Location  |   Ship Plan  |   People  |   Atlas  |   Glossary
 December 7, 2000   1   2   3 

... Further evidence is needed
While Argo II and Alvin continue photographing and visiting the mountain, and geochemical teams continue evaluating the returned rocks, scientists from the Scripps Institution of Oceanography have been doing different and more computer-intensive investigations.

As Atlantis towed the DSL-120 side-scan sonar over and around the massif, another device being pulled piggyback-style by the sonar itself was sending signals back to Gee. That instrument is a magnetometer, which measures magnetic field variation in the terrain it overpasses.

Using electromagnetic sensors as well as a direction-finding gyroscope, this "vector"-style magnetometer can detect subtle differences in the orientations of magnetic fields in all three dimensions.

A record of Earth's own magnetic field gets frozen into rocks at the time they cool down from the molten form. "They don't get magnetized until below 600 degrees Centigrade," Gee said.

Because scientists have amassed separate records of how our planet's field reverses itself over long time spans, magnetometers can thus pinpoint the magnetic interval that rocks are born into, and hence their approximate ages. Photo by Monte Basgall.

Along the Mid-Atlantic Ridge, where new ocean crust gets created and transported as if on a treadmill, such information can also help determine how far rocks have moved over time. For the same reason, magnetometers like Gee's may help estimate how rocks have been rotated in space as well, he added.

Fig. 6. Jeff Gee's gravity map.

Analyzing and cross-matching returns from all the DSL-120 runs, Gee has put together a colorful, almost surrealistic, map of magnetic change across the massif. Yellow and red colors indicate "positive" readings, pegged to the "polarity" (spatial orientation) of today's magnetic field. "Negative readings," the reverse of today's, are recorded as dark blue or purple.

Earth's magnetic magnetic field most recently reversed itself about 780,000 years ago. It was similar to today's polarity between 990,000 and 1.07 million years ago, and again starting about 730,000 years earlier than that.

Since the crust is moving away from the Mid-Atlantic Ridge at a rate of about 12 millimeters per year, crust just west of the massif's center should date to that first magnetic reversal and the crust at the mountain's western edge to about 1.8 million years ago, Gee said.

The map his magnetometer made fits in well with the time-line for magnetic reversals except over the dome, which gives a strong negative reading while the massif's southern edge just below is positive. Gee acknowledged he doesn't know what that might signify.

Meanwhile, Scott Nooner, a Scripps graduate student working with Sasagawa, has been pulling late nighters on a computer revising earlier estimates of "gravity highs" on the massif. The purpose is to reflect more-precise readings Alvin divers have recorded with the Scripps gravimeter. Photo by Monte Basgall.

Displayed on Sasagawa's laptop, the new readings of tiny variations in the pull of gravity are clustered in three areas: to the northwest, on top, and to the southeast of the mountain's center dome.

Fig. 7. Glenn Sasagawa with the Scripps gravimeter in the main lab.

Blackman's and others' 1998 analyses of previous gravity measurements done at the ocean's surface showed the mountain registering a "gravity high," a measure of unusually dense rock, over its southeastern flank - possibly meaning the presence of mantle material.

According to that study, coauthored by Blackman, Cann and other scientists, the high density area "can be interpreted as lower crustal rock which has been uplifted and rotated counterclockwise during exposure along a detachment that forms its eastern boundary."

That earlier study used a mathematical model that was valid for ocean surface, where the earlier gravity readings were done. But the model "does not work at the sea floor," Blackman said aboard Atlantis. Nooner's new evaluations show "the shape of the high density is going to have to be modified," she added. "It's a great training project for Scott."

Today's pages: Drawing parallels | Where's the fault? | Further evidence needed



     ©2000 by the Regents of the University of California. All rights reserved.