DID THE EARTH'S UPPER MANTLE SLIP BY 90 DEGREES relative to the rest of the Earth 530 million years ago? (Think of an orange's skin becoming detached from the pulp beneath and sluing around to the side.) This breathtaking hypothesis, advanced by Caltech scientist Joseph Kirschvink, is based on his study of the pattern of fossil magnetism in rock samples. The colossal slippage of the outer part of the Earth relative to the inner, a process called true polar wander, would have produced a rapid (by geologic standards) and global redistribution of surface topology, although the planet's spin axis would have remained fixed. The crustal plates, which normally float about on top of the mantle at a pace of perhaps centimeters per year would, under this new geodynamic regime, have moved at rates up to a meter per year since the mantle itself (or least part of it) underneath the plates had become unmoored and was moving (and bringing the crust along with it) relative to the rest of the Earth's bulk. All of this happened, according to Kirschvink, about 530 million years ago for a period of 15 million years. The ensuing grand trek of continental landmasses was thus simultaneous with, and might have influenced or caused, the "Cambrian explosion," the greatest evolutionary proliferation of diverse living organisms in history. (Science, 25 July 1997.)