March 7, 2008

Researchers study Hawaiian islands' formation

In the dead of winter, Chicago feels a long way from luaus and leis, but a pair of U of C physicists have spent the season studying the forces that may have created the Hawaiian Islands millions of years ago.

Hawaii rests atop what physicists call a geographical “hot spot,” an area of high volcanic activity. The Aloha State is unique, however, in that its hot spot lies in the middle of a tectonic plate.

The earth’s crust consists of about a dozen plates that float on a sea of molten rock called the mantle many miles beneath our feet. Most volcanoes form when magma sneaks up from the mantle to the surface around the edges of plates.

But geophysicists think Hawaii’s hot spot was formed by a stream of magma—a “mantle plume”—that burst through the earth’s crust in the middle of the Pacific Plate.

“Chains of islands are then formed as the tectonic plate slowly moves over the stationary column of magma below,” said Laura Schmidt, a physics graduate student, in an e-mail interview.

Schmidt conducted the research with assistant physics professor Wendy Zhang.

But this theory raises a difficult question: How do the plumes remain stable for the millions of years required to form the islands? It is this issue which Zhang and Schmidt sought to tackle.

They hypothesize that the mantle is a viscous mixture of molten rock heated by the earth’s core.

“A familiar example of such a flow is a lava lamp: the slow psychedelic flow you see is driven by heating at the base of the lamp, which is like the hot core of the earth driving flows in the mantle above it,” Schmidt said.

When you boil a pot of water on a stove, the liquid becomes turbulent and the cooler upper layer mixes evenly with the hotter lower one. But in a mixture with distinct layers, such as the earth’s mantle, something different happens.

In experiments conducted by Anne Davaille of the University of Paris which tracked the dynamics of heated layers of salt and fresh water, Davaille noticed that instead of mixing evenly, the lower layer was actually drawn into the upper layer via thin tendrils.

“Surprisingly, these thin tendrils persist over many convection cycles, despite the presence of fluctuations in both the temperature and the velocity fields,” Schmidt said.

Some tendrils lasted for hours, despite changes to experimental conditions. The remarkably stable mantle plumes may mimic these tendrils on a much larger-—and hotter—scale.

But why are they so stable? Critically, the tendrils flare out at their bases like a trumpet. Schmidt and Zhang repeated Davaille’s experiments using oil and water and then, through mathematical analysis, found that the flares actually anchor the tendrils to the boundary between the liquids. This may explain how the tendrils and mantle plumes remain so stable.

“Translating this result to reality means that hot spots like the one under the Hawaii Islands aren’t likely to change any time soon,” Schmidt said. “The islands should continue being created slowly and steadily as they have been for millions of years.”

Schmidt and Zhang’s work appears in the February 1 issue of Physical Review Letters.