GEOFORM 2009
8Oct/096

On to Farallon Basin

With all this dredging, I'm becoming familiar with the deep rumbling of the bow thruster and the sudden jerks of the ship when there are big pulls on the wire. After all, we are currently transiting to our 13th dredge target. There are just four full days of dredging left, but at this rate we should be able to get a lot more done.

Conditions have varied somewhat since we entered the Gulf, but last night the sea looked as calm as a lake, with the moonlight reflecting off the water. Only slightly marring the weather were visits by numerous seabirds, who must see the New Horizon as an ideal place to rest. This proclivity is unfortunate for the crew, whom I overheard musing about the benefits of working on deck in ponchos.

3D image of Farallon spreading center

3D image of Farallon spreading center

Today we entered the Farallon Basin (check ship track for exact location) in the southwestern Gulf. The Farallon Basin has a spreading axis at its center, so the seafloor here is oceanic crust, transitioning into thin, rifted continental crust at the basin's edges. Spreading centers in the Gulf, like this one in Farallon, are different from more well known spreading axes like the Mid-Atlantic Ridge (MAR) and East Pacific Rise (EPR), since they form as deep rift valleys rather than bathymetric highs. Some of these rift valleys are over 3,000 meters deep! One difference is that the Gulf spreading centers are so close to land. Unlike the MAR or EPR, these spreading centers are inundated by massive amounts of sediment that pour into the Gulf. One familiar source of terrigenous input to the Gulf is the Colorado River, which empties into the northern Gulf (though no longer consistently, as a result of overuse for irrigation). Further south, especially in the eastern Gulf, the Mexican Sierra Madre Occidental mountains are a more important source of sediment.

The second major source of sediment is pelagic rain from highly productive surface waters. This "rain" is actually organic debris from biota. High surface water productivity is a result of strong wind-driven upwelling, which causes phytoplankton blooms. Diatoms, or microscopic algae, are one of the major components of these blooms. They are also one of the most important photosynthesizers in the ocean. When conditions are right, diatoms can reproduce extremely quickly. Upwelling provides these conditions by replenishing nutrient-depleted, warm surface waters with deeper waters that are filled with important nutrients like nitrate and phosphate from the decomposition of organic matter. Diatoms form a test (or outer shell) made of silica, so sediments in the Gulf are silica-rich.

All this sediment is important because it influences the way new ocean crust forms. Out on the MAR or EPR, underwater eruptions bring basalts into contact with cold water, causing the formation of pillow lavas. These distinctive shapes form as the outer surface of the lava cools and hardens into a roundish pillow shape, at which point hotter lava breaks through and builds another pillow on top. My advisor, Dick Norris, has a fantastic visual aid for demonstrating pillow lavas, which I highly recommend should you have an extra bag of marshmallows handy while camping. After squishing up the marshmallows into one mass (handily achieved by sitting on the bag), simply throw the gooey ball onto a campfire. Next, watch as the top of the marshmallow ball burns; then, flows of heated marshmallow break the surface and begin to form pillows. Anyway, those layers of sediment on the Gulf seafloor prevent the formation of pillow lavas. Magma can't extrude onto the seafloor surface; rather, it injects vertically into the sediments as dikes, then spreads out horizontally as sills. I'll get into sills more later, since this is a favorite topic of Jared's.

Rock sorting table in the lab

Rock sorting table in the lab

Our most recent dredge was over a seamount in the Farallon basin that looked curiously like a sombrero. Our next target will be another seamount, 1,900 meters deep and surrounded by a moat-like ring. Our rock collection is growing rapidly!

Comments (6) Trackbacks (59)
  1. Period 3,
    The food is amazing–we have a great cook! We get three meals a day, with lots of variety, and there’s always plenty of desert. We’ve had Mexican food, ribs, pizza, hamburgers, stir-fry, and more.

    In the Gulf, it’s almost like being in a calm lake. There is very little swell, so the boat ride is nice. On the transit down, due to strong winds and the CA current, there was a larger swell, so the ride wasn’t quite as comfortable. The R/V New Horizon is a smaller ship, so it moves quite a bit.

    I always enjoy being out at sea, and we’re getting good data, so I’m having a great time. I wouldn’t be doing this research if I didn’t think it was fun!

    No one fishes off these ships, or goes swimming, but we have seen some interesting marine life. Today, we think we saw a giant purple eel off the side of the ship.

    The total cruise is 11 days, so we have about 3 1/2 days remaining. There are 24 people on the ship, including both the ship’s crew and the scientific crew.

    Finally, we’re collecting rocks in order to figure out the composition of the seafloor and to understand the history of the Gulf’s formation.

  2. Kylee,
    At spreading centers we generally find volcanic igneous rocks, whereas at subduction zones we also find metamorphic rocks. Most of the rocks we find are igneous rocks. On this cruise, we’ve gotten rocks from 2,500 meters (around 1.5 miles deep). We choose our dredge sites based on bathymetric and sonar data we’ve collected previously (this data shows us the shape and structure of the seafloor and below). Most of the seafloor is muddy, so without this data it would be hard to find rocks. Every dredge target is still a bit of a mystery, so we’re always just hoping to bring up rocks in the dredge. We’ve already collected roughly 1,000 pounds of rocks during this cruise. My favorite site so far was our 14th dredge and the 3.5 kHz sonar survey we did right after, since both were a huge success. I’d have to say my favorite mineral is quartz, since it can crystallize in multiple forms, like jasper, onyx, or chert. Thanks for the questions!

  3. MMHS P3: How is the food? What is the boatride like? Are you having fun?
    P4: Does anybody do any fishing? See any sharks? See any whales? Do you really work 24 hours? Do any fish get caught in the dredge? Anybody go swimming?
    P5: How long is your science trip? Why are you collecting rocks? How many people are on the ship?

  4. Jared, I am currently taking a geology lab and we just did our section over igneous, metamorphic, and sedimentary rocks so I know a good basis over them. When your on this cruise do you find more igneous or metamorphic rocks? Do you have a favorite mineral? What is the deepest you have ever recieved a mineral from or deepest you have gone to get a rock? When your looking for rocks I always wondered, how do you know where to look for the rocks since the ocean is so big; is it hard to find them? When you go to certain locations do you have certain things your looking for or just in hope to find minerals? When your on this trip how many rocks do you think sample? Last question would be where has been your favorite place so far on your trip?

  5. 1) Technically I’ve never been seasick. I guess this is a sliding scale, because I haven’t always felt 100% out at sea. It really depends on the size of the ship you’re on. The bigger the ship, the less it moves, the less likely you are to get sick. 2) I get paid the same amount at sea as I do on land (which isn’t much)! 3) We are finding all kinds of rocks that are associated with two major plate boundaries–a subduction zone and a spreading center. Rocks include volcanic and intrusive igneous rocks as well as metamorphic rocks.

  6. Period1 MMHS 1) Jared, do you get seasick? 2) How much do you get paid?! 3) What type of rocks are you finding?


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