Geology

After World War II, geophysical evidence began to accumulate that confirmed the lateral motion of continents and indicated the young age of oceanic crust. This evidence led to the theories of seafloor spreading and plate tectonics in the 1960s. American marine geologists Robert S. Dietz and Harry H. Hess proposed the seafloor spreading hypothesis, the concept that the oceanic crust is created as the seafloor spreads apart along midocean ridges. American oceanographers Bruce C. Heezen, Marie Tharp, and others prepared detailed maps of the ocean floors and the mid-Atlantic ridge and rift system, a mountainous chain found throughout the ocean. These maps provided additional evidence that seemed to support the continental drift theory. Further evidence came from paleomagnetism, the record of the orientation of earth's magnetic field recorded in rocks. In the 1950s, British geophysicist S. Keith Runcorn determined that this evidence indicated that the continents had moved relative to the earth’s magnetic poles and to each other. British marine geophysicists Fred J. Vine and Drummond Matthews described the record of changes in the earth’s magnetic field when they discovered “magnetic stripes” formed at spreading centers of the mid-ocean ridges, leading to the Vine-Matthews hypothesis. Magnetic stripes were also independently described by Canadian geophysicist Lawrence Morley and confirmed by American marine geologist Walter Pitman and others. These stripes indicated reversals of the direction of the earth’s magnetic field recorded in rock as new ocean crust was created at mid-ocean ridges. Scientists used paleomagnetism and seafloor spreading to determine that the continents had moved relative to the magnetic poles and to each other.

Canadian geophysicist J. Tuzo Wilson and American geophysicist Jason Morgan, among others, proposed the outline of the theory of plate tectonics in the 1960s. This theory stated that the earth’s lithosphere is made up of several rigid plates. These plates slide and move over a less-rigid layer called the asthenosphere. A plate may be composed entirely of oceanic crust, like the Pacific Plate, or of part ocean crust and part continental crust, like the North American Plate. New ocean crust is generated at ocean ridges (underwater mountain chains formed by the young ocean crust). Older ocean crust sinks down, or subducts, into the earth’s mantle at subduction zones, which are found at the deepest parts of the ocean, called trenches. As the plates move, they collide and form mountains. The plates recycle crust, generate volcanoes, and move past each other along faults. Using satellites, scientists can now measure movement of the continental plates in centimeters per year. Plate boundaries are the sites of most of the earth's earthquakes and the majority of earth's volcanoes. The continents are made of remelted sediments and partially melted oceanic crust, forming a lower density layer that has collected through time. The mechanism that drives the earth’s crustal plates is still not known, but geologists can use plate tectonics to explain most geologic activity. See also Earth.

The full recognition by scientists of earth as a planetary body, combining the fields of solar-system astronomy and geology, is perhaps the latest revolution in the earth sciences. Although scientists have recognized earth as a planet for centuries, space exploration that began in the 1960s created a new view of the earth. Photographs of earth taken from space had a profound effect on how people saw the earth. The exploration of neighboring moons and planets has led to a new understanding of the earth as an evolving planet.