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.)

THE WORLD'S FASTEST SEMICONDUCTOR has been devised by physicists at the Weizmann Institute in Israel. Using an ultra-clean vacuum system and highly purified materials, the researchers (Moty Heiblum, heiblum@hpl.hp.com) created a sandwich consisting chiefly of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) layers. Cooling the material to 0.1 K and subjecting it to a small electric field, electrons at the GaAs/AlGaAs interface drifted in the direction of the field at speeds 40% greater than those in the best previous GaAs-based materials. The electrons journeyed an average of 120 microns before colliding with anything. In general, electrons tend to scatter less in GaAs than in silicon; moreover, this material greatly outperformed its GaAs-based cousins because it had significantly fewer impurities. This is not the case at room temperature, however, where thermal vibrations of the semiconductor crystal diminish the importance of purity. Additionally, the researchers detected possible signs of large negative magnetoresistance, in which magnetizing the cold sample actually appeared to decrease its electrical resistance substantially ---something not seen before in similar AlGaAs/GaAs materials. (V. Umansky et al., Applied Physics Letters, 4 August.)

SMART MATTER , material utilizing microelectromechanical systems (MEMS), is almost sentient in that is can sense (strain, temperature, pressure, motion, etc.), actuate (push, squeeze, deflect, switch, etc.), communicate (with fibers, antennas, wires, etc.), and calculate (with microprocessors). Machines or even arrays of millimeter and micron-sized machines on a chip, made with integrated-circuit technology, are still at an early stage of deployment, but researchers foresee a micro-industrial revolution: clouds of meteorological smart dust sent to keep an eye on a hurricane, programmable silicon cilia to sort blood cells or position tiny machine parts, and microflaps to control a plane's wing shape. (Science News, 26 July.)

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