Page 401 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 12 The Dynamic Planet 365
Volcanic crater
Ridges
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Rift valley
Hess and other geologists then faced a new problem: If seafloor spreading and the creation of new crust are ongoing, then old ocean crust must somewhere be con- sumed; otherwise, Earth would be expanding. Harry Hess and another geologist, Robert S. Dietz, proposed that old seafloor sinks back into the Earth’s mantle at deep-ocean trenches and in subduction zones where plates collide. Scientists now know that in the areas of ocean basins farthest from the mid-ocean ridges, the oldest sections of oceanic lithosphere are slowly plung- ing beneath continental lithosphere along Earth’s deep- ocean trenches.
Magnetic Reversals Earlier we discussed the history of reversals in Earth’s magnetic field. As seafloor spreading occurs and magma emerges at the surface, magnetic par- ticles in the lava orient with the magnetic field in force at the time it cools and hardens. The particles become locked in this alignment as part of the new seafloor, creating an ongoing magnetic record of Earth’s polarity. Using isotopic dating methods, scientists have estab- lished a chronology for these reversals—that is, the ac- tual years in which the polarity reversal occurred. Ages for materials on the ocean floor proved to be a fundamen- tal piece of the plate tectonics puzzle.
Figure 12.16 shows a record from the Mid-Atlantic Ridge south of Iceland, illustrating the magnetic stripes preserved in the minerals—the coloured bands are areas of reversed polarity, the areas between them have nor- mal polarity. The relative ages of the rocks increase with distance from the ridge, and the mirror images that de- velop on either side of the mid-ocean ridge are a result of the nearly symmetrical spreading of the seafloor. These
60° W 30° W 0°
4 km
▲Figure 12.15 The Mid-atlantic ridge. A 4-km-wide image of the Mid-Atlantic ridge shows a volcanic crater, a rift valley, and ridges. The image was taken by the TOBI (towed ocean-bottom instrument) at approximately 29° n latitude. [Image courtesy of D. K. Smith, Woods Hole Oceanographic Institute.]
the deeper lower mantle. As upwelling occurs, the new seafloor then moves outward from the ridge as plates pull apart and new crust is formed. This process, now called seafloor spreading, is the mechanism that builds mid- ocean ridges and drives continental movement.
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30° W
60° W 30° W 0°
Mid-ocean ridge
60° N
▲Figure 12.16 Magnetic reversals recorded in the ocean floor. Magnetic reversals recorded along the Mid-Atlantic ridge south of Iceland; coloured bands indicate magnetic stripes on the seafloor with reversed polarity, the areas between have normal polarity. Similar coloured bands on either side of the ridge indicate symmetrical seafloor spreading with oldest rock bands farthest from the ridge. [Adapted from J. r. Heirtzler,
S. Le Pichon, and J. g. Baron, Deep-Sea Research 13, © 1966, Pergamon Press, p. 247.]
Greenland
Mid-Atlantic Ridge
Iceland
Europe
Depth (metres)
Older
Older
Ridge axis