October 11- October 17, 2003
October 16, 2003
David Clague writes:
We completed our final dive today at 3 pm and are now underway back to Moss Landing, where we should arrive Friday afternoon. The final dive was located on the northeast side of the seamount and addressed several questions about the formation of the seamount. We started in an amphitheater that looks like a large landslide scar that dissects two volcanic cones. The interpretation, based entirely on morphology, turned out to be correct and the fault cutting the deeper cone exposed a stack of thick lava flows. The thickest flow was about 50 meters thick and several others were more than 20 meters thick. Such thick flows are consistent with low-temperature viscous (pasty) lava. We recovered samples from the interiors of these flows, which are usually suitable for radiometric dating. The shallower cone was also dissected by the fault bounding the landslide, but the cross-section through the flank of the cone consisted of a thick series of volcaniclastic layers, ranging from breccia to sandstone. The shallower cone apparently erupted explosively throughout its history.
The dive continued upslope across a thick sequence of fine sandstones that are probably beach sands. Several thick flows crossed the ancient shoreline. The dive continued across the northern part of the gently domed summit to a small rough region that we had identified as a probable volcanic vent and thick flow. The dive ended on this flow. During the dive we collected 41 rock samples, about half are lava fragments and the rest are either fine volcanic sandstones that were most likely beach sands and coarser-grained volcanic breccia to sandstone that probably formed during explosive eruptions.
The dives on Rodriguez accomplished a number of objectives, including confirming that the amphitheater was indeed a landslide scar, that the flat top of the volcano was planed at sea level, and that Rodriguez was an island surrounded by sandy beaches. These beaches consisted entirely of volcanic grains and were similar to black sand beaches formed in Hawaii where lava flows into the ocean. With time, the glass altered and cemented the sand to form hard sandstone. We also determined that many, if not most, of the cones on Rodriguez erupted explosively in water as deep as about 500 meters (at their time of formation) and produced abundant volcaniclastic rocks ranging from breccia to sandstone. The present depth of the summit, coupled with the identified shoreline at about 700 m depth, indicates that Rodriguez subsided at least this far since the volcano was active, about 10-11 million years ago. Much of the hard work now begins as we prepare samples for microscopic examination, chemical analysis and radiometric age dating. The dive video will also be used to map the distribution of the different eruptive products (lava flows, breccia, sandstone) in detail to evaluate the role of lava composition and eruption depth on the eruptive style. The final report tomorrow will summarize the biological observations and collections.
As we prepared to depart last Saturday to work at the Taney Seamounts due west of Moss Landing, we had no idea that bad weather would so modify our plans that no dives were actually completed at the Taney Seamounts, one dive would be done at Pioneer Seamount northwest of Moss Landing, and four dives would be done on Rodriguez Seamount off Point Conception. However, the ship’s crew and ROV pilots adjusted schedules and plans so that we could still succeed and forward our project goals to better understand submarine volcanic processes.