENTALLY DIFFERENT RELATIVE AGES OF NEWLY SURVEYED SEAMOUNTS WITHIN THE JOHNSTON ATOLL UNIT OF THE PACIFIC REMOTE ISLANDS MARINE NATIONAL MONUMENT

During the FK161229 expedition, Schmidt Ocean Institute's R/V Falkor embarked on a scientific transit-cruise en route from Guam to Hawaii. The goal of the cruise was to map geomorphic and subsurface seamount structures within the newly expanded Johnston Atoll Unit (JAU). The cruise collected over 11,000 km² of high-resolution multibeam data within the targeted survey area. A detailed geologic map was illustrated that differentiates between volcanic, sedimentary, and erosional units of the seamounts. The unit contacts were drawn using morphological relationships observed from 20-meter bathymetric and 5-meter backscatter resolution grids. Structural relationships were mapped including individual volcano-specific contacts, numerous landslide scarps, faults, and paleoshorelines. Along-track magnetic data show contrasting magnetic anomalies designating two main relative ages of emplacement for these seamounts. These new maps reveal additional complexities of a previously assumed simplistic model of volcanic growth. At least ten individual volcanoes built the four seamounts surveyed, a constructional evolution similar to that of the Hawaiian volcanoes. The high-quality data display the detailed nature of the contacts between these volcanoes and show multiple periods of subaerial growth. The subaerial regions with flat or gradually sloping carbonate and sediment-capped summits are present at three main depth ranges, indicative of three different subaerial growth durations. Numerous post-erosional monogenetic cones that erupted through the carbonate caps are the most notable features of the summit morphology. Over 50 individual summit volcanic cones were identified. A post-subsidence, submarine emplacement of the summit cones is hypothesized due to the preservation of the cones’ smaller size and location where subaerial erosion could have easily erased these features from the geologic record. These features are also observed on the summits of other seamounts within the JAU seamount group. The frequency of occurrence of summit cones on JAU volcanoes suggests that the eruptive mechanism generating the cones has been long-lived throughout the formation of these seamounts and differs from the equivalent rejuvenated stage of Hawaiian volcanoes.