MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 30 update posted by Dave Clague
Tiburon dive 891 - Exploring the central vent area of the Escanaba Trough
We completed the fifth and final dive at the NESCA eruption site today, with a dive near the inferred eruption vents. During the dive, we continued our collection of sediment cores to determine the sizes and shapes of volcanic particles in the sediment, as well as the total amount of glass deposited close to the vents. We also performed several biological transects over different type of flows to see how much variation in community structure there was between this flow and the other younger flows we have seen during this cruise.
The lava flow that we explored today surrounds several uplifted hills of sediment. We believe that these hills were uplifted by magma that flowed beneath the surface to form shallow sills. One of the small sediment hills was bounded by a lava-block talus slope rather than an in-place lava flow, which suggests that this particular hill was uplifted after the lava flow was emplaced. However, the fact that the flow also flowed around the hill suggests that the hill began to uplift before the emplacement of the flow around it. Taken together, these two observations provide evidence that the eruption and the local uplift caused by lava-sill emplacement occurred at about the same time.
In several places we observed that the early lava flows took the form of flat, folded, or jumbled sheet flows, but later flows contained more pillow lavas. This sequence is what we would expect if large amounts of lava flowed from the vent at the beginning of the eruption, but the flow rate decreased as the eruption proceeded. This eruption pattern is common for volcanoes on land, in Hawaii and elsewhere in the world.
During our transects across the lava flow, we spent a lot of time discussing what the different pillow-lava shapes and textures could tell us about the rates and styles of lava emplacement. Many of the lava pillows had thick "bread-crust" rinds. These rinds suggest that the pillows inflated slowly at first, allowing a thick crust to form, then reinflated due to a later lava surge, which caused them to split open. Other pillows were smooth-skinned.
During our five dives in the Escanaba Trough, we collected about 135 push-cores of sediment and 25 new samples of volcanic rock. These samples will be analyzed along with previously collected samples from this area to help us evaluate the homogeneity of the flow and to determine if the different flow morphologies reflect differences in lava temperature and emplacement rates. The push-core sediment samples we collected have all been sieved to separate the sand- and silt-sized grains of glass from the mud, which makes up more than 95% of the samples. Our next step will be to determine the largest grain sizes found at each location as well as the distribution of the grain sizes and the amount of glass in each sample. After we get all this done, we will be able to build computer models that relate the dispersal of the volcanic glass particles to the style and rate of the initial eruption.
At the end of today’s dive, we revisited a hydrocarbon-seep site where we had collected chemosynthetic clams in 2002. The samples from the 2002 dive smelled like diesel because they contained "cracked" hydrocarbons (essentially crude oil, which was created when hot hydrothermal fluids "cooked" the organic carbon buried in the Escanaba Trough sediments). It took us several hours to relocate the site, but when we did, all we found were dead clamshells. We also didn't see the bacterial mats or black, hydrocarbon-rich sediments that were present in 2002. This suggests that the seep stopped flowing some time during the last 3 years.
As on previous days, we observed several new and strange animals today, including “the basketball”--a large, round, bumpy, reddish-brown benthic mollusc, which we carefully scooped into the biobox on the ROV. We also collected a branched animal (a gorgonian coral?) with small balls on the end of each branch. Both are completely new to all of us. Finally, we collected several sea stars--a white one that occurred everywhere on the lava flow and a second, larger species that was unlike any we have seen previously. We also saw found additional examples of some of the new animals from yesterday's dive, including a diaphanous benthic mollusk that we collected yesterday and a large (30-cm) white polychaete worm we saw but could not collect because it was too quick for us.
On the way back to the surface, we stopped to buy a postcard showing a few more of the animals of the Escanaba trough (thanks to Janice Fong for the inspiration).