July 24 - August 6, 2006
August 5 update
Tiburon dive T1014, Vance G Seamount
Dave writes: Today was our final dive for the cruise and we elected to examine the easternmost volcano in the Vance chain. This volcano is a broad shield only 180 meters tall and 3.7 kilometers (NW-SE) by 2.4 kilometers (NE-SW). The entire volcano, including a broad, elevated, underlying plateau, has a volume of only about 15 cubic kilometers compared to volumes of the larger volcanoes with calderas of 23 to 34 cubic kilometers. The volume of the shield above the plateau is very small - no more than a few cubic kilometers. The shield is offset by two normal (vertical) faults that drop the southeastern side downwards. In addition, the southeast flank of the shield has two linear ridges that may represent fissure eruptions that occurred along faults parallel to the nearby Juan de Fuca Ridge axis. These are the only structural features evident in the bathymetry.
As I explained yesterday, the dive today was much shorter than normal so that we could begin our transit back to port in Newport, Oregon to arrive by 5 pm Sunday. To compensate a little, we began the dive a half hour earlier than usual, while ending the dive at 2 pm instead of the normal 7 to 8 pm. Thus we had slightly more than half the time on the bottom as a normal dive day. We planned the dive to cut across the two fissure zones and the three faults to maximize our sampling opportunities.
The dive began as expected with a low ridge of young, nearly unsedimented sheet flows followed by another of pillow lava. As we continued towards the northwest, however, the terrain shifted to old, sediment covered, volcaniclastite with pillows projecting up through the clastic layers. The sediment we collected was deeper than the 30 cm length of the pushcores and ended in very light-colored foraminiferal ooze that must be at least Pleistocene in age, as today planktonic foraminifers dissolve before they sink to the bottom at the depth of this dive. We spent much of the dive traversing regions with this sediment cover and small outcrops of volcaniclastite draped over older pillow or sheet flow lavas. The gentle slopes we expected based on the bathymetry turned out to almost always consist of fault scarps up to 30 meters tall, exposing sections of talus below interbedded stacks of pillow and sheet flows. The larger faults evident on the bathymetry were simply the largest of these faults, but numerous others cut the region, with all oriented roughly parallel to the nearby Juan de Fuca Ridge axis.
We expected this entire shield volcano to consist of young lavas like we found in the beginning of the dive, and the older age of most of it came as a surprise. In addition, the thickness of both volcaniclastite and hydrothermal manganese crusts was unexpected since we expected much younger lava flows. We speculate that the collapse of the caldera at nearby Vance F (our very first dive site at the Vance Seamounts) may have produced volcaniclastic material that draped not only the summit and caldera floor of Vance F, but also the adjacent areas, including Vance G. The breach in the caldera on Vance F to the southeast would enable transport of the fine volcanic particles towards, and probably beyond, Vance G. We can test this hypothesis by seeing if the glass particles at both sites have the same compositions.
The dive ended with a flourish as we collected 6 rock samples on a vertical scarp in the final 11 minutes, perhaps setting a record for sample collection rate during a dive. Only a few years ago, collecting 6 samples would have been a normal recovery for an entire dive. Such efficiency reflects on the skills of the pilots using the extremely capable ROV Tiburon, and the increasing sophistication of robotic technologies.