Monterey Bay Aquarium Research Institute

Keck Expedition 2004
August 31, 2004 Day 2

Update for Tuesday August 31—the Search for Dacite Hill

Thanks to the fine weather the ROV was in the water by 6:30 in the morning diving to the curved ridges that mark the intersection of the Juan de Fuca Ridge and the Blanco Transform Zone. If you look at the image posted on yesterday’s update, you will see that these thin ridges that curve around the deep “nodal basin” appear knobby. One of the knobs sampled in 2000 provided a few shards of silica-rich glass called dacite- a rock type rarely found at mid-ocean ridges. Based on this one small sample we designed a dive to investigate several of the knobs and ridges to look for the unusual rocks.

ROVcloseup.jpg (103700 bytes)The ROV was set up to maximize our ability to collect rock samples. The drawer was outfitted with a complete set of 14 partitions for keeping the rocks separated. To this we added five PVC containers (aka biotubes) to the front of the drawer for individual fragile samples. We also placed five wax-tipped push cores on the swing arm that could collect small amounts of glass.

But how would we recognize the dacites without taking them back to the laboratory for analysis. Three of us—Gill, Stakes and Perfit—used our field experience to propose a diagnostic morphology for dacite flows. Small broken giant_pillow.jpg (44265 bytes)pillows, I said. No, giant pillows, said Mike. Jim was just as certain that they would appear as thick sheets. So, we decided that we would look for and sample any lava that looked different, especially if it had lots of gas holes in it. Why the gas holes? Evolved rocks like andesites and dacites contain volatiles such as water and carbon dioxide dissolved in them prior to their eruption. When they do erupt the dissolved gas separates from the still viscous magma, leaving behind a hole where the gas bubble used to be. If the lava is viscous enough, the holes can get elongated or even flattened as the flow moves after the gas separates.

The dive track placed us near the bottom of Dacite Hill so that we could climb up the side looking for “anything different”. This could prove to be a very long day.   Near the bottom of the slope the rocks looked definitely normal with tectonized slices of broken pillows, which is in fact EXACTLY tubes.jpg (33105 bytes)what you would expect to find in a transform zone. A large pillow or two was seen, but still nothing extraordinary. Then we began to see unusually fresh rocks with common morphologies, such as giant pillows and long lava tubes.  These were topped by astonishingly perfect elongate pillow tubes (see image to right) which certainly did not look normal.  

The summit of Dacite Hill was in fact a dacite dome with sheets (see dome_sheet.jpg (55866 bytes)image to right) and thick layers of gas-rich dacite (see image below left). The peculiar structures were related to the viscous nature ofthe lava that do not flow easily and in dacite_dome1.jpg (35706 bytes)some places could only push the surface rocks up into a dome-like structure. The day was completed by tracing the transition of the silicic rocks into the more normal structure of the southernmost spreading center.  We took samples of everything, a record number of 39 samples for the day.

RockSorting_1.jpg (85254 bytes)By dinner time the vehicle was on its way to the surface. So how do you identify 39 samples stuffed into 19 compartments? We had made “mug shots” of each sample before stowing it into the ROV drawer. During the vehicle’s ascent, I printed out copies of the sample images so that we could identify each specimen. This was still a daunting process that took a couple of hours. The image shows Mike Perfit trying to decipher which sample is which along with the assistance of Frank Ramos, George Kamenov, Sarah Langberg, and Lonny Lundsten. Once the rocks were finally indentified, the rock describing process began and continued long into the night.  We now have a wealth of exciting samples to keep the students busy for quite some time.

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