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The Calcite Stress Gauge

News from the edge


plate deformation map

Red pattern shows where "rigid" plates are widely deformed.

US Geological Survey

Plate tectonics started from a very few rules suggested by the data: Plates (1) are rigid pieces of the Earth's outer shell (2) that have exactly three kinds of boundaries—spreading ridges, subducting trenches, and side-slipping transform fault zones.

This simple picture was a good start, but on closer inspection the two rules are too simple. The boundaries have a lot of variations: spreading ridges that open like zippers, subduction that's oblique and sloppy, fault zones that are wide splintery gouges and not clean lines. And neither are the plates truly rigid. The plates deform.

Calcite: A Mineral Gauge of Deformation

Ben van der Pluijm published a low-tech way to measure plate deformation in the 8 August 1997 issue of Science. He and coauthors John Harris, Brita Graham, and John Craddock focused on calcite, the most common mineral in plate interiors.

Calcite warps so easily, under such small stresses, that you can do it in the classroom using a vise. Even better, you can nick it with a knife and watch the mineral on one side of the nick glide all the way to the edge, as if it were a stack of playing cards.

Stress and Strain Both

Van der Pluijm's team collected rocks in a long line from the northern Rockies across the Great Plains, hoping to measure the stresses associated with mountain-building, or orogenesis. They did the same along a line from the Appalachians toward the northwest. Using the petrographic microscope, they found that the calcite grains detected stresses nearly 2,000 kilometers away from the mountain ranges.

They were also able to measure orogenetic strain. (Stress is a force, and strain is the actual deformation that a stress causes.) Well away from the stressful edge of the North American plate, van der Pluijm measured strains of several percent. That's a significant amount of give—something like the difference between a face with no expression and one with a Mona Lisa smile. Work like this is showing that every place on Earth is affected by plate-tectonic activity.

Action on the Plate Edges

Many workers have been concentrating on those active plate edges, the continental margins. There are so many new questions to be asked that in 1991 a worldwide voluntary committee formed to start organizing the research. It set about writing a program called the MARGINS Initiative, and its efforts are beginning to pay off. (A similar committee for core and mantle research called SEDI, for Study of the Earth's Deep Interior, has been active for more than 10 years.) The MARGINS Web site contains the full text of the Initial Science Plan, and just like that, you can be informed on a vast spectrum of frontier research into the cutting edge of Earth science.

PS: Brita Graham, coauthor of van der Pluijm's calcite-deformation paper, was still an undergraduate. It's not unique for a sophomore to gain authorship on a paper in a major journal, but it's quite a feat just the same. If you're a college student yourself, or if your child wants to be one, then Graham's story should be a lesson in getting ahead in a geological career—look for chances to do real research.

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