Vance Expedition
July 24 - August 6, 2006
August 3 update
Tiburon dive T1012, Vance B Seamount
Map of Vance B "seamount" and our dive track today (black line). We started at the floor of the small cone-shaped structure in the upper left and explored a number of lava-pillow ridges and mounds to the east and south.
Dave writes: Today we dove, not on a discrete seamount, but in a region with abundant volcanic activity that produced many small cones and ridges. This region is located between the oldest seamount (seamount A, our second dive on the Vance Seamounts) and the seamount we will dive on tomorrow (seamount C). Despite the rather non-descript bathymetry of "seamount" B (no circular flat-topped seamount with calderas) the volume of excess lava erupted on the seafloor is similar to that of two of the large volcanoes, suggesting that the rate of magma production along the chain did not vary widely. The primary dive objective was to determine if the lavas differ significantly from those that form the larger volcanoes in the chain. On most of the Vance Seamounts, magmas passed through chambers beneath the calderas. Evacuation of magma from these chambers created the space that became the caldera after collapse of the surface. Some processes that modify magmas, such as cooling and crystallization, magma mixing, and incorporation of surrounding rocks are more efficient if the magmas pass through and reside in such chambers prior to eruption. The area we studied today, unlike all the other areas studied during this cruise, should be lacking magma chambers, so these processes should be minimized or eliminated, allowing direct assessment of the composition of the magmas produced in the Earth’s mantle. When we get all the samples back to our laboratories, we will analyze them chemically to determine if we our ideas about magma chambers are viable. We think that the lavas collected today will have higher magnesium contents and therefore higher eruption temperatures, compared with those from the large circular seamounts.
In the meantime, we made some interesting observations today that characterize the style of eruptions in this area of dispersed volcanic activity. All we saw all day was pillow lava and talus (and some sediment), so these smaller cones apparently did not produce volcaniclastite during explosive eruptions. The absence of sheet and lobate flows suggests that eruption rates were low. Many of the eruptive vents were quite small and many had very steep sides with talus at the base. Usually, one would interpret talus as evidence for faulting and breaking of rock after eruption. Last year we studied the 1996 eruption on the northern Gorda Ridge, where vertical walls with talus at their base were constructed during the eruption. Today we saw clear evidence that the same occurred here, as we found talus draped by elongate pillow flows from the same eruption. Such steep-sided vent structures most likely form during eruptions with very low eruption rates, so that the flows never advance far from the vent. A model we are discussing is that this area formed by many small frequent eruptions, perhaps influenced by abundant crustal faults, whereas the large circular volcanoes are built by infrequent very large eruptions fed through a few faults. When all the data is pulled together, including the chemistry to be completed ashore, we hope to be able to address such questions.
Once again, we observed and collected numerous animals. The summits of several of the cones were home to colonies of large and diverse deep-sea corals and sponges. We found 6 more gorgonians and coralliums and observed at least one more that we could not reach to collect. In addition, we captured a number of polychaetes using the suction sampler, collected another enteropnuest (acorn worm), a small squat lobster, a sponge that looks like a fringed lollipop, and the usual brittle star assortment on the rock samples.
Summit of a mound of pillows, which looks like many we saw today. Typically, each mound was more than 30 meters (~100ft) high, and the mounds were often built on top of each other so we would dip into a sedimented gully next to one and then climb the sheer cliff of the next. These steep-sided, relatively small cones are probably individual eruptive vents.
On the steep flank of a pillow mound, this lava pillow had cracked open and drained during the eruption. The rind at the top of the broken pillow makes a convenient grip for the manipulator to collect a sample; most of the rest of the lava is well-cemented to the earth.
Dave composing his summary of today's dive in the dry lab on the ship.
Jenny writes: It got rough last night but settled down a bit today so we were able to dive. The sun was out this afternoon (it may have been out more than that today, but I was cocooned in the dark ROV control room and didn't take much notice). When the sun is out, the sea out here is a pure, brilliant, transparent blue that is hard to imagine until you see it. When it is stormy, so is the color of the sea: an angry gray. But up here in this part of the North Pacific, while the sun brings out the beauty of the water, it also means the wind is going to blow hard. So it is better for it to be overcast (just not stormy, please).
I would like to take this opportunity to laud the video capabilities of the ROV Tiburon. The images in these cruise logs have been down-sized so they can be sent by satellite to shore and then upload in your browser without taking forever and costing a fortune, but they don't do justice to the beautiful imagery of the new high-definition TV camera on the Tiburon. The video we see and record in the control room is spectacular: high resolution, wider perspective, and no distortion. Especially in these clear waters, we can see so much more detail and a greater amount of seafloor than was possible before. Coupled with the cameras the pilots use to fly the vehicle, which point in other directions and are also displayed in the control room, it is almost like being there.
The map above shows the area to be complicated compared to the other seamounts. However, the terrain we explored today was far more complicated than the 30m resolution bathymetric data on the map suggests: for example, at one point we thought the ROV should be on a ridge, but it was in a gully facing a sheer cliff with another behind it. The depth of the 30 meter x 30 meter pixel on the map within which our real-time GIS was placing the Tiburon was an average of the wide range of depths of these pillow mounds and didn't begin to represent the actual topography. The map was made with one of the best surface-based multibeam mapping systems available, but its resolution is not quite good enough to show what the bottom here is actually like!
Today was Dave Clague's birthday! We sang to him TWICE (got it on tape in the control room!) and this evening had a special dessert of ice-cream sundaes with his favorite toppings. He said no one played any practical jokes, but he got plenty of "old guy" remarks.
Christoph decanting excess water from a push-core sample he has just sieved. The sample will be dried and we will examine it under the microscope tomorrow.
Christoph writes: Well, I have to admit, this is my first research cruise to the middle of the wide and wild Pacific Ocean. And wild it was indeed, the fist couple of days. It took my stomach and my feet a while to get used to the moving ground, but now I have to say, I do understand why people go crazy about this kind of fieldwork. It truly is some kind of comfortable field work – no dirty hands and you never have to worry about good food. And that we get served every single day, eating like kings we do, thanks to our master of the kitchen. The whole crew is a really nice “bunch” of great people and perfectly organized, too.
It is real good fun and an awesome experience at the same time to climb up one or more caldera walls a day in a depth of about 2000m - watching everything through the eyes of Tiburon our little submersible. Actually it is not that small. We passed by some beautiful volcanic outcrops on one of the Vance seamounts showing huge pillow basalts, strange-looking lobate basalts, massive sheet flow sections or layers of volcanic sediments, evidence of the wild explosive life parts of the volcano.
On Axial seamount, an active caldera located exactly on a mid-ocean-ridge, we sampled little piece of basaltic volcanic glass, produced by the volcano when the erupted hot lava was cooled extremely quick by the seawater. In those samples we found fascinating forms of glass pieces. Some in the shape of a hair, really not much thicker, some of them are even curled. Other fragments are thin plates with sharp edges, slightly bent, making them look like fragments of tiny beer bottles, since they are brownish transparent, too. The only drawback about most of the samples taken on the flanks of this volcano is, these were taken in long aluminium tubes by simply pushing them into the sediment and these are not to be opened on the ship – so we don’t now yet what we got!! This is going to be the big surprise at the very end of the trip. Patience required therefore!
Small outcrop of elongate pillows. The manipulator arm is poised to begin searching for a sample that it can break free from the outcrop and from the tenacious cement of the manganese crust on these old rocks. The sample it will succeed with is in the lower center of the photo, a twisted tube that budded from the pile.
Sample of the elongate pillow bud in the manipulator's grasp. We photograph each sample as it is collected to assist us with figuring out which black rock is which in the ROV's hydraulic drawer after the dive.
Lava sample collected in the accompanying photos, as pictured in the lab. The glassy, black surfaces along the top are the freshly broken places where it was attached to the outcrop. The brown, duller texture of the rest of the rock is manganese crust.
Closeup of a large gorgonian (sea fan) with a white sponge visible at its base on the rocks. The red laser dots are 29cm apart for scale.
A ray swimming past pillow lavas draped with sediment.