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SPAZIO CLIL Paleomagnetism W e have seen repeatedly how the geologic record of ancient magnetism, or paleomagnetism, has become a crucial source of information for understanding Earth’s history. Magnetic stripes mapped on oceanic crust confirmed the existence of seafloor spreading and still provide the best data to explain how plate motions have evolved since the breakup of Pangaea 200 million years ago. The paleomagnetism of old continental rocks has been essential for establishing the existence of earlier supercontinents, such as Rodinia. Scientists have also used paleomagnetism to reconstruct the history of Earth’s magnetic field. The oldest magnetized rocks found so far formed around 3,5 billion years ago and indicate that Earth had a magnetic field at that time similar to the present one. The presence of magnetism in the most ancient rocks is consistent with the ideas of Earth’s differentiation discussed in Chapter 1, which imply that a convecting fluid core must have been established very early in Earth’s 4,5 billion year history. Let’s delve a little more deeply into the rockforming processes that have allowed geologists to draw these remarkable conclusions. Recall that high temperatures destroy magnetism. An important property of many magnetizable materials is that, as they cool below about 500°C, they become magnetized in the direction of the surrounding magnetic field. This happens because groups of atoms of the material align themselves in the direction of the magnetic field when the material is hot. When the material has cooled, these atoms are locked in place and therefore are always magnetized in the same direction. This process is called thermoremanent magnetization, because the magnetization is «remembered» by the rock long after the magnetizing field has disappeared. Thus, the Australian student was able to determine the direction of the field when the stones cooled after the last fire and took on the magnetization of Earth’s magnetic field of that time (see figure 1). The discovery of magnetic reversals and the means to decipher them was a key ingredient in formulating the theory of plate tectonics. Some sedimentary rocks can take on a different type of remanent magnetization. Recall that marine sedimentary rocks form when particles of sediment that have settled to the seafloor become lithified. Magnetic grains among the particles (chips of the mineral magnetite, for example) be- Fantini, Monesi, Piazzini - Elementi come aligned in the direction of Earth’s magnetic field as they fall through the water, and this orientation can be incorporated into the rock when the particles becomes lithified. The depositional remanent magnetization of a sedimentary rock results from the parallel alignment of all these tiny magnets, as if they were compasses pointing in the direction of the field prevailing at the time of deposition (see figure 2). 30 000 years ago Today Figure 1 Earth’s magnetic field 30 000 years ago was the reverse of today’s, as evidenced by the discovery of reversely magnetized rocks found in the fireplace of an ancient campsite. The rocks, cooling after the last fire, became magnetized in the direction of the ancient magnetic field, leaving a permanent record of it, just as a fossil leaves a record of ancient life. di Scienze della Terra • Italo Bovolenta editore - 2013 1 SPAZIO CLIL Figure 2 Newly formed sedimentary deposits can become magnetized in the same direction as the contemporaneous magnetic field of the Earth. Magnetic mineral grains transported to the ocean with other sediments become aligned with the Earth’s magnetic field while settling through the water. Direction of magnetic field Ocean Magnetic particles in ocean sediment This orientation is preserved in the lithified sediments, which thus “remember” the field that existed at the time of deposition. Fantini, Monesi, Piazzini - Elementi Reversals of Earth’s magnetic field are clearly indicated in the fossil magnetic record of layered lava flows. Each layer of rocks from the top down represents a progressively earlier period of geologic time, whose age can be determined by radiometric dating methods. The direction of remanent mag netism can be obtained for each layer, and in this way the time sequence of flip-flops of the field (that is, the magnetic stratigraphy) can be deduced. Geologists can also get data on Earth’s magnetic reversal history by mapping magnetic stripes on the seafloor. From a combination of these data, they have worked out a detailed history of reversals for the last 200 million years. This information is of use to archeologists and anthropologists as well as geologists. For example, the magnetic stratigraphy of continental sediments has been used to date sediments containing the remains of predecessors of our own species. About half of all rocks studied are found to be magnetized in a direction opposite that of Earth’s present magnetic field. Apparently, then, the field has flipped frequently over geologic time, and normal (same as now) and reversed fields are equally likely. di Scienze della Terra • Italo Bovolenta editore - 2013 2