Participant Perspective - July 15, 2001

Interview with Volcanologist Bill Chadwick

Jeff: What are you hoping to find out from the extensometers placed at the South Cleft segment of the Juan de Fuca Ridge?

Bill: It's a long-term monitoring experiment to try and measure actual seafloor spreading events. This has never been done before. Seafloor spreading occurs episodically, once every twenty years or so depending on the spreading rate. All of a sudden it will open up several meters. So, even though the average spreading rate here is 6 cm per year, it's not spreading all the time. Extensometers have a fairly long life time so we're hoping to have them down long enough to catch one of these spreading events. Those data will tell us exactly how spreading is accommodated at the ridge crest. We're also monitoring the hydrothermal vents nearby so we can see the relationship between spreading events and changes in venting activity.

Jeff: How do you retrieve data from these instruments?

Bill: These instruments are designed so their optimal communication distance is fairly short (100-200 meters) because they use high frequency sound that has a limited travel distance. However, the high frequency does give you high resolution distance measurements across the axis. To span the whole axis we needed a lot of instruments in a line. We'll have twelve of them in a row to span the 1.4 km axis. To get the data from these instruments we put an infrared sensor carried by ROPOS next to the extensometer. We then communicate with the instruments without touching them. Last year after we deployed them we went back to the first ones placed and and saw that at least some of the extensometers had ranged successfully. They've been out there for a year and hopefully they've been making a measurement a day, but we won't really know until we get there and see if they're full of data or not. So, we've got our fingers crossed.

Jeff: Could you tell us a little bit about the new NeMO Explorer website you've been working on?

Bill: The original NeMO website has a lot of information. We've been more ambitious with the NeMO Explorer website and put virtual views of the seafloor on the web so that people can see what it really looks like. We've also put a lot more movies and video clips on the site and tried to link it all together. It's pretty neat. I hope people check it out. From the beginning we've tried to gear the NeMO website at least partially teachers and students. I've also been working on some curriculum for teachers based on NeMO. We designed it around a story of a rumbleometer that got stuck in a lava flow so it's pretty interesting. I hope teachers will discover it, use it, and give us some feedback.

Jeff: How is Axial Volcano similar to and different from other mid-ocean ridge eruptions?

Bill: Axial is different in terms of a place or setting than other mid-ocean ridges. It has a higher magma supply because it occurs where a hot spot is superimposed on the ridge. The hot spot gives it an enhanced magma supply. It has a caldera, which most of the ridge doesn't have. It obviously has a centralized magma core that feeds magma down the rift zones. We don't understand how the magma is distributed at most mid-ocean ridge segments, whether magma comes up all along the whole segment or it's local and gets intruded by dikes. Axial is just more robust magmatically and probably has more frequent eruptions and a centralized system instead of a linear system.

Jeff: Has activity slowed since the '98 eruption at Axial?

Bill: Things have cooled down some, but at one particular vent the temperature had been dropping for half the year and then it plateaued out for the rest of the year. The area where the '98 eruption occurred had lots of hydrothermal vents before the eruption so it could be that there's going to be sustained hydrothermal vent activity there after the eruption too. Axial might be losing the extra oomph from the eruption and returning to a more steady-state level of venting. We wouldn't necessarily expect all the vents to lose activity. Most of the vents were still going strong last year.