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How Robert Horvitz Discovered Seismic Topography

Robert Horvitz is our man in Washington, DC, and our farranging scout for the unreported story. He's been reviewing unconventional art and science for Whole Earth since 1976. He captioned the picture to the right: "Peep," a 5-year old Guimo (guinea pig) from Peru, with his pet, "Robert Horvitz."

What am I working on now? Researching topics that may tum up as articles in these pages sooner or later Don't want to blow any surprises, but one item on the list may work best as a capsule preview anyway.

An article by Philip J. Hilts in the Washington Post last June described a new remote-sensing technique called "seismic tomography." Developed about two years ago, the concept is similar to the CAT scans used in medical diagnostics, But instead of X-rays, seismic recordings from monitoring stations around the globe are integrated by computer to create 3-D images of the Earth's interior. So for the first time, we can peer into this huge hidden volume, which is the bulk of our planet, Compared to the vague generic model of the underworld we were taught in school, some of the features revealed by seismic tomography seem like science fiction. Take the Earth's core - a red-hot sphere of liquid iron, right? Wrong, magma-breath. According to Adam Dziewonski, a geophysicist at Harvard, vibrations passing through the core behave as though the iron at the very center is compressed into "a single thousand-mile-wide crystal" swaddled in liquid iron. The rocky mantle around the core seems to have an irregular in-facing surface some 1,800 miles below us, "Antimountains" six to seven miles tall poke into the potato-shaped core, along with "anti-continents" that may be ringed by "anti-oceans" of iron, As the Earth spins, the mantle's rough underside could generate currents and turbulence in the core. That may account for small fluctuations observed in the lengths of days, and the geomagnetic "storms" that disrupt our radio communications from time to time.

It used to be that we could only guess about the circulation of matter in the Earth's mantle. But differences in density and temperature make it possible to track these processes tomographically. Our understanding of plate tectonics and continent formation should improve rapidly as we fill in the blank regions in our 3-D map of the planet.

Reading about seismic tomography for the first. time gave me a rush of awe quite like the first photos of the Earth from orbit. As the "picture" of our planet's no-longer-inscrutable, interior gets clearer, it's likely to have a similar unifying and reorienting effect. Since tomographs are more indirect and artificial than photographs, they may never be quite so compelling. But that also may stop them from becoming cliches.

In any event, completing the image of the Whole Earth that initially inspired this Publishing Empire is a mighty fine way to mark our 20th anniversary. For the 30th, let's add animation.