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The Theory of Plate Tectonics (3) Seafloor Spreading and the Recycling of Oceanic Crust  | | A figure showing the relationship between seafloor spreading and the recycling of the ocean crust. Figure source: http://pubs.usgs.gov/publications/text/dynamic.html |
The observation of symmetrical magnetic stripes on the seafloor led to the development of the seafloor spreading hypothesis. Starting in 1961, seafloor spreading was hypothesized to explain these remarkable magnetic stripes that are symmetrical about the mid- ocean ridges. Mid-ocean ridges are structurally weak zones in the ocean floor, and where magma rises to form new oceanic crust. This process, called seafloor spreading, has built the present system of mid-ocean ridges. Seafloor spreading is supported by several pieces of evidence: (1) starting from the ridge and moving outward in both directions, the oceanic crust progressively gets older as the rocks move away; (2) the youngest rocks, located at the ridge crest, have present day normal polarity; and (3) each individual zebra-like strip has alternated magnetic polarity (i.e. normal-reversed-normal), suggesting that the Earth's magnetic field has reversed many times.
Two geologists, Harry H. Hess and Robert S. Dietz, who first made up the term "seafloor spreading", were especially interested in the implications of the phenomenon. For the earth to produce new oceanic crust in a conveyor belt-like motion, it must shrink elsewhere for it not to expand. This takes place at ocean trenches which are very deep canyons, like along the rim of the Pacific Ocean basin. Subduction zones are plate boundaries where old oceanic crust is recycled back into the mantle. This explains why the Earth's circumference does not expand with the production of new oceanic crust and why, in comparison to continents, the seafloor has very little accumulation of sediment. Lastly, the recycling of the oceanic crust through subduction zones and the production of new crust from the mid-ocean ridges, explain why the seafloor is much younger than the continents.
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