Leg 7: May 1, 2012
Day 11: Cruise summary
We are sailing towards La Paz at the end of a very successful cruise. Using the remotely operated vehicle (ROV) Doc Ricketts, we explored and sampled the geology of Alarcón Seamount and the Alarcón Rise, the northernmost segment of the East Pacific Rise before it enters the Gulf of California. Everyone is tired and will sleep well tonight knowing that we have had a great cruise with many discoveries.
We started with a general concept of what we wanted to accomplish, but specific dive planning could not begin until a few days before we sailed, when the first autonomous underwater vehicle (AUV) surveys of high-resolution mapping data were collected. The last of the surveys were completed after we sailed on the 21st of April.
We extend our thanks to Hans Thomas, Duane Thompson, Doug Conlin, collaborators Carolina Nieves Cardoso and Miguel Santa Rosa-del Rio, and the crew of the R/V Zephyr for collecting these data, and especially to Dave Caress for his additional work processing the data into usable maps in far too little time.
With one-meter resolution maps in hand, we identified dive sites to sample the youngest lavas along the entire ridge segment, determined if many small positive relief features were active or inactive hydrothermal chimneys, and decided to explore an unusual irregular, rough, high-relief feature in the northern part of the ridge. The ROV program could not have achieved but a tiny fraction of what we did without such maps.
Our first dive was on Alarcón Seamounts, a group of flat-topped volcanoes that formed adjacent to the mid-ocean ridge, where the AUV mapped around several large, nested, and overlapping calderas. We have been working for many years on three similar chains of volcanoes in the northeast Pacific named the Vance, President Jackson, and Taney Seamounts. These volcanoes tend to have widespread deposits of fragmental volcanic rocks whose mode of origin is not well understood. In each case, these deposits occur on the rims surrounding the calderas, suggesting that the deposits might form during caldera collapse, or perhaps during mildly explosive eruptions following formation of the caldera. We wanted to see if the Alarcón Seamounts had the same features. That first dive showed that the largest of three calderas was indeed surrounded by fragmental deposits, but a much smaller one was not.
Our second dive, and first dive on the ridge segment, was on the south-central portion of Alarcón Rise. It is dominated by an extensive sheet and lobate flow that erupted along nine kilometers (5.6 miles) of fissures oriented slightly oblique to the main ridge trend. Two hydrothermal chimney groups were identified within this flow, with one along the shallowest portion of the fissure and the other about 400 meters (1,300 feet) northwest of the fissure, in the middle of the flow field.
This first ridge dive was on the group of chimneys containing the tallest one—a 23-meter-tall (75-foot-tall) chimney as measured in the AUV data. We were not on the bottom for long before we encountered abundant dead and broken clam shells, and then clusters of Riftia tube worms. This was indeed an active hydrothermal field with several tall “black smokers” and an abundance of chemosynthetic animals. With advice from our Mexican collaborators, we decided to call the active field Meyibó, meaning “time of favorable sun, implying happiness or joy, celebration of harvest, and a time of new learning” in the local native Kiliwa language. The individual chimneys will be named after native Indian groups from Baja California and Sonora.
The site brought joy to our group and was definitely a time for new learning for our science party of geologists. With a sole biologist as our guide, we admired, tried to spell the names of the animals correctly in the log books, and collected samples on and around the chimneys. We spent a good part of the dive collecting high-definition video of each chimney and the animals on it, collecting some sulfide and lava samples, and collecting a selection of the animals for taxonomic and genetic study by colleagues back at MBARI. We continued the dive exploring and sampling the southwestern part of the fissure system and some older pillow mounds cut by the fissure.
All but one dive that followed had the primary objective of collecting samples of different lava flows and short sediment cores. The cores will be used to determine a minimum age of those flows using carbon-14 dating of foraminifera—a group of microscopic animals that make their shells of calcium carbonate. Most foraminifera live in the surface waters and their shells sink to the ocean floor when they die, where they are preserved in the sediments. Others live on the seafloor in the sediment. Either can be used to determine an age of the sediment overlying a lava flow. We distributed the dives along the ridge axis to determine if activity along any portion was younger than elsewhere along the ridge. All of the flows we explored had some sediment cover, so probably none are younger than perhaps 50 to 100 years.
With but nine dives to explore the ridge axis, we could not go everywhere and sample everything, so each evening we supplemented the dive samples with what turned out to be a dozen rock crusher collections. The rock crusher was described in some detail earlier, but it is basically a wax-tipped corer that smacks into the bottom, and broken bits of glass stick in the wax to be recovered with the corer. When all was said and done, we collected 194 lava samples, including the dozen from the rock crusher. We also collected sediment cores from almost all flows that had thick enough sediment to retain in the core tube. These amounted to 87 mostly short cores, with a few longer cores where the sediment was especially thick.
Three dives included observations and sampling at what we had identified as additional hydrothermal chimneys. All to the north were inactive, although several still retained fields of broken, dissolving clam shells, and so had become inactive fairly recently. The second site on the large sheet and lobate flow was active as well. More video, animal, and sulfide collection followed on that dive, and then we explored the northwestern part of the fissure system and some older adjacent flows to the northwest.
The dive to this second cluster of active chimneys happened right after a dive that collected a few samples of an unusual, rough mound towards the northern end. Some samples were comprised of almost clear glass, so they are silicic lava such as dacite or rhyodacite. Such lavas are rare along spreading centers and usually found near boundaries at transform faults, but at Alarcón, the 1,200 by 500 by 50-meters-tall (4,000 by 1,640 by 165-foot-tall) dome is located about 6 kilometers (3.7 miles) from the northeast end of the spreading segment. After this dive, we changed our plans our final dive, to further explore and sample this dome instead of returning to Alarcón Seamount.
As fate would have it, the ROV suffered a major hydraulic failure just before reaching bottom on the next-to-last dive, thereby preventing us from exploring the northernmost part of the ridge axis. Instead, we used the day to collect rock crusher samples on many of the cones we had planned to go to with the ROV, while the pilots repaired the sub. The final dive began on schedule Monday morning at 6:30 a.m. and proceeded without further incident. We sampled all lobes of the dome and the nearby surrounding lava flows. Much of the exposed lava is angular blocks that appear to be shed from a growing dome, much like the one that formed inside Mount St. Helens following the 1980 eruption. We have some evidence that explosive activity played a role here at 2,400 meters (7,800 feet) depth as well, as some lava fragments are bubbly and others have smooth glass surfaces typical of small blobs of melt ejected from the vent into the seawater and quenched.
During the past week, you have met many of our science party through their writing. It takes a lot of work to execute the dives and process all the samples that come onboard with each dive. I would like to thank Jenny Paduan, Julie Martin, Ryan Portner, and Lonny Lundsten from MBARI, who are the glue that makes all this happen, and collaborators Brian Dreyer from the University of California, Santa Cruz, Pat Castillo from Scripps Institution of Oceanography, Julie Bowles from University of Minnesota, Ronald Spelz from Centro de Investigación Científica y de Educación Superior de Ensenada, and Rigoberto Guardado and Hiram Rivera from Universidad Autónoma de Baja California in Ensenada. These are the people who did all the hard work while I digested the new mapping data and planned our dives and rock core sites.
Of course, the best science party is still sitting around on the boat with little to do without ship’s crew and ROV pilots who literally take us where we want to go. Thanks to Knute Brekke, Eric Martin, Randy Prickett, Bryan Schaeffer, and Marco Talkovic, who remotely take us to the bottom of the ocean and do miraculous things to collect delicate animals and glassy rocks. We appreciate all the effort you put into your work.
The ship’s crew under Captain Ian Young, First Mate George Gunther, and Second Mate Andrew McKee get us where we need to be, the engineers Matt Noyes, Lance Wardle, and Dan Chamberlain work behind the scenes and keep everything from engines to plumbing to computer systems running, and steward and chef supreme Patrick Mitts keeps us (too) well fed. The rock crusher team on the winch Jason Jordan, Craig Heihn, and Olin Jordan, were successful on every deployment. Finally, we are thankful to the weather gods, who gave us 11 warm (a few perhaps a bit too warm) sunny days with mild winds and small seas.
Most of what we will learn about Alarcón Rise and Seamount is still to be discovered as we examine and analyze the samples collected and refine the maps and study them in more detail. However, being the first people to see new places and things is always a thrill. It is what keeps us all going when the days are long and the nights are short at sea. I hope we have shared some of that excitement with you through the daily updates and that you have also been able to “remotely” explore the strange but beautiful world below all that water. In the end, we hope to learn more about how volcanic systems work, and to help build the foundation for understanding the hazards posed by eruptions of different types and how to reduce risk from future eruptions under the sea or on land.