Q: How do you detect undersea volcanic eruptions?
Kelley: We’re actually really lucky here in Seattle, because we’ve got an array — called a SOSA array. It’s a hydro-phone array the Navy used to use to look for Russian submarines off the Pacific Northwest coast. In 1993 the Navy started letting NOAA [the National Oceanic and Atmospheric Administration] review the data from the array. And it turns out the listening devices in this array also pick up earthquakes. So, since eruptions on the sea floor produce a very characteristic pattern of earthquake, we locate the eruptions through their signature earthquakes.
Q: And these earthquakes occur in places where the tectonic plates that form the ocean floor are spreading apart?
Kelley: Yes. The holding tanks of submarine volcanoes — where the melt is stored — are called magma chambers. The reason submarine volcanoes — or any volcano — erupts is, the gasses in the melt expand. And periodically melt upwells in those areas where the plates are spreading apart. And if enough melt collects, there’s a volcanic eruption, and that forms the top crust of the sea floor.
Q: I read that one kind of eruption on the ocean floor involves hydrothermal vents, these black chimneys, called “black smokers.” How does that happen?
Kelley: The heat given off from the magma chamber melt below an underwater volcano can drive the circulation of fluids beneath the seafloor. When the hot fluids exit the seafloor they may form a vent.
Another way vents form is through the residual heat in the rocks below, from the melt cooling and crystallizing. So as the plates spread apart, seawater penetrates down to depths of around two to eight kilometers, where it is warmed up by the cooling melt. And this creates a series of convection cells. The fluids in those convection cells eventually vent onto the sea floor. Those fluids are 400 degrees Celsius or so, so when they hit the two-degree seawater, all of their metals precipitate out. When you see black smoke in the water column above a vent, you’re seeing metals precipitating out of those venting fluids. But not all the metals go into that plume of black smoke. Some form the chimney itself.
Q: What do we know about these hydrothermal vent systems?
Kelley: We have a pretty good general idea of how the systems work, but if you start looking at the details, we don’t know very much (laughs). We know why volcanoes erupt. We know how often you might expect an eruption on the sea floor. We’ve actually never seen an eruption on the floor. Even though we’ve been going out there for — what, 100 years or more? — we’ve never ever yet been out there right during an eruption.
Q: At the end of 2000 you found a type of undersea hydrothermal vent that no one had ever seen before. What was that?
Kelley: We found a new kind of hydrothermal vent that we believe forms from circulation of fluids in deep rocks, called peridotites, that underlie submarine magma chambers. In certain places in the Mid-Atlantic ridge, there can be 50,000 to 100,000 years between volcanic events. But the plates are always spreading apart. So when they do finally rupture, very large faults form, through processes that we don’t fully understand. But those faults end up stripping off the top, say, eight kilometers of rock on the ocean floor. That exposes very deep underlying rocks that have a very different chemistry, and that are chemically unstable on the sea floor. The interaction of seawater with the unstable peridotites generates heat. So it’s a very different process than the generation of heat from cooling of a magma chamber. This is a chemical reaction that’s generating the heat.
Q: So the heat comes from a chemical reaction between seawater and these unique rocks that usually reside at such great depth?
Q: And when the heat from this reaction drives fluid circulation, the fluids venting on the seafloor form a different type of structure?
Kelley: Yes. Because the peridotite rocks have a unique chemistry, the warm fluids that interact with them also have a different chemistry than typical black smokers. When the warm fluids mix with seawater, calcium carbonate crystallizes out of the seawater and forms limestone. So these towers that we discovered are basically made from the same elements that form limestone, the limestone formations you see in caves.
Q: And what do they look like?
Kelley: They are very steep-sided pinnacles rising from the ocean floor. They look like the skyscrapers of an underwater limestone city. We called it “the Lost City.” One was 180 feet tall. That’s the largest hydrothermal vent structure ever found. Previously, the tallest known hydrothermal vent was a black smoker called “Godzilla.” That one was forty-five to fifty meters tall (about 150 feet). So these things can get quite large.
Q: How did you happen to find the Lost City hydrothermal vents?
Kelley: Well, one characteristic feature of the mid-Atlantic ridge is these very large mountains, sort of the size of Mt. Rainier, about 14,000 feet tall. And they form through this faulting activity that we don’t quite understand. Our group was out there trying to figure out how these mountains form.
During the days we would go down in a three-person submarine, called Alvin. At night we would do camera surveys with a deep-sea camera system connected to the ship by a fiber-optic cable. At night we took remote video and electronic still images. And one night we just happened to come across —
Q: These structures?
Kelley: Yes, one of these. I was actually in my cabin at the time, and my friend Gretchen came running in saying, “I think we found something here that doesn’t look like anything we’ve seen before.” And then there was just a lot of excitement and craziness all that night on the ship. We basically spent all that night, Jeff Karson and I and the whole group, looking at these vents.
Q: Had hydrothermal vents of this type ever been discovered before?
Kelley: No. We knew of some carbonate chimneys on the margin of the U.S., but nothing like this. Nothing in this kind of environment, or this tall.
Q: How long did it take them to form?
Kelley: We don’t know. We don’t even know how long black smokers take to form. Unlike black smokers, which are hosted on the youngest and hottest rocks on the planet, these carbonate vents are on rocks that are a million to a million and a half years old. But we don’t have a good idea how old the carbonate chimneys are.
Q: What’s it like going down in Alvin the submarine to investigate the underwater mountain.
Kelley: It’s always exciting. I would drop anything and jump in whenever I could. You start off in this really beautiful light blue water. Then as you drop down, it’s almost like a sci-fi movie, like you’re going through the stars. You see these little organisms and chains and weird shapes floating by the window as the submarine descends. They’re bioluminescent — they glow in the dark. Then about 300 feet off the bottom they turn the lights on outside the sub. It’s a very different experience.
Q: Is the submarine uncomfortable?
Kelley: Kind of. It’s a small titanium sphere, about six feet across. The whole back end is filled with electronic gear. The pilot looks out the front view port, and there’s two side view ports, for the scientists, which are a few inches off the floor. So you’re in a fetal position for eight hours or so. All the light are off.
Q: Do you at least get a pad to sit on?
Kelley: There’s a little black pad to sit on. You drop down through the water and you’re sitting in a metal ball, right? In the Atlantic Ridge the seawater might be almost freezing by the time you get to the bottom. So there’s a lot of condensation and it’s kind of damp in there. And it’s incredibly busy. One analogy is if you crammed three people in the front of a Volkswagen Bug and went up to the Cascades at night without a map, and just drove around with a flashlight and tried to figure out where you were — that’s kind of what it’s like. In some places, where the topography is very steep, you don’t have very good navigation, you’re kind of feeling your way around. The visibility is maybe thirty feet.
Q: You can see out of this thing?
Kelley: Yes, there are lights on the front of the sub.
Q: And it has windows?
Kelley: It has viewports of maybe six to eight inches across. You see out. The scientists on the side can’t see out front, and the pilot, in front, can’t see out the side. So there’s lots of talking. You know, “What do you see, what do you see?” There are probably eight cameras going continuously. So we’re changing tapes out, taking pictures. You’re continually talking into a microphone to take notes, trying to figure where you are and describing what you are seeing. Particularly in a new area — like where we discovered the Lost City, and later went down in person — there’s a lot of anxiousness and excitement. We knew we only had one dive to see the Lost City in person, that was all we were going to get. It’s just an absolutely intense time. Surprisingly, though, almost everyone I know falls asleep on the way up. You just curl up in a blanket (laughs).
Q: Because the excitement is so exhausting?
Kelley: Yes! You don’t realize it at the time. If anyone had ever told me I would fall asleep inside the submarine, but yeah… and then as soon as you get done, it’s like, “Oh, I have to get back down again!”
Q: Can you hear any sounds?
Kelley: There’s a lot of sound inside the submarine. There are a whole bunch of sonar systems, so it’s kind of like the old fashioned war movies, you know, where you hear these echoes, this pinging going on.
Q: I was going to ask if it compares to those scenes in “Das Boot.”
Kelley: It’s certainly not the intensity of that (laughs). We’re not worried about getting bombed or anything. But it is intense in a different way, you know, in the very best way of doing science. That’s the intensity that you have down there.