(or ) The Fresh Cut Surface of Beautiful Ocean Rock
These photographs show some of the dredges in which we obtained rocks from the seafloor. The photograph below shows a first look at our first big dredge of basalt.Generally, once we have an initial look at the rocks out on deck, we follow a series of steps to carefully classify, describe, and document the samples. I want to share that process with you now in words and in photographs.
I will describe what we do with the rocks after we bring them up from the seafloor. Basically, after the dredge basket is brought up, we pull out all of the rocks and lay them out on the fantail of the ship near the hanger where we keep the rock saws. In the photo below, Sarah, Holly and I are sorting a large dredge. We were still north of the equator and the weather was great that day!
After some initial sorting, we have to cut all of the rocks open with brick saws in order to identify and describe them.
Large rocks that won't fit in the saw are first photographed and then smashed open with a sledgehammer. Then, we take one of the rock fragments and saw it open to expose a fresh surface. The surfaces of the rocks are weathered, altered, and covered in black manganese, so we can't see the mineralogy of the rocks unless we cut them open. To some degree, we can identify rocks before we saw them open. We generally can tell which rocks are sedimentary and which are igneous based on the shape, color, and density of the samples. However, to identify a rock in detail we must expose a fresh surface.
Check out this boulder. Ron, our dredge expert, is awesome at finding rocks. Later we had to break this rock open, but not before Ron tried to pick it up!
Looking at the fresh surfaces of the sawed rocks is always exciting. We will not identify the rocks definitively for several hours to several days, depending on the dredge. However, we can gain a sense of what the rocks our in those initial few minutes after the dredge is put on deck, and we start up the saws. Because we often complete several dredges in a row, we generally start sawing the rocks open within minutes of pulling them out of the dredge basket and laying them out on the ship's fantail.
I like to use the rock saw... maybe because of the stylish apron and ear protection you get to wear!
Even without the time constraints of dredging, however, I think we would saw the rocks open right away. I know I would. I am always excited to see what we have brought up from the deep. Sawing the rocks open and exposing their fresh surfaces becomes a sort of quest, for me.
I saw and saw, eagerly looking for a fresh plagioclase crystal or a highly-deformed breccia or an alteration rim that has an unusual color. I saw through the samples in a fury, pausing now and then when I think I may have opened up something beautiful, unusual, or scientifically valuable. After a few moments pause, however, I return to sawing. I keep sawing because I never know what might be hidden under a thick manganese crust or inside a weathered cobble.
My curiosity compels me, like an addiction.
Fortunately, for all but the largest dredges we saw every single rock open, anyway, and I am not the only geologist on board who is addicted to the rocks. After we cut the rocks, we wash them first in salt water and then in fresh water. We then bring the rocks into one of the labs and lay them out on brown paper to dry.
A few hours later, we look at the cut rocks carefully. Before describing the rocks or even deciding on their lithology, we first just look all of the samples and put similar-looking rocks together. Sorting the rocks can be an exhausting process. Sometimes when I am sorting rocks, I felt as if I was participating in the "what does not belong" skit on Sesame Street. In the Sesame Street skit, a group of objects such as an orange, an apple, a pear, and a beach ball appear on the screen. Then, a pleasant, child-friendly voice asks what doesn't belong.Rock sorting involves separating breccias from basalts and carbonates from volcaniclastics instead of separating beach balls from pieces of fruit, but the basic principle is the same. You have to think about why you're putting rocks together and decide which rocks to exclude.
Sometimes, the groups are not obvious or intuitive. With the Sesame Street sorting, one could easily exclude the pear because it isn't a sphere and group the beach ball with the apple and orange. Clearly, grouping the fruit together makes more sense, but this is not always intuitive to a three-year-old programmed to see shapes. In a similar way, sorting rocks is not always intuitive, especially when we find a new type of rock we haven't seen before in our dredges or when the rocks are very altered.
Often, we go through several rounds of grouping before deciding on the best way to sort the samples.
People have different opinions and sometimes fights break out. These are not fist fights (yet), just heated verbal battles that involve much passing back and forth of samples and hand lenses and extended consultation of the petrology textbook. Pens and other small objects have been thrown at people in the heat of the grouping process, but eventually we all agree on groups for the rocks. These groups have titles such as "palagonite (altered glass) breccia" and "highly plagioclase-phyric, massive basalt" and "lithified calcareous ooze (chalk)."
We group the rocks for several reasons. First, this makes our later description much easier as we can describe one sample and then basically draw arrows down the columns of the description sheets for the following samples. Also, it is important to have some quantitative idea of how much of each rock type we obtain in a dredge. If a certain rock type is rare at one location but common at another, this information can potentially lead to important geological insights about the ridge. We also group the samples because sometimes we don't have any idea what a sample is. Often, describing several rocks together is easier than describing individual rocks. This is because some samples may be more altered than others or may have important contacts that can help us identify the lithology.
If I don't know what a rock is, I try to find another rock that looks similar that I maybe can identify.
Next, we describe all the rocks in detail. If there are too many rocks in the dredge to describe them all, we pick a subset of 50-60 samples to describe. We first try to identify the lithology of the rock, which is easy to do for some samples but can be very challenging for the more altered and weathered samples. We guess if necessary. Next, we describe the sample's mineral content, degree of alteration, any veins or cracks, any deformation, the overall shape of the rock, and the surface characteristics such as alteration rims and manganese crust. To help our identification, we often wet the surfaces of the rocks as it is easier to see the crystals this way. We use small hand lenses and also the two microscopes we brought with us to the ship to help us see the crystals. We have a small collection of mineralogy and petrology books we can consult if we are uncertain about a vein, mineral, or other feature of one of the rocks. For some of the samples, we need to make thin sections (thin slabs of rock you can look at under the microscope) or do chemical analysis in order to identify the mineralogy with certainty. We cannot make thin sections or do chemistry out here at sea, so we just have the hand samples. Here on the ship, we just do the best we can with identification.
We'll do more thorough identification later on when we have access to more resources.
After the rocks have been fully described, we select a subsample of rocks which are good for geochemistry, dating, or both. We put this subsample aside then pack up the rest of the dredge. Before packing the rocks, we photograph them. The rocks we describe we photograph individually, and we also take group photographs of the dredge laid out on the tables.
Every rock sample is different, and many are beautiful in their own ways. Some of the samples are very colorful because of trace elements or alteration. Some of the basalts have weathered green, orange-red, and purple. The breccias can be pink, green, and multicolored. We had two breccias in one dredge that we dubbed "candy rocks" since they contain clasts of all colors: green, blue, red, orange, purple, black, white, and so forth. However, often the plain or ugly samples are the most important ones, scientifically. A plain gray basalt is excellent for geochemistry but is not particularly photogenic. These plain gray basalts have stories to tell, we hope. With a little coaxing and perhaps the help of some acid and a mass spectrometer, these rocks may tell us the age of the seafloor and also the story of these volcanoes: how they grew and evolved.
Labeling and packing the dredge is done very carefully.
Unless we do this, our rock samples are useless if we don't know exactly where they came from. First, we paint a small area of the sample with white latex paint. Once the paint dries, we use a sharpie marker to write the sample number on the rock. Some of the softer rocks, such as the carbonates, are not painted. We never paint the rocks on which we plan to do chemical analysis since paint and ink contain high amounts of trace elements that can affect our analysis. We are careful not to mix up these samples, though, and each sample also is given two tyvek tags. Tyvek is a wonderful material that resists tearing and is water-resistant. We write on the tyvek in permanent marker. Each sample goes in its own bag, and we place one tyvek tag inside the bag. The other tag is taped or attached with wire to the outside of the sample bag. Then, many sample bags are placed in a large, woven plastic bag. These large bags, which weigh a hundred pounds or so apiece, are placed in large metal baskets. We have four of these giant baskets, and they can hold up to 4,000 lbs. of rock. At the end of our trip, we'll use a pallet jack to move the baskets of rocks out of the lab and a crane to move them off the ship. The rock baskets will be sent by container ship from Singapore to New York and will arrive in the states about a month later.
Okay, I had better get back to work now. I've taken too long a break from my rock group descriptions already, and we've had three successful dredges in a row the last two days. We have about 700 lbs. of rocks out on the tables now to describe and put away!
Evy
Updated July 26, 2007 from the Indian Ocean
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