Theory of Plate Tectonics

Evidence for the theory of seafloor spreading came from many sources: (1) there is high heat flow out of the mid-ocean ridge; (2) the youngest oceanic rock was found in the ridge, and progressively older rock was found farther out from the ridge; (3) the youngest rocks at the ridge crest always have present-day (normal) polarity; (4) stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has flip-flopped many times; and (5) mid-ocean ridges have very little sediment, while sediment thickness increases on either side of the ridge. The age, sediment thickness, and magnetic patterns are all nicely explained by seafloor spreading. Additional evidence was gathered by the research vessel Glomar Challenger in the late 1960s. This ship drilled into the ocean floor to collect rock samples that were analyzed and dated, providing irrefutable evidence of seafloor spreading.

Once scientists realized that Hess was right, it did not take long for them to resurrect Wegener’s idea. If the seafloor were splitting apart, wouldn’t that cause the continents to move apart also? The reasoning went something along these lines: If the seafloor were spreading, then this action would provide the motion that would cause the continents to move. The continents weren’t pushed across the seafloor by some enormous force, as Wegener suggested, but were like rafts moving with the spreading seafloor.

The stage was set for the development of a new theory. The ideas of continental drift and seafloor spreading were combined into a new theory called plate tectonics.


Source: http://pubs.usgs.gov/gip/dynamic/historical.html (public domain)