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A Rare Glimpse Into the Ocean's Crust

Exploring rocks' magnetic signals, a WHOI graduate student reconstructs how the seafloor forms

Source: Oceanus Magazine

About one and a half million years ago, a great hidden piece of the ocean’s crust uplifted and rotated, giving Clare Williams a window and a time machine into Earth’s mysterious mantle.

About halfway between Florida and Africa on a massive seafloor fault, a seismic slippage caused part of the crust’s top layers of basalt lava to slough off. It exposed a raised, domed section of what lies beneath—an intact, mountain-sized piece of lower crust and upper mantle rising from the seafloor. In a monumental shrug, the piece rotated as it was uncovered, so that the layers in the geological layer-cake that is Earth’s crust were laid on edge, oldest to youngest.

The place is called the Kane Megamullion, for a characteristic corrugated appearance that looks like mullions, the slender vertical slats that divide windows. “It’s like a window into the lower crust,” Williams said, “with all the basalt removed by the fault movement.”

An award from the Geological Society of America

For Williams, a graduate student in the MIT/WHOI Joint Program in Oceanography, the fuel that propels her time machine is magnetism. Molten rock rising from the mantle and onto the seafloor cools and solidifies to form new ocean crust, and magnetic material in the rocks aligns with Earth’s magnetic field. But over Earth’s history, the magnetic field has periodically reversed direction, with the north and south magnetic poles changing places. That leaves a magnetic record in the rocks—alternating bands of magnetism, oriented in one direction or the reverse, that chronicle when the lava hardened into rocks.

“We know that lava flows come out onto the seafloor periodically, at different spots, to build up crust,” Williams said, “and you can tell an old flow (which has a lower-intensity magnetic signal) from a new flow.” Magnetism lets her unravel the patterns of lava flows to learn how the crust builds up over time.

But at Kane Megamullion, she can see even deeper into the crust’s structure. Because the basalt—usually about a kilometer thick—is gone, Williams and colleagues could collect a trove of more than 1,500 rock samples of usually covered, rarely available pieces of the upper mantle and lower crust. Their magnetic properties could reveal volumes about their ages and histories.

Williams quickly wrote a proposal to the Geological Society of America (GSA) for funds that would enable her to analyze the magnetic properties of these rock samples. The GSA grants student awards to “provide partial support of master's and doctoral thesis research in the geological sciences.”

She not only got the funding, but on Oct. 24 at the GSA meeting in Philadelphia, she was presented with the GSA’s 2006 ”GSA Geophysicics Student Award” for an “outstanding student research grant project which involves the application of the principals and techniques of geophysics.”

Drawn to marine magnetics

Williams grew up in a seaside town in England and studied geophysical sciences for her bachelor’s degree at the University of Leeds. Like most budding geophysicists, she experienced that “plate tectonics moment,” she said, when they first feel the wonder that Earth’s surface is dynamic—composed of tectonic plates that move and shift, driven by geological forces in Earth’s mantle.

“As a student I really liked math, physics, and geography,” she said. “I was also a keen sailor all through university and represented my university in sailing, so I’m very comfortable with the water and boats. With all that, it wasn’t a stretch to think about ocean geophysics.”

While in college, she spent a year at the University of California, Santa Barbara, and went on a research cruise to the East Pacific Rise and the Galápagos Islands with a Woods Hole Oceanographic Institution (WHOI) science team led by Dan Fornari, current director of the Deep Ocean Exploration Institute at WHOI. The cruise was one of WHOI’s “Dive and Discover” series of online expeditions for students and educators, and Williams was profiled on the expedition website.

She loved being at sea, loved the research experience, and loved the work, especially seeing images of the seafloor taken with the WHOI towed camera systems (DSL-120 and Argo II). As soon as she returned home, she applied for oceanographic graduate programs in the United States. She is now finishing her Ph.D. degree in the Geology and Geophysics Department with her advisor, WHOI marine magneticist Maurice Tivey.

In her research, Williams used magnetometers—instruments that measure the strength of the magnetic field—at the sea surface and near the seafloor, towing them from ships and from both tethered and autonomous robotic vehicles (Jason and ABE, the Autonomous Benthic Explorer). The magnetometers recorded the varying magnetic signal from the lower crust and upper mantle rocks below.

As a second source of information, she also collected seafloor samples and analyzed their magnetic signals. The analysis is not straightforward: Rocks’ magnetic signals decay over time, as the rocks are “weathered,” but rock can also become chemically or thermally altered in ways that increase the magnetic signal.

The unique features at Kane Megamullion gave Williams a rare and important opportunity to untangle the sources of the crust’s magnetic signal. “It’s a great location to figure out how Earth’s changing magnetic field affects the lower crust and upper mantle,” she said, “and how much these underlying rocks contribute to the total magnetic signal of the crust.”

Williams' research is funded by the National Science Foundation and the Geological Society of America.

Originally published: November 6, 2006