Archive of September 29th, 2007

Teacher at Sea log for September 29, 2007 by Linda Sciaroni

We have been traveling to different locations and submerging the Jason 2 ROV so we can look for rocks to sample. I thought you would enjoy seeing how different the seafloor can look at all of the places we have seen while collecting samples. The ship is networked so we can see what the cameras on Jason 2 are recoding on our personal computers. We call that the "Virtual Van," because the place where the crew operates Jason 2 from is called the "Control Van." I went to the Virtual Van and copied these images of the varying places Jason 2 has been. One very common feature is that all of the rocks are covered with manganese coating. Manganese coating thickens with time and paves our dive sites like cement, making it difficult to pry samples loose. In 36 hours, we collect between 30 to 60 rocks; that works out to just one or two samples an hour. What do you think is the history of the rocks in these photos? Do you think they came from a volcano?

Science Update by Todd Bianco

The images today show slices into two basalt samples. The upper two images are of sample J2-300-01, a pillow toe collected from Site 05, northwest of Kaula. The speckled brown material on the side view image is palagonite, which is glass that has been altered by water. The front view show that the palagonite is very thin, so the sample has not has enough time on the seafloor to undergo extensive alteration. Therefore, we suspect the sample is relatively young compared to the closest islands of Kaula and Niihau. The rest of the sample is almost completely glass. Glass forms when liquid magma cools so quickly that there is no time for crystals to form, so we suspect that this basalt extruded onto the seafloor. The shape of the sample, a pillow toe, also leads us to that conclusion. The bottom two images are of sample J2-301-26, a large angular clast collected from an outcrop at Site 06, southeast of Middle Bank. In the original image, the cross section shows the surface of the rock is thick (6-12 mm) manganese coating. Also, this rock has vesicles (bubble holes) and altered olivine phenocrysts. Phenocrysts are large crystals, and olivine is a mineral found in many basalt samples. Olivine can form in magma as it cools slowly because there is time for crystal growth. Therefore the amount of olivine is connected to eruptive and magma chamber processes. Olivine is normally green, but the close-up image shows that our sample has brownish colored olivine. This is another alteration process, and we see that some phenocrysts are more altered than others. The alteration occurs because olivine is stable in a magma chamber environment, but at the temperature and pressure of the seafloor, it is unstable. As with the palagonite above, the level of olivine alteration is an indication of the amount of time the olivine has been on the seafloor.
Therefore, by describing the amount of glass and olivine, and the level of glass and olivine alteration in our samples, we can already begin to understand differences in the volcanic history of our sample sites. Gathering this information during the cruise rather than later in the lab is important. It gives the team a developing picture of the range of where we have been and what we have sampled, which is not always controlled by moving around to many distant locations.

Presented by the School of Ocean and Earth Science and Technology at the University of Hawaii, with financial support from the National Science Foundation.