concerned solely with photon emission and absorption. Molecular light emission is particularly appealing for sensing purposes owing to its near-ultimate detectability, off/on switchability, and very high spatiotemporal resolution, including video imaging.

Neurobiology, say the authors, has been a special beneficiary of fluorescence-sensing techniques combined with high-resolution microscopy. This is so because the central nervous system is a highly complex network of billions of cells, each with an elaborate morphology. Enhanced understanding of the central nervous system requires more intense study of neuronal activity at the network level as well as increased subcellular resolution.

Neotectonics

Recent advances in Global Positioning System (GPS) technology have made it possible to detect millimeter-scale changes in Earth’s surface. According to Clement et al., these advances have made it possible to detect relative motion between the large plates of the outermost rigid layer of Earth.21 These motions previously had only been inferred from indirect evidence of the plates’ motions. A remarkable piece of knowledge from these studies is that the plate motions are nearly continuous, not episodic, processes, even on human time scales. Analyses of these motions indicate that much of the motion between plates occurs without producing earthquakes. In addition to monitoring interplate motions, GPS arrays are making it possible to study present deformation occurring within mountain belts. The strain-rate models obtained from the GPS arrays then can be used to test specific theories of crustal deformation in mountain belts.

The theory of plate tectonics that revolutionized the earth sciences during the 1960s was based primarily on indirect evidence of past crustal movements. Motions of the seafloor crust were inferred from the magnetization of the crust that recorded polarity reversals (of known age) of Earth’s magnetic field. The symmetric magnetic patterns suggested that new crust was being created at the mid-ocean ridges and then was spreading away from the ridges. The seafloor-spreading hypothesis was successful in explaining many longstanding problems in earth sciences and became the basis of a new paradigm of crustal mobility.

Most plate boundary zones accommodate motion along relatively narrow regions of deformation. However, plate boundary zones involving continental lithosphere absorb relative motion by deforming over broad zones that are hundreds to even thousands of kilometers wide. An understanding of the forces at work in these zones is important because many of the most damaging earthquakes occur within these zones, say Clement et al. At present, little is known of how the

21  

Frontiers of Science/1999. Bradford Clement, Rob McCaffrey, and William Holt, at <http://www.pnas.org/cgi/content/full/96/25/14205>.



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