Historical Development Of The Plate Tectonic Paradigm

The plate tectonic paradigm was developed from a number of different models, ideas and observations that were advanced over the prior century by a number of scientists on different continents. Between 1912 and 1925, Alfred Wegener, a German meteorologist, published a series of papers and books outlining his ideas for the evolution of continents and oceans. Wegener was an early proponent of continental drift. He looked for a driving mechanism to move continents through the mantle, and invoked an imaginary force (which he called Pohl uicht) that he proposed caused the plates to drift toward the equator because of the rotation of the Earth. Geophysi-cists showed that this force was unrealistic, and since Wegener's idea of continental drift lacked a driving mechanism, it was largely disregarded.

in 1929 British geologist Arthur Holmes proposed that the Earth produces heat by radioactive decay and that there are not enough volcanoes to remove all this heat. He proposed that a combination of volcanic heat loss and mantle convection can disperse the heat, and that the mantle convection drives continental drift. Holmes wrote a textbook on this subject, which became widely used and respected. Holmes proposed that the upwelling convection cells were in the ocean basins, and that downwelling areas could be found under Andean-type volcano chains.

Alex du Toit was a south African geologist who worked on Gondwana stratigraphy and published a series of important papers between 1920 and 1940. Du Toit compared stratigraphic sections on the various landmasses that he thought were once connected to form the supercontinent of Gondwana (Africa, south America, Australia, india, Antarctica, Arabia). He showed that the stratigraphic columns of these places were very similar for the periods he proposed the continents were linked, supporting his idea of an older, large, linked landmass. Du Toit also dem-

a a Major thrust fault <Cj Displacement direction of major continental block

A A Subduction zone ^ ^ Region of extension "iin Normal fault

India Asia Collision Tectonic
Map of central and eastern Asia collision showing the wide area affected by the collision of India with Asia

onstrated that the floral distributions had belts that matched when the continents were reconstructed, but that appeared disjointed in the continents' present distribution.

In the 1950s, paleomagnetism began developing as a science. The Earth has a dipolar magnetic field, with magnetic field lines plunging into and out of the Earth at the north and south magnetic poles. Field or flux lines are parallel or inclined to the surface at intermediate locations, and the magnetic field can be defined by the inclination of the field lines and their deviation from true north (declination) at any loca tion. When igneous rocks solidify, they pass through a temperature at which any magnetic minerals will preserve the ambient magnetic field at that time. In this way, some rocks acquire a magnetism when they solidify. The best rocks for preserving the ambient magnetic field are basalts, which contain 1-2 percent magnetite; they acquire a remnant inclination and declination as they crystallize. Other rocks, including sedimentary red beds with iron oxide and hematite cements, shales, limestones, and plutonic rocks, also may preserve the magnetic field, but they are plagued with other problems hindering interpretation.

In the late 1950s, British geophysicist Stanley K. Runcorn and Canadian geologist Earl Irving first worked out the paleomagnetism of European rocks and discovered a phenomenon they called apparent polar wandering (APW). The Tertiary rocks showed very little deviation from the present pole, but rocks older than Tertiary showed a progressive deviation from the expected results. They initially interpreted this to mean that the magnetic poles wandered around the planet, and the paleomagnetic rock record reflected this wandering. Runcorn and Irving made an APW path for Europe by plotting the apparent position of the pole while holding Europe stationary. However, they found that they could also interpret their results to mean that the poles were stationary and the continents were drifting around the globe. Additionally, they found that their results agreed with some previously hard to interpret paleo-latitude indicators from the stratigraphy.

Next, Runcorn and Irving determined the APW curve for North America. They found it similar to Europe's from the late Paleozoic to the Cretaceous, implying that the two continents were connected for that time period and moved together, and later (in the Cretaceous) separated, as the APW curves diverged. This remarkable data set converted Runcorn from a strong disbeliever of continental drift into a drifter.

In 1954 Hugo Benioff, an American seismologist, studied worldwide, deep-focus earthquakes to about a 435 mile (700 km) depth. He plotted earthquakes on cross sections of island arcs and found earthquake foci were concentrated in a narrow zone beneath each arc, extending to about a 435 mile (700 km) depth. He noted that volcanoes of the island-arc systems were located about 62 miles (100 km) above this zone. He also noted compression in island-arc geology and proposed that island arcs are overthrusting oceanic crust. Geologists now recognize that this narrow zone of seismicity is the plate boundary between the subducting oceanic crust and the overriding island arc, and they have named this area the Benioff Zone.

Development of technologies associated with World War II led to remarkable advancements in understanding some basic properties of the ocean basins. In the 1950s, the ocean basin bathymetry, gravity, and magnetic fields were mapped for the U.S. Navy submarine fleet. After this, research scientists from oceanographic institutes such as scripps, Woods Hole, and Lamont-Doherty Geological Observatory studied the immense sets of oceano-graphic data. As the raw data was acquired in the 1950s, the extent of the mid-ocean ridge system was recognized and documented by American geologists including Bruce Heezen, Maurice Ewing, and Harry Hess. They also documented the thickness of the sedimentary cover overlying igneous basement and showed that the sedimentary veneer is thin along the ridge system and thickens away from the ridges. Walter Pitman from Lamont-Doherty Geological Observatory in New York happened to cross the mid-oceanic ridge in the South Pacific perpendicular to the ridge and noticed the symmetry of the magnetic anomalies on either side of the ridge. In 1962 Harry Hess from Princeton proposed that the mid-ocean ridges were the site of seafloor spreading and the creation of new oceanic crust, and that Benioff Zones were sites where oceanic crust was returned to the mantle. In 1963 American geologists Fred Vine and Drummond Matthews combined Hess's idea and magnetic anomaly symmetry with the concept of geomagnetic reversals. They suggested that the symmetry of the magnetic field on either side of the ridge could be explained by conveyor-belt style formation of oceanic crust, forming and crystallizing in an alternating magnetic field, such that the basalts of similar ages on either side of the ridge would preserve the same magnetic field properties. Their model was based on earlier discoveries by a Japanese scientist, Motonori Matuyama, who in 1910 discovered recent basalts in Japan that were magnetized in a reversed field and proposed that the magnetic field of the Earth experiences reversals. Allen Cox (Stanford University) had constructed a geomagnetic reversal time scale in 1962, so it was possible to correlate the reversals with specific time periods and deduce the rate of seafloor spreading.

With additional mapping of the seafloor and the mid-ocean ridge system, the abundance of fracture zones on the seafloor became apparent with mapping of magnetic anomalies. In 1965, Canadian geologist J. Tuzo Wilson wrote a classic paper, "A New Class of Faults and Their Bearing on Continental Drift," published in Nature. This paper connected previous ideas, noted the real sense of offset of transform faults, and represented the final piece in the first basic understanding of the kinematics or motions of the plates. Lynn Sykes and other seismologists provided support for Wilson's model about one year later by using earthquake studies of the mid-ocean ridges. They noted that the ridge system divided the Earth into areas of few earthquakes, and that 95 percent of the earthquakes occur in narrow belts. They interpreted these belts of earthquakes to define the edges of the plates. They showed that about 12 major plates are all in relative motion to each other. Sykes and others confirmed Wilson's model, and showed that transform faults are a necessary consequence of spreading and subduction on a sphere.

See also convergent plate margin processes; divergent plate margin processes; geodynam-ics; transform plate margin processes.

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