Evidence that three seamounts off southern California were ancient islands

Many seamounts world-wide exhibit features that suggest they were once islands. Rodriguez and San Juan Seamounts and Northeast Bank, within 250 km of the continental margin of the western US, were explored with the ROV Tiburon and found to have guyot morphology, beach deposits and erosional features, degassed volcanic lavas and pyroclasts, and cooling and alteration patterns unusual for submarine erupted lavas. These features are evaluated as evidence that the volcanoes stood above sea level. The summit of Rodriguez Seamount is a smooth, gently domed platform. Rough hills stand above the platform and consist of dense, degassed ‘a‘a flows erupted subaerially. Bedded sandstones and rounded cobbles interpreted to be beach deposits occur near the top of a major break-in-slope at about 700 m depth. The summit of Northeast Bank is also a smooth, gently domed platform. Beach deposits were observed just below the pronounced break-in slope at about 500 m. The summit of San Juan Seamount is a northeast-southwest trending series of rough ridges rather than a dome. However, subaerially exposed ‘a‘a-like lava flows were found above 700 m. Oxidized olivine, degassed volcanic glass, and secondary phosphatization of the lavas are found both shallower and deeper than the breaks-in-slope, and their presence may not be robust indicators of subaerial exposure. Rodriguez stood at least 70 m above sea level and formed a small island 6.8 km2 in area prior to subsiding at least 700 m. Northeast Bank was a large island perhaps 200 m above sea level and 90 km2 in area. The summit ridge of San Juan emerged as a line of eight small islands with a total area of about 2.8 km2, and the tallest island rose at least 140 m above sea level. The subsidence of Northeast Bank within the California borderland, Rodriguez Seamount on the continental slope, and San Juan Seamount on the adjacent ocean crust occurred since they formed 7 to 11 million years ago. The minimum subsidence is consistent with cooling and contraction of the underlying lithosphere.