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Hydrothermal Vent Formation

 

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Research vessel Atlantis.
Photo © University of Washington, American Museum of Natural History, and Pennsylvania State University.


Greetings from aboard the Atlantis on a chilly day in June. It was a cold morning, so I had to bundle up in my fleece jacket and pants. Mornings are always chillyand so are the damp nights! Last night I stayed up late working in the labs; just like everyone else, I had on the standard fleece vest and a long-sleeved shirt. Things usually warm up by afternoon and we can switch to short sleeves. But if the sea gets rough, we have to be careful on the lower deck; there's some pretty cold spray coming off the water!

 I've been thinking about temperatures all day because I've been (very carefully) preparing temperature probes for deployment on tomorrow's dive. The researchers will use the thermometers I prepare to study minute temperature variations around the hydrothermal vent site, so there's no room for mistakes! You see, water comes out of the chimneys' hydrothermal vents (hydro = water, thermal = hot), but it quickly mixes with the surrounding ocean water, which is only a few degrees above freezing. The thermometers I prepare will help monitor the currents that pull the hot chimney water into the cold ocean; in other words, they'll measure how fast it is cooled off and mixed. This information will be used in the study of life at the vents; it's also used to monitor changes in the effluentin other words, to examine, over time, the chemistry of the mineral-rich waters that spew from the chimneys and mix with the cold ocean water.

To prepare the thermometers, I have to make sure that each one has exactly the right amount of lead weight added to it. The thermometers have to stand upright, with the lead balls anchoring the bottom end of the thermometer to the sea floor. The thermometer setup consists of pieces of PVC pipe with a weight attached to the bottom; the actual recording instruments slide inside the pipe. I spent the morning cutting pieces of lead, taping them onto the PVC, and then putting the pipe into a tube to check that it would stand upright properly. I had to add or remove lead as needed to keep the thing upright.

I don't know if I'll get picked to go to the bottom, but at least the thermometers I prepare will get there. The ALVIN will position the probes at a specific deep sea vent site; the thermometers will spend the year recording deep ocean temperatures at the vents until they are retrieved next year. So I'll be back in Pennsylvania, teaching high school science and talking about my time aboard the Atlantis (and, hopefully, the ALVIN), while my thermometers remain perched on the vents, measuring away. Hopefully there won't be any major volcanic eruptions or earthquakes to destroy the vents and bury my thermometers.

My monkey's fists won't stay down all year, but they'll get to go down on a few dives. No, I'm not talking about my own fists, but the seaman's knots I had braided onto the equipment baskets full of materials to be deployed at the bottom of the ocean. At the vents, the ALVIN's mechanical arm sets each of the baskets down in place on the floor of the site. To do this, it picks up the baskets by grabbing the monkey's fists. Well, "it" doesn't pick them up; the pilots do! I've heard that they are so good at their work that the arm seems to be just an extension of the pilot's own arm outside the sub. They use the arm also to pull on bungee chords inside the equipment basket. The bungee cords spring out, pulling with them the temperature probes from the baskets. Cool beans!

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An active black smoker venting "smoke" (hot water with tiny minerals suspended in it), taken by observers in the submersible ALVIN on the seafloor surrounding the Main Endeavor Hydrothermal Vent Field on the Endeavor Segment of the midocean ridge at 47°57' N & 129°6' W.
Photo © J. R. Delaney, University of Washington.


It's incredible to think of the dynamic forces creating volcanic mountains, new sea floor, large cracks or fissures, deep sea vents, and even the rocks that my monkey's fists will help the researchers collect. But it's important to remember that deep sea vents aren't volcanoes. Don't picture each deep sea vent as a little volcano! They are submarine hot springs, spewing out mineral-rich hot water; they sit on the flanks of underwater volcanoes, but they are not volcanoes. While they're not the same thing, they are related; the hot water that spews from hydrothermal vents is heated by the same magma that forms submarine volcanoes.

You see, as mid-ocean ridges form, the sea floor spreads away from the ridge; as it cools and solidifies, cracks and fissures form along its surface. Cold sea water percolates down into many, many tiny cracks, sinking by the force of gravity. Cold water that sinks down far enough gets close to molten rock (or magma) near the base of the oceanic crust. The water gets heated. It also reacts chemically with the rock through which it is flowing. The hotter the water gets, the more minerals it can dissolve,so as it heats up, it is also becoming more and more rich in minerals. Eventually, the mineral-rich seawater, or hydrothermal fluid, gets so hot that it becomes buoyant; in other words, it "wants" to rise back up through the rocks of the oceanic crust, and it "looks" for a channel to get back up. Although cold seawater percolates down through many cracks, the hydrothermal fluid flows up through a much smaller number of channels, creating the submarine hot springs that dot the flanks of mid-ocean ridges.

When the incredibly hot fluid spews out of the hydrothermal vent into the frigid waters of the deep ocean, it mixes rapidly with the cold water and its temperature drops. Remember what I told you about hot fluids being able to dissolve more minerals? Well, that fluid isn't hot anymore! Suddenly cooled, the fluid can no longer keep all the chemicals and minerals it was carrying in solution. Those materials precipitate out of the vent fluid as it mixes with the sea water, forming tiny mineral grains. That's why deep sea vents seem to spew black smoke; the dark color results from minerals precipitating out of the vent fluid.

So volcanic activity on the sea floor forms the mid-ocean ridges and, together with plate spreading, creates new ocean floor. As part of the bargain, we get these amazing deep sea hydrothermal ventswhat a bonus! The heat that drives mantle convection, which in turn causes ocean floor spreading and melts rocks, is the very same heat that transforms seawater into hydrothermal fluid in the oceanic crust, which in turn eventually spews out of the vents. This heat that drives both mantle convection and hydrothermal vent formation is also one of the reasons that the vents themselves don't last very long. Dynamic forces create constant change at the sea floor. With just little shifts in seismic activity, underlying molten rockand thus the heat sourcegets cut off from an existing vent. At that point, all the living organisms in the vent community die; they rely on the heat source and the chemical composition of vent-heated seawater. On the other hand, if volcanic activity shifts and the area gets too hot, everything gets cookedand possibly buried! Hopefully my thermometers won't suffer that fate!

Last night we watched video from yesterday's ALVIN dive through the vent fields; everywhere we looked, there were plumes of hot vent fluid spewing from the vents like smoke. I had to remind myself again about the really tiny mineral particles precipitating out of the fluid as it cools; these minerals had also built up in deposits to create the chimney-like structures we could see on the video. In one sense, the same "smoke" that came out of the black smokers was also helping to build them higher! Sometimes the researchers collect pieces of the chimneys in order to learn about the chemistry of hydrothermal fluid before it mixes with seawater. No one is collecting chimney samples on this mission, but in the summer of l998, scientists actually brought four huge pieces of these chimneys all the way to the surface so they could study them better. Want to see them? You can actually get a look at them at the American Museum of Natural History's Hall of Planet Earth. Or check them out virtually on the museum's Web site.

Or, if you're one lucky REVEL teacher, go down in ALVIN and see them for yourself! I hope that's me, and by tomorrow, I'll find out for sure!

Pat

Deep Sea Vents was developed and produced by the Education Department of the American Museum of Natural History in collaboration with the REVEL program of the University of Washington. Deep sea vents image courtesy of Dr. Michael Perfit, University of Florida, and NOAA VENTS program.
Deep Sea Vents materials developed with the support of the National Aeronautics and Space Administration.
© 2002 American Museum of Natural History. All rights reserved.

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