Sea floor spreading

In the early 1960s, Dietz (1961) and Hess (1962) had proposed that continental drift might be accomplished by a process that Dietz termed sea floor spreading (Section 1.2). It was suggested that new oceanic lithosphere is created by the upwelling and partial melting of material from the asthenosphere at the ocean ridges. As the ocean gradually grows wider with the progressive creation of lithosphere, the continents marginal to the ocean are moved apart. The drift between North America and Europe, for example, would have been accomplished by the gradual growth of the Atlantic Ocean over the past 180 Ma. Since the Earth is not increasing in surface area by any significant amount (Section 12.3), the increase in size of those oceans growing by sea floor spreading would be balanced by the destruction of lithosphere at the same rate in another, shrinking, ocean by subduction at deep sea trenches situated around its margins.

The driving mechanism of these movements was believed to be convection currents in the sub-litho-spheric mantle (Fig. 1.5). These were thought to form cells in which mantle ascended beneath ocean ridges, bringing hot material to the surface and giving rise to new lithosphere. The flow then moved horizontally

Mid-age of sliding window (Ma)

Figure 4.4 Estimated frequency of geomagnetic reversals over the past 160 Ma (redrawn from Merrill et al., 1996, with permission from Academic Press. Copyright Elsevier 1996).

Mid-age of sliding window (Ma)

Figure 4.4 Estimated frequency of geomagnetic reversals over the past 160 Ma (redrawn from Merrill et al., 1996, with permission from Academic Press. Copyright Elsevier 1996).

away from the ridge, driving the lithosphere laterally in the same direction by viscous drag on its base, and finally descended back into the deep mantle at the ocean trenches, assisting the subduction of the lithosphere. This possible mechanism will be discussed more fully in Section 12.7.

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