Keck Expedition 2004
September 4, 2004 Day 6
Update for September 4 by Debra Stakes
Today we accomplished our cross-ridge transect of the Endeavour Segment despite marginal weather. If you look at a map of the Endeavour segment, the first thing that you will notice is that it looks like a canyon (the axis valley) between parallel ridges running north and south. On the map the two parallel ridges are a light green color with lighter green summits. The seafloor around the ridge is deeper and shows up as blues and grays on the map. Mid-ocean ridges are always shallower than the surrounding seafloor due to the thermal bulge of the upwelling mantle beneath. The mantle upflow is what delivers the small amount of melted rock that forms the ocean crust. Such “mantle convection” is one of the primary forces of our planet. The yellow triangles show all of the sites that we had hoped to sample, but we do not have enough time to accomplish all of those dive tracks. So together we must formulate a prioritized list. The plan that we finally decide upon is to spend the first dive crisscrossing the northern axial valley that has been the most neglected by previous studies. Remember that the axial valley is thought to contain the youngest erupted volcanic rocks and is thought by most geologists to be the ONLY location for mid-ocean ridge eruptions. Our next priority was to look at the range of relative ages and types of volcanism across the entire area. Today we essentially completed this transect, following the centermost line of yellow triangles shown on the map. Like true explorers we traveled a part of the seafloor that had never been seen before, recording our observations by digital imagery, slowly building up our model of how all of these volcanic events must have occurred to build such as edifice. We saw folded sheet flows in most of the deeper areas (see image to right) and the steeper flanks built of piles of pillow tube (see image to left).
In between were areas of pillows drained of magma (see drained_pillows) topped with flattened pillows (see fish_pillow.jpg). We have grand ideas of how it all fits together, but that must wait for the chemical analyses to be done after we return home. Like any other exploration, we stumble across unexpected discoveries. We find an unreported but beautiful low temperature vent covered with graceful chemosynthetic tubeworms (see bottom left) and old sulfide chimneys covered with animals including red seafans (see bottom center). At the end of this transect we were rewarded by a visit by a mysterious octopus (see bottom right).
From co-Chief Scientist Jim Gill:
To people who have used other vehicles to explore the sea floor, the Western Flyer and Tiburon, plus the very high resolution bathymetry, are a stunning combination that creates opportunity and adds confidence to unprecedented levels. Those of you that use to using GPS in your car to keep track of your location might not be impressed that we are sometimes uncertain where the ROV is to within 20+ meters, but anyone who works on the seafloor finds that remarkable. In places the bathymetry we are using has a resolution of a few meters, so there is constant chatter in the control room about where we think the ROV is. Sitting in the control room, surrounded by wide screen monitors, you think you are actually sitting in the ROV Tiburon hovering slightly about the sea floor 2000 meters below. When it accidentally bumps bottom, you almost expect to feel it.
There are also constant trade offs about how many rock samples to take, where, and how long to spend looking for the “right one”. We take about one rock every 15 minutes, or about 40 altogether per dive. It takes two to ten minutes per sample. The pilots and scientists point to the screen like back seat drivers saying “How about that one?” or “This looks like it might break off quickly” or “That one has the kind of glassy edge I want”.
The combination of knowing exactly where the rock is from and choosing the “right one” matters a lot to what happens next. Each rock is carefully prepared on shore for chemical and microscope study. It takes hours per sample. All surface coatings must be removed and only pure unaltered material used. Choosing the best sample at an on-land volcano usually takes more time than we can spend at each spot on the sea floor, plus sea water and hydrothermal venting create mischief for the sea floor rocks. As a result, much of what we collect cannot be used, and a few grams of pure “kernels” from each rock are precious. Once isolated, they are cleaned in distilled water and leaching lightly in several acids to be sure any extraneous coatings are removed. (See Jim Gill hard at work in the image to the left) Then, chemical and microscope study takes days to weeks of work per sample. That’s why there is so much chatter about which one to take. You cannot afford to invest that much effort into the wrong one.
The “right one” tells an integrated story about the history of the earth. Here the rocks are basalts, volcanic rocks formed by melting of the mantle as it decompresses during ascent beneath the ridge. There are larger variations in the chemical and isotopic composition of the basalts at Endeavour than is usual at mid-ocean ridges. We will use those variations to understand how the volcano was constructed and how long it took, just as one does for volcanoes on land. In addition, the variations will tell us about the mantle that was melted – its history, how it melted, and how the magma made its way to the surface. All that comes from the chemical and isotopic composition of the basalts, often from “trace elements”, those present at part per million or even per billion levels in the rocks. We cannot afford to risk contamination even at those levels!
Before this cruise, not one basalt from Endeavour whose location was known to within a km or so had been thoroughly analyzed. Now, if we are collecting the “right ones”, then after a year or more of laboratory work, we may have scores that are thoroughly known, winnowed from maybe over 100 that are studied in more reconnaissance fashion. But it all starts with the map, the navigation, and the choice of the rock on those TV monitors.