Attracted to Magnetics
A conversation with WHOI geologist Maurice Tivey
Maurice Tivey has probably endured more than a few bad puns, like the one in our headline, after he tells people what he does for a living. A geologist at Woods Hole Oceanographic Institution (WHOI), Tivey specializes in the magnetic properties of rocks.
When magma solidifies, magnetic crystals form in rocks and become magnetized in the direction of Earth’s prevailing magnetic field. Over our planet’s 4.5-billion-year history, the field has flip-flopped many times, with magnetic north and south reversing direction. These magnetic polarity reversals have been well-documented, and so by analyzing rocks’ magnetic properties, Tivey can date them and chronicle the geological processes in which they formed.
English by birth (his given name is pronounced “Morris”), Tivey has focused on rocks atop the seafloor, which form when magma from deep in Earth’s crust and mantle rises to the surface, spills out, and cools to create new volcanic ocean crust. But he has been involved in a long-term project to drill a hole through the seafloor at a place called Atlantis Bank in the Indian Ocean, where scientists hope to retrieve previously inaccessible rocks from deeper levels of the crust and at the Moho, the still-mysterious boundary between crust and mantle. These rare samples could help unveil the geophysical and geochemical processes that are happening there.
Oceanus: Tell us about Atlantis Bank.
Tivey: It’s a very interesting place. Instead of having to drill through all the volcanic lavas that are usually on top of the seafloor, the upper crust is removed on Atlantis Bank, and so you can get directly into the lower crust and eventually, we hope, to the Moho. The Moho is one objective, but the road to the Moho is important, too. Upper ocean crust is formed by lava that spills onto the seafloor. It is quenched quickly by cold seawater and its magnetic properties are locked in fast. But the lower crust is closer to the hot magma chamber and cools slowly, and so it may be delayed in magnetizing. There is a magnetic reversal recorded in the rocks within the lower crust at Atlantis Bank. We want to understand how the lower crust becomes magnetized and how a magnetic reversal is recorded by these rocks.
Oceanus: What is your theory?
Tivey: Magnetism does not exist above a temperature called the Curie point, 580 degrees Celsius for ocean crust. As magma cools down, the solidifying rock eventually passes through the Curie point and starts to become magnetized in the direction of Earth’s magnetic field. We think that this cooling happens slowly and over a wide expanse in the lower crust. I doubt that there’s a sharp magnetic boundary. We think there’s going to be some sort of transitional zone, a wide cooling front that could be ten meters, or one hundred meters, thick. We don’t know where or how that magnetic reversal boundary forms in the lower crust.
Oceanus: And what about the Moho?
Tivey: The Moho has been defined as a seismological boundary: Seismic waves travel faster through the mantle below the Moho because it is made of peridotite, a denser rock than the gabbroic rock above the Moho. Peridotite is not magnetic, but when seawater percolates down and encounters peridotite, chemical reactions occur and create magnetic minerals. Peridotite changes into serpentinite, which can be very magnetic. So for me, the Moho may also be a magnetic boundary.
Oceanus: What it’s like on the JOIDES Resolution drillship?
Tivey: I’ve sailed on many research cruises, but this was my first experience on the drillship. It’s a fantastic system. The labs on board, the technology, the drillers, and how everything works is really impressive. The roughnecks are continually adding on new sections of pipe that hang below the ship to the seafloor. As you drill down, a core of rock goes up into a core barrel that’s nine point six meters long— thirty feet. Then they send down a wire that clips on, and they pull it up on deck and give the cores to the scientists who are oohing and aaahing.
But it’s a bit stressful, to be honest. Everything revolves around one drill hole. If something goes wrong with it, that hole is done. I helped with the survey in 1998 to prepare for this expedition, and it was 18 years before we actually got back to the site. So I have a lot of mental investment in this. Henry Dick, to his credit, got the scientific community geared up to go back here and try again. We are trying to go deeper, where nobody’s gone before.