Here’s another great missive from Evelyn, our favorite geology grad who is currently in the Indian Ocean studying an underwater volcano chain! Enjoy!
(part 1 is here)
Hard to believe that July is here and that I’ve been at sea for over two weeks! I am still spending five more weeks onboard the ship and then traveling for two weeks, so my journey is far from over. Already, though, it feels as if I have been at sea for a very long time. We are generally surrounded by endless blue. We can see about three miles to the horizon, I am told. Every so often, we see other ships. We have seen several container ships and even a couple in a small sailboat. The captain was very surprised to see the cuple in the small sailboat. He said they were sitting ducks for the pirates that roam this part of the world! Fortunately, we have not encountered any pirates. We have also had a few animal visitors. Every so often dolphins follow our ship, and we have also seen numerous birds and a couple of sperm whales. The weather has been mostly calm although yesterday the seas were quite rough. Most of us have adjusted to the rocking of the ship, and so very few people became seasick yesterday. However, many of us are bruised from being slammed into the sides of hallways or the furniture. Walking around the ship when it is really rocking is quite a challenge!
I have been keeping very busy with work the last week or so. The first week or so of the cruise was largely devoted to a geophysical survey of the northern part of the ridge, so the petrology (i.e. rock description) group had it fairly easy. We just had three or so dredges to describe, photograph, and catalog and one of those dredges weighed less than two kilograms. The second week of the cruise has been very busy with dredges, though, and we now have eight total and are looking for more places to dredge. One day we pulled up three dredges the same day! I slept well last night, but for a week or so I was surviving on two to three hour power naps as well as large quantities of tea and diet coke. The ship costs $25,000 a day or more to operate, so the scientists do not want to waste any time. Sleep is a luxury, and if there’s work to be done one needs to work instead, within reason. As I know I still have five long weeks ahead of me, I have been sleeping when I’m able.
Some of you are probably wondering what is involved in the dredging process. The concept is fairly simple, actually. Basically, we throw a large metal basket with teeth overboard, scrape the basket along the seafloor for two to six hours, and then pull the basket in to see if we’ve gathered any rocks. This technique sounds primitive, and it is. Much more sophisticated ways of gathering rocks do exist. Scientists can gather rocks from the seafloor by making use of tools such as underwater robots and submersibles like Alvin. When scientists go down to the bottom of the seafloor in Alvin, they are able to see all of the rocks on the seafloor and use a metal claw to select samples. Going down in Alvin is an ideal way to gather rock samples. However, this technique is extremely expensive. I think each dive in Alvin costs about $50,000. Also, rocks cannot be collected below certain depths as Alvin can only dive so deep. Since we can’t afford to bring out fancy robots or submersibles, we have our economical metal basket. Even the simple dredge is expensive, though. In addition to the $25,000 a day cost of running the ship, we have to pay for two technicians to operate the dredge and for all of the students (or at least their airfares) helping us with the dredging process and rock description.
The term “dredge” refers to both the metal basket we throw overboard and to the group of rocks collected in a single dredge. In some dredges, we collect 1000 lbs. of rock or more while in others we bring up nothing but mud and a few pieces of calcium carbonate, a type of lithified seafloor sediment. Finding a good place to dredge is an art. Basically, we look for a deep, steep slope along the side of one of the volcanoes we’re studying. We look for steep slopes because these are generally not covered in thick sediments, and also eroded pieces of the volcanoes often fall down and settle at the bottom of these slopes. The dredge basket can break off some weaker pieces of the seafloor, but mostly the dredge basket picks up pieces of rock that are already sitting loose on the seafloor. An ideal place to dredge is the bottom of a landslide scarp. Landslide scarps are difficult to identify based on topography alone, but generally they form a small, steep area surrounded by more gradual slopes. Most places, we only have topography to pick our dredge sites. We have the large-scale topographic maps from satellite altimetry and also the detailed multibeam bathymetry maps we’re making all the time as the ship moves. The multibeam maps are made with sonar– we bounce soundwaves off the seafloor and are able to map an approximately 12 km wide swath of the seafloor as the ship moves. At some sites, we also have seismic data that lets us measure the depth of the sediment. The less sediment, the better when it comes to dredging. Since the volcanoes along the Ninetyeast Ridge are very old with ages randing from 40 to 80 million years, the sediments are quite thick here and dredging is a challenge. That we’ve managed to gather any rocks at all is a testament to the ability of our dredging technicians.
We have had a fair amount of success. Two of our eight dredges brought up nothing but mud and carbonate, but all of the other dredges had at least 50 lbs. of volcanic rock. Two dredges brought up an enormous amount of basalt. One dredge brought up nearly 1000 lbs. of basalt while the other brought up close to 500 lbs. We’re still busy describing these two dredges! To describe the rocks, we first cut 100 or so rocks open using a brick saw that we’ve outfitted with a diamond-coated sawblade. We have to cut the rocks open to view a fresh surface as identification of the rocks from the outside is nearly impossible. Old rocks that have been sitting on the seafloor for a long time are generally discolored and may also be coated in a thick, black layer of manganese that precipitates onto the rocks because this element is saturated in seawater at the bottom of the ocean. This manganese layer can be inches thick, in places. Even when rocks are not coated in manganese, they are difficult to identify because they have been weathered and rounded on the outside. We have brought up a few big boulders in our dredge basket, too. These we first attack with a sledgehammer and then saw open some of the pieces. A few members of the crew have been working hard to impress some of the female scientists with their sledgehammering ability.
After the subset of rocks have been sledgehammered and sawed, these sawed rocks are dried, labeled, weighed, photographed, and described in detail. Before describing the rocks, we put similar-looking rocks together into groups such as basalt, pumice, breccia, sedimentary volcaniclastic, etc. We describe the original lithology, the weathering and alteration processes, the surface characteristics and coatings, the number and size of vesicles, the veins and cracks, and many more aspects of the rocks. After the description forms have been filled out, the rocks are carefully packed in thick plastic bags, labeled a second and third time with tyvek tags, and then put in large, plastic burlap sacks for storage. Before the bulk dredge is packed, a subset of samples is selected for dating and geochemistry. The least-altered basalts with the highest amounts of plagioclase (for dating) are selected and cut into smaller pieces. A couple of pieces are taken for geochemistry and dating, and a very small piece is taken for the making of a thin section, a very thin slab of rock that can be examined under a microscope and used to identify various textures and minerals. This special subset of samples will be hand-carried by the scientists after the cruise. The rest of the rocks, likely 10,000 lbs. or more, will be shipped by sea in a container.
Well, I had better wrap up this email and get back to work. Attached are a couple of photographs. The first (ed: above) is a picture of me with one of the dredge baskets and the second (ed: below) show me with one of the large boulders we brought up in a dredge.