Alum Uses Remote Sensing to Make Sense of Ocean Basins

Tuesday, October 25, 2011
At right: Will Sager on the R/V Marcus G. Langseth research ship with Honolulu in the background, just before heading out to Shatsky Rise in the Pacific, July 2010.
When Will Sager was majoring in physics at Duke, he wanted to go into astronomy, but an intro geology course opened up a new vista. “I took Geo 101 at Duke and it was really interesting,” Sager says. “The professor said to me, ‘You ought to consider geophysics,’ and I said, ‘What’s that?’” Today, Sager is a professor in the departments of oceanography and geology and geophysics at Texas A&M, where he also holds the Jane and R. Ken Williams ’45 Chair in Ocean Drilling Science, Technology and Education. He says, “I’m a geologist who uses physics principles to explore the structure and history of ocean basins.” After graduating from Duke in 1976, Sager went to the University of Hawaii to study marine geophysics. “Before too long, I was out on a research ship,” he says, “and I thought it was really cool—collecting data in blank spots in the ocean, exploring places people have never been.”
He’s now been on more than 40 scientific cruises, most recently a trip to the middle of the Pacific Ocean in the summer of 2010. Sager and his colleagues used sound waves to map the structure of Shatsky Rise, an underwater mountain range the size of Arkansas. Air guns on the ship produce sound waves that travel through the crust and reflect back to the ship, where they are picked up by 6-kilometer-long “streamers” of hydrophones trailing in the water behind the ship. “Sound waves allow us to make an image of what’s below the surface,” he says. “It’s like an X-ray of the crust beneath the ocean. We can see sedimentary layers on top, and packages of lava flows under that.” Instruments on the ship also continually measure gravity and magnetic properties of the crust. Gravity data gives information about the density of the rock layers, and magnetic data helps locate past locations of mid-ocean ridges, which are plate boundaries where new oceanic crust is churned out. The image that Sager has put together using the data collected on the trip indicates that Shatsky Rise is formed of volcanic rock that erupted over a relatively short period of time from a central source—a so-called “supervolcano.” This fits with the hypothesis that the mountain represents a mantle plume—a huge “bubble” of molten rock that rises through the mantle and erupts on top of the Earth’s crust. The magnetic data indicates that Shatsky Rise formed at the junction of three ancient mid-ocean ridges. Sager is trying to piece together how mantle plumes fit in with the theory of plate tectonics, and whether Shatsky Rise is related to those nearby plate boundaries. “We thought we understood this when I was a graduate student, but then we learned we didn’t,” he says. “Things look simple when you don’t have much data, but when you look closely, it’s more complicated.” Shatsky Rise could be an ancient analog to Iceland, which is thought to represent a mantle plume on the mid-Atlantic ridge. “ Or,” says Sager, “it may be another thing entirely.” Although there’s a way in which studying the ocean bottom with remote sensing is like studying far-off planets, one big difference is that Sager can occasionally get his hands on samples of the oceanic crust he is studying. In 2009, he traveled to Shatsky Rise on a drill ship that collected rock samples. These samples allow him to corroborate the data he collects from remote sensing. It’s not easy studying such inaccessible parts of the planet. Shatsky Rise is about 3,000 miles west of Hawaii; just to get there took nine days of cruising. And it’s certainly not a pleasure cruise. “When we go out, we’ve spent years getting ready,” Sager says, “We work 24 hours a day, seven days a week. If I’m up, I’m on duty. It’s intellectually stimulating, but it’s pretty tiring.” Because of a convergence of funding, Sager has three cruises scheduled for the next six months. In February, he will fly to Cape Town for a seven-week cruise to investigate a volcanic chain in the South Atlantic. He says, “Then I have to figure out how to get from Cape Town to Yokohama in 24 hours [for the next cruise to Shatsky Rise].” It may seem a long way from the Duke Physics building to a scientific research ship in the middle of an ocean, but Sager says when he first became interested in the stars, “I was told, ‘Get a good background in physics and you’ll be set for whatever you want to do in science.’”