plate tectonics, theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history.

Development of Plate Tectonics Theory

The beginnings of the theory of plate tectonics date to around 1920, when Alfred Wegener, the German meteorologist and geophysicist, presented the first detailed accounts of how today's continents were once a large supercontinent that slowly drifted to their present positions. Others brought forth evidence, but plate tectonics processes and continental drift did not attract wide interest until the late 1950s, when scientists found the alignment of magnetic particles in rock responded to the earth's magnetic field of that time. Plotting paleomagnetic polar changes (see paleomagnetism) showed that all continents had moved across the earth over time.

Synthesized from these findings and others in geology, oceanography, and geophysics, plate tectonics theory holds that the lithosphere, the hard outer layer of the earth, is divided into about 7 major plates and perhaps as many as 12 smaller plates, c.60 mi (100 km) thick, resting upon a lower soft layer called the asthenosphere. Because the sides of a plate are either being created or destroyed, its size and shape are continually changing. Such active plate tectonics make studying global tectonic history, especially for the ocean plates, difficult for times greater than 200 million years ago. The continents, which are c.25 mi (40 km) thick, are embedded in some of the plates, and hence move as the plates move about on the earth's surface.

The mechanism moving the plates is at present unknown, but is probably related to the transfer of heat energy or convection within the earth's mantle. If true, and the convection continues, the earth will continue to cool. This will eventually halt the mantle's motion allowing the crust to stabilize, much like what has happened on other planets and satellites in the solar system, such as Mars and the moon.

Plate Boundary Conditions

There are numerous major plate boundary conditions. When a large continental mass breaks into smaller pieces under tensional stresses, it does so along a series of cracks or faults, which may develop into a major system of normal faults. The crust often subsides, forming a rift valley similar to what is happening today in the Great Rift Valley through the Red Sea. If rifting continues, a new plate boundary will form by the process of seafloor spreading. Mid-ocean ridges, undersea mountain chains, are the locus of seafloor spreading and are the sites where new oceanic lithosphere is created by the upwelling of mantle asthenosphere.

Individual volcanoes are found along spreading centers of the mid-ocean ridge and at isolated "hot spots," or rising magma regions, not always associated with plate boundaries. The source of hot-spot magmas is believed to be well below the lithosphere, probably at the core-mantle boundary. Hot-spot volcanoes often form long chains that result from the relative motion of the lithosphere plate over the hot-spot source.

Subduction zones along the leading edges of the shifting plates form a second type of boundary where the edges of lithospheric plates dive steeply into the earth and are reabsorbed at depths of over 400 mi (640 km). Earthquake foci form steeply inclined planes along the subduction zones, extending to depths of about 440 mi (710 km); the world's most destructive earthquakes occur along subduction zones.

A third type of boundary occurs where two plates slide past one another in a grinding, shearing manner along great faults called strike-slip faults or fracture zones along which the oceanic ridges are offset. Continental mountain ranges are formed when two plates containing continental crust collide. For example, the Himalayas are still rising as the plates carrying India and Eurasia come together. Mountains are also formed when ocean crust is subducted along a continental margin, resulting in melting of rock, volcanic activity, and compressional deformation of the continent margin. This is currently happening with the Andes Mts. and is believed to have occurred with the uplift of the Rockies and the Appalachians in the past.

Movement of the Continents

According to plate tectonics, the ocean basins are viewed as transient features that have periodically opened and closed, first rending and then suturing the continental masses, which are permanent features on the earth's surface. Geologists now believe that the continents were sutured together 200 million years ago at the beginning of the Mesozoic era to form a supercontinent named Pangaea. Initial rifting along the Tethys Sea formed a northern continental mass, Laurasia, and a southern continental mass, Gondwanaland. Then plate movements caused North American and Eurasian separation coincidentally with the separation of South America, Africa, and India. Australia and Antarctica were the last to separate. The major plates are named after the dominant geographic feature on them such as the North American and South American plates.

Plate motions are believed to have transported large crustal blocks several thousand miles, suturing very different terrains together after collision with a larger mass. These "exotic" terrains may include segments of island arcs quite unrelated to the history of the continent onto which they are sutured. Some geologists believe that continents grow in size primarily by the addition of exotic terrains.

plate: see electron tube.

Sheffield plate, metalware of copper, silver-plated by fusion, originated at Sheffield, England. This process of plating was discovered c.1742 by a Sheffield cutler, Thomas Boulsover, who found while doing repair work on silver and copper that they fused at high temperature and could be hammered and shaped as one metal. He used his discovery to make buttons and buckles, but an apprentice, Joseph Hancock, grasped the broader application and began the production of tableware and other domestic articles that won wide popularity as substitutes for the more expensive solid silver. The manufacture spread not only in England, where Birmingham became an active center of production, but to the Netherlands, Russia, and Poland, where English methods and patterns were adopted. Similar ware was produced in France by a different process. Sheffield plate followed, in general, the contemporary styles in silver, but some original designs were used and in the 19th cent. characteristic flat-chased pieces developed. Early ware was plated on one side only, but c.1765 a method for plating both sides was introduced. Edges were at first soldered, then concealed with plated wire and finally with applied silver edges. Additional silver was embedded in areas to be engraved. German silver, an alloy of nickel, zinc, and copper, came into common use c.1835 and was preferred to copper as a base, since it showed less where the plating wore off. Special hallmarks were used after 1784. Sheffield plate was superseded c.1840 by the cheaper electroplating method.

Plate, River: see Plata, Río de la.

Theory that the Earth's lithosphere (the crust and upper portion of the mantle) is divided into about 12 large plates and several small ones that float on and travel independently over the asthenosphere. The theory revolutionized the geological sciences in the 1960s by combining the earlier idea of continental drift and the new concept of seafloor spreading into a coherent whole. Each plate consists of rigid rock created by upwelling magma at oceanic ridges, where plates diverge. Where two plates converge, a subduction zone forms, in which one plate is forced under another and into the Earth's mantle. The majority of the earthquakes and volcanoes on the Earth's surface occur along the margins of tectonic plates. The interior of a plate moves as a rigid body, with only minor flexing, few earthquakes, and relatively little volcanic activity.

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Terminal or electrode from which electrons leave a system. In a battery or other source of direct current, the anode is the negative terminal. In a passive load it is the positive terminal. In an electron tube, electrons from the cathode travel across the tube toward the anode; in an electroplating cell, negative ions are deposited at the anode.

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Articles made of copper coated with silver by fusion. The technique was discovered circa 1742 by the Sheffield (Yorkshire, Eng.) cutler Thomas Boulsover, who noted that the combination of fused silver and copper retained the ductility of both metals and acted as one when manipulated. Other workshops in Britain, continental Europe, and North America also produced cooking and eating utensils of Sheffield plate. After the introduction of electroplating in 1840, production of Sheffield plate declined; by the 1870s it had all but disappeared. Admired for its soft, glowing gray lustre, Sheffield-plate ware soon came to be prized and collected.

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Estuary of the Paraná and Uruguay rivers, between Uruguay and Argentina. It is about 180 mi (290 km) long, with a maximum width of 136 mi (219 km) at its mouth, but it narrows gradually to the northwest as it approaches Buenos Aires. Discovered by the Spanish in 1516, it was explored by Ferdinand Magellan in 1520 and by Sebastian Cabot (1526–29). The first permanent settlement in the area was at Asunción in 1537.

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