ASU Research E-Magazine: Born To Be Crust

Rocks from outer space have become almost humdrum in ASU’s Department of Geology. But midocean ridge basalts, now those are something special.

Rocks from outer space are special, too, of course. ASU’s Center for Meteorite Studies has the world’s third-largest collection of meteorites. No other university anywhere can match it.

Nevertheless, meteorites come easily to hand at ASU, while midocean ridge basalts, or MORBs, as they are called, do not. MORBs are famous among geologists as new lava formed on the sea floor, but few ever actually get to see a sample. That’s why most of the department lined up to touch the MORB that graduate student Tracy Gregg brought to her lecture on sea-floor lava flows.

"No one has ever seen a volcanic eruption on the sea floor," says Gregg. "It’s a really fundamental process because it basically covers the birth of the Earth."

The eruption and spread of lava at midocean ridges on the sea floor lays at the heart of plate tectonics, the model that geologists use to explain how the Earth works. As the lava spreads it sends plates of the Earth’s crust pressing into one another or sliding past each other.

Either way, the results are easy to see. Collision of plates produces mountain ranges such as the Rockies and the Himalayas, and earthquakes such as the ones that devastated Kobe, Japan.

"What you don’t see is where these plates start out, where they begin on the sea floor," says Gregg. She has staked out that area as her specialty. "It’s the other half of the plate tectonics story that we haven’t been able to study because of technological constraints."

Geologists have classified lava flows according to the form they take after they have cooled. Until now, the process has been more descriptive than scientific. Leveed flows follow a channel. Lobate flows resemble noodles. Pillow flows look soft and puffy. The list goes on.

"We’re trying to place some numbers on what kind of eruption rates are associated with each of those types, because the types can be mapped over fairly large areas," says Gregg’s adviser, geology Professor Jonathan Fink.

"If we could come up with correlations, then it would let us characterize different parts of the ocean floor as having different eruption rates. That’s useful for models of how plate tectonics works and how crust is formed in the ocean," Fink says.

Gregg’s research builds on the work of Fink and Ross Griffiths of Australian National University in Canberra. Fink has spent eight months working in Griffiths’ lab in recent years, learning how to model lava flows in the laboratory.

"I went down there because Ross has the best geological fluid dynamics group in the world," Fink says.

Fink built his own lava-flow simulation laboratory after returning to ASU. Gregg’s experiments in the lab have helped her to understand what she saw on the ocean floor during three Pacific Ocean cruises in 1994.

She made two dives on the Alvin research submarine to the East Pacific Rise about 500 miles south of Acapulco, Mexico. As new sea floor slowly forms along the rise, it drives huge plates of the Earth’s crust in opposite directions. The Cocos plate to the east of the rise pushes beneath the Andes Mountains. The much larger Pacific plate on the other side of the rise pushes west.

Gregg also studied the Juan de Fuca Ridge off the coast of Oregon from a surface vessel. The ridge drives one of the plates that forms the Cascade volcanoes, including Mount St. Helens and Mount Rainier.

The experiments take only 30 minutes to run but about 3 hours to set up. They entail pumping hot wax onto the sloping floor of a tank full of sugar water. She uses sugar water—each experiment consumes 25 pounds of sugar—because it allows her to chill its temperature below freezing without forming ice.

"It’s no good to look at this and say, ‘Oh, what a neat picture,’" Gregg said. "You have to measure everything."

Gregg’s measurements have allowed her to distinguish how differences in flow rate and lava temperature distinguish one type of flow from another. Pillow flows, for example, erupt more slowly and at lower temperatures than do leveed flows. She and Fink published their results in the January 1995 issue of the journal Geology.

Their work has applications both to helping understand volcanic hazards on Earth and planning missions to other planets.

Fink and a researcher at the University of Tokyo have studied the lava dome at Mount Unzen in Japan. Unzen’s growing lava dome has collapsed several times since 1991, producing deadly flows of fiery rock and ash. Japanese officials would like to know in which direction the future collapse will occur.

"We were able to pin down what the role of a cooling surface crust is on the way lava domes grow. We’ve come up with some new models that I think are applicable to the interpretation of all lava flows," Fink says.

For now, though, the growth of Unzen’s dome has slowed. "The Japanese have other geologic hazards that they’re concerned about at the moment," Fink adds.

Some of Gregg’s experiments can help solve the trickier question of lava composition.

"The reason you study lavas on other planets is because the lavas had to come from the inside and they tell you what’s inside," Gregg explains. "If you want to go to Mars, you’d like to know what Mars is made out of so you know what you can mine there.

"Will we be able to get building materials if we ever decide to colonize Mars? The only way we can tell that is by what we see on the surface."—Steve Koppes