Earth Story

Photograph

By: Dean Hollingsworth

Date: 2003

Source: Hollingsworth, Dean. "New Scientist, Earth Story." The Dallas Morning News.

About the Photographer: Dean Hollingsworth is a graphics and news designer for the Dallas Morning News, a Texas-based newspaper with a daily circulation of over 100,000.

INTRODUCTION

Plate tectonics refers to the movement of rigid plates constituting Earth's lithosphere, and is the unifying theory of modern geoscience. There are a dozen major plates, all but one associated with continents, and several minor plates. The only major plate not associated with a continent is the Pacific Plate. The word "tectonics" is derived from a Greek root meaning "one who builds," and is sometimes used in reference to architecture (for example, architectonics). Used in a geological sense, however, tectonics refers to the architecture or construction of Earth's crust and lithosphere.

The earth's crust is defined in terms of the chemical composition of its rocks. It overlays the mantle, the boundary between the two being marked by an abrupt change in the speed of seismic waves known as the Mohorovicic discontinuity (often referred to simply as the Moho). The lithosphere, in contrast, is defined in terms of its brittle response to applied stresses and lies above the ductile—meaning soft but not liquid—asthenosphere. Although the term "plate tectonics" is often used synonymously with continental drift, geoscientists have largely abandoned use of the latter term. When the term "continental drift" is used, it refers to the movement of continents without regard to the underlying mechanism.

Geoscientists explain earthquakes, volcanoes, and the origin of most mountain belts in terms of interactions between adjacent plates. The earthquake-prone San Andreas fault, for example, is the boundary between the North American and Pacific plates. Volcanic eruptions and great earthquakes of the circum-Pacific Ring of Fire occur as the Pacific and Nazca plates are forced beneath adjacent plates and melted. The Himalaya, Earth's tallest mountain range, is being formed as the Indian plate pushes northward against the southern edge of the Eurasian plate.

The origin of tectonic features, such as mountain ranges and continental-scale sedimentary basins, and events such as earthquakes and volcanic eruptions have long been of interest to geoscientists. One of the major gaps in geoscientific knowledge before the discovery of plate tectonics was the absence of a mechanism that could explain geologic observations. Processes such as the shrinking of a gradually cooling Earth were proposed to explain some features of mountain belts, for example the buckling of layered sedimentary rocks into folds. Before the elucidation of plate tectonics, however, prevailing theories could account for only a limited number of observations and were generally unsatisfactory.

The idea that continents move is not new. Sir Francis Bacon (1561–1626), Galileo (1564–1642), and Benjamin Franklin (1706–1790) all noted the curious similarity between the east coast of South America and the western coast of Africa. Franklin, in particular, suggested that Earth's crust floated on a liquid core. During the early years of the twentieth century, German meteorologist Alfred Wegener (1880–1930) used the occurrence of identical fossils on continents separated by oceans as the basis for his hypothesis of continental drift. Geologists had long explained the distribution of fossils by postulating the existence of land bridges between the continents. Wegener, however, used the ages and distributions of the fossils to infer that the currently existing continents had once been part of a single supercontinent named Pangaea, which had broken apart into smaller continents about 200 million years ago.

Wegener was ridiculed by the scientific establishment and froze to death on a meteorological expedition to Greenland more than thirty years before his hypothesis was substantiated. Some of the most important data that led to the acceptance of moving continents was collected by American geologist Harry Hess (1906–1969), who towed a depth sounder behind a transport ship he commanded during World War II. Hess later published a paper in which he suggested that Earth's oceans are underlain by rocks that are pushed apart by magma welling up along the mid-ocean ridges, a process known as seafloor spreading. His idea was substantiated when British geologists Drummond Matthews (1931–1997) and Frederick Vine (1939–) discovered that ocean floors were composed of rock with alternating magnetic polarity. The theory of plate tectonics also proposed that oceanic crust is consumed when it is subducted beneath the edge of an adjacent plate, explaining curious patterns of earthquake occurrence along the edges of continents observed by seismologists Hugo Benioff and Kiyoo Wadati. In large part, the discoveries that led to widespread acceptance of plate tectonics were made possible by the development of instruments such as sensitive magnetometers and the global deployment of seismographs in the decades after World War II.

By 1970, a preponderance of scientific evidence led to the almost universal acceptance of plate tectonics. Technological advances such as global positioning system (GPS) receivers with millimeter accuracy and seismic tomography have since allowed geoscientists to measure plate movements and produce images of plate edges disintegrating as they are subducted.

PRIMARY SOURCE

MAP OF TECTONIC PLATES

See primary source image.

SIGNIFICANCE

Plate tectonics is the unifying theory of modern geoscience, and is used to explain phenomena such as earthquakes, volcanic eruptions, and the origin of mountain ranges. It also serves as a theoretical framework for natural resource exploration, helping to locate petroleum reservoirs and mineral deposits. Although some of the ideas that led to the discovery of plate tectonics date back as far as the seventeenth century, most scholars now recognize Alfred Wegener's early twentieth-century work as the foundation for subsequent studies that solidified the theory during the second half of the twentieth century.

FURTHER RESOURCES

Books

McPhee, John. Annals of the Former World. New York: Farrar, Straus and Giroux, 2000.

Oreskes, Naomi. Plate Tectonics: An Insider's History of the Modern Theory of the Earth. Cambridge, MA: Westview Press, 2003.

Web sites

Sample, Sharon. "On the Move Continental Drift and Plate Tectonics." National Aeronautics and Space Administration (NASA). 〈http://kids.earth.nasa.gov/archive/pangaea〉 (accessed March 8, 2006).

Kious, W. Jacquelyne, and Robert I. Tilling. "This Dynamic Earth: the Story of Plate Tectonics." U.S. Geological Survey, February 1996. 〈http://pubs.usgs.gov/gip/dynamic/dynamic.html〉 (accessed March 8, 2006).