History of plate tectonics

Plate tectonic theory had its beginnings in 1915 when Alfred Wegener proposed his theory of "continental drift." Wegener proposed that the continents plowed through crust of ocean basins, which would explain why the outlines of many coastlines (like South America and Africa) look like they fit together like a puzzle. Wegener was not the first to notice this puzzle-like fit of the continents (Magellan and other early explorers also noticed this on their maps), but he was one of the first to realize that the Earth's surface has changed through time, and that continents that are separated now may have been joined together at one point in the past.

Paleontologists had also found that there were fossils of similar species found on continents that are now separated by great geographic distance. Paleoclimate studies, which concerns examining the climate in Earth's past, revealed that glaciers covered large areas of the world which also are now separated by great geographic distances. These observations seemed to indicate that the Earth's lithosphere had been moving over geologic time.

Wegener's ideas were very controversial because he didn't have an explanation for why the continents moved, just that there was observational evidence that they had. At the time, many geologists believed that the features of the Earth were the result of the Earth going through cycles of heating and cooling, which causes expansion and contraction of the land masses. People who believed this were called the anti-mobilists. The mobilists were in the opposite camp and supported Wegener's ideas, since many of them had seen evidence for continental motion, especially in the Alps.

Although Wegener's "continental drift" theory was later disproved, it was one of the first times that the idea of crustal movement had been introduced to the scientific community; and it laid the groundwork for the development of modern plate tectonics. As years passed, more and more evidence was uncovered to support the idea that the plates move constantly over geologic time.

Paleomagnetic studies, which examine the Earth's past magnetic field, showed that the magnetic north pole seemingly wandered all over the globe. This meant that either the plates were moving, or else the north pole was. Since the north pole is essentially fixed, except during periods of magnetic reversals, this piece of evidence strongly supports the idea of plate tectonics.

Following World War II, even more evidence was uncovered which supports the theory of plate tectonics. In the 1960's a world-wide array of seismometers were installed to monitor nuclear testing, and these instruments revealed a startling geological phenomenon. It showed that earthquakes, volcanoes, and other active geologic features for the most part aligned along distinct belts around the world, and those belts defined the edges of tectonic plates.

In addition, further paleomagnetic studies revealed a striped pattern of magnetic reversals in the crust of the ocean basins. Basalt contains a fair amount of magnetic minerals called magnetite. When the lava from spreading centers in the oceans forms and cools, these minerals align to the north pole. The Earth has undegone several magnetic reversals in the past, in which the north and south poles are reversed for a period of time. When geologists and geophysicists discovered that the crust in the ocean recorded these reversals, it was even more positive proof that the lithosphere had to be in motion, otherwise there would be no "stripes" of normal and reversed polarity crust.

These were some of the final pieces of the puzzle that led to the development of modern plate tectonic theory. Since its emergence in the 1960's, plate tectonic theory has gained wide-spread acceptance as the model of Earth processes.

Structure of the Earth History of plate tectonics Plates Plate boundaries Forces in the Earth Faults Hypercard Resources

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Last modified on 8/13/98 by Maggi Glasscoe (scignedu@jpl.nasa.gov)