December 13, 2004
Ancient islands off Southern California
SAN FRANCISCO—The first-ever video images from submersible dives on southern California seamounts revealed wave-cut rock surfaces, eroded beach sand, and rounded pebbles—clues that these undersea volcanoes were once islands. Geologists from the Monterey Bay Aquarium Research Institute (MBARI) present compelling evidence for this conclusion this week at the American Geophysical Union 2004 Fall Meeting in San Francisco.
Using MBARI’s research vessel Western Flyer and remotely operated vehicle (ROV) Tiburon, MBARI researchers, led by volcanologist David Clague, collected video images and rock samples during expeditions to seamounts off Southern California in October 2003 and April 2004. The eleven Tiburon dives provided the first-ever video observations of Rodriguez Seamount, San Juan Seamount, and Northeast Bank, undersea volcanoes about 300 kilometers west of Los Angeles. Prior to the MBARI expeditions, only a handful of rock samples had been dredged from these sites in the late 1960’s to mid-1980’s. Some of the seamounts, like San Juan Seamount, had never been sampled before.
Clague’s research team, including technician Jennifer Paduan, first author of the AGU poster, were surprised to find apparent beach features more than 500 meters below the sea surface. As shown in the accompanying photographs, ROV Tiburon recorded images of beach sands and wave-sculpted lava from Rodriguez Seamount at 680 meters depth. Such deposits are too deep to have formed during low sea levels caused by glacial events. These features were observed on Rodriguez Seamount, San Juan Seamount, and Northeast Bank, but not on Central California seamounts that Clague has been studying since the late 1990’s, or on deeper Southern California seamounts that the team investigated during their recent expeditions.
The team collected many rock samples from the seamounts. Later chemical and mineralogical analyses of these samples in the laboratory suggested that the rocks had been exposed to air, as would be expected if the volcano had erupted above water.
Video and still images of the seamounts also revealed rich communities of deep-sea animals, including large deep-sea corals and sponges. Seamounts are believed to be biological hotspots and are of growing interest to researchers worldwide. Future explorations using deep-diving vehicles and improved seafloor mapping will no doubt reveal new information about these prominent but little-known seafloor features.
Research presentation at AGU 2004 Fall Meeting:
Evidence that three seamounts off Southern California were ancient islands, *J B Paduan, D A Clague, A S Davis, Thursday 1340 V43E-1463 MCC level 2 (poster)
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When MBARI researchers dove on Rodriguez Seamount they were surprised to see rocks such as these, which appear to have been sculpted by wave action, but are currently 635 meters below the sea surface (far below the reach of the largest storm waves).
Like many organisms living on seamounts, this “mushroom coral” captures minute food particles that drift past on the ocean currents.
Large deep-water sponges often provide sanctuaries for many smaller animals such as these three crabs.
For additional information or images relating to this article, please contact: Kim Fulton-Bennett