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Transcript
SPAZIO CLIL Seismic tomography The four maps show the variations in S-wave speeds at depths of 70 km, 200 km, 500 km, and 2 800 km in the mantle derived from seismic tomography. Regions with faster S-wave speeds indicate relatively colder, denser rock. Regions with slower S-wave speeds (red and yellow) indicate relatively hotter, less dense rock. (S-wave speeds: G. Ekström and A. Dziewonski, Harvard University; maps: L. Chen and T. Jordan, University of Southern California) S 70 km. Near Earth’s surface, hot rocks in the asthenosphere slow S waves, as revealed by the warm colors (red and yellow) along oceanic spreading centers. 200 km. Moving deeper, we see the cold lithosphere of continental cratons (blue and purple regions) and warmer asthenosphere beneath ocean basins (red regions). 500 km. Deeper in the mantle, the features no longer match the continental positions. 2 800 km. Near the mantle-core boundary, the S-wave patterns reveal colder regions around the Pacific that may be the «graveyards» of sinking lithospheric slabs. Fantini, Monesi, Piazzini - Elementi eismic tomography is an adaptation of a method now commonly used in medicine to map the human body, called computerized axial tomography (CAT). CAT scanners construct threedimensional images of organs by measuring small differences in X rays that sweep the body in many directions. Similarly, geologists use the seismic waves from earthquakes recorded on thousands of seismographs all over the world to sweep Earth’s interior in many different directions and construct a three-dimensional image of what’s inside. The method enables them to find places where seismic waves speed up or slow down. They make the reasonable assumption, consistent with laboratory experiments, that regions where seismic waves speed up are composed of relatively cool, dense rock (for example, subducted ocean plates), whereas regions where seismic waves slow down ind icate relatively hot, buoyant matter (for example, rising convection plumes). Seismic tomography has revealed features in the mantle clearly associated with mantle convection. Figure 2 presents a tomographic model of Swave speed variations in the mantle constructed by researchers at Harvard University. The model is displayed as a series of global maps at depths ranging from just below the crust down to the core-mantle boundary. Near the surface, you can clearly see the structure of plate tectonics. The low S-wave speeds caused by the upwelling of hot asthenosphere along the mid-ocean ridges are shown in warm colors; the high S-wave speeds from cold lithosphere in the old ocean basins and beneath the continental cratons are shown in cool colors. At greater depths, the features become more variable and less coherent with the surface plates, reflecting what geologists infer to be a complex pattern of mantle convection. Some largescale features stand out, however. For example, you will notice that, just above the core-mantle boundary, there is a red region of relatively low S-wave speeds beneath the central Pacific Ocean, surrounded by a broad blue ring of higher S-wave speeds. Seismologists have speculated that the high speeds represent a «graveyard» of oceanic lithosphere subducted beneath the Pacific’s volcanic arc, the Ring of Fire, during the last 100 million years or so. di Scienze della Terra • Italo Bovolenta editore - 2013 1