Spatial Distribution, Density Structure, and Relationship of Intrusive and Extrusive Volcanics of Seamounts Along the Northwest Hawaiian Ridge

The Papahānaumokuākea Marine National Monument (PMNM) is a 362,073 km2 conservation area encompassing islands and seamounts with prodigious diversity of size and morphologies formed by Hawaiian hotspot volcanism 7-31 Myr ago. During the winter and spring of 2014, we collaborated with the Schmidt Ocean Institute to conduct a detailed bathymetric mapping and geophysical survey of the PMNM on board the R/V Falkor. On two cruises, we collected 14,585 km of gravity data using UNOL's BGM-3 marine gravimeter and magnetic data with the University of Hawaii's G-882 cesium magnetometer. We will present these new data paired with a select set of existing NGDC marine geophysical data. Using these new gravity and magnetics data, we will investigate the internal density structures of these seamounts and the oceanic lithosphere upon which they were emplaced. The locations and volumes of the dense intrusive magmatic centers and rift zone dike complexes of these seamounts will be determined to investigate whether the ratio of intrusive to extrusive volume varies with time and volcano size. One of the largest rift zones of Hawaiian volcanoes is that of the St. Rogatien volcano ( ~125 km in length), and preliminary results from the Bouguer gravity anomaly suggest it is largely made of a dense dike complex. This rift zone, and other long rifts within the Monument will be compared to and contrasted with previously studied rift zones in main Hawaiian Islands to more completely characterize these features along the chain. Few volcanoes within the Monument that have been sampled. Some of these have yielded Cretaceous ages (e.g., O'Conner et al. 2013) and therefore it is unclear how many volcanoes within the Monument were formed by the Hawaiian mantle plume. The gravity data will be used to identify which of the undated seamounts are compensated by an uncharacteristically thin elastic plate and thus, did not form as a part of the hotspot chain. These results will help refine models of Hawaiian mantle plume productivity through time, absolute plate motion, and better define the volcanic loads for studying the rheology of the flexing lithosphere beneath these volcanoes.