zone [Gr.,=girdle], in geography, area with a certain physical and/or cultural unity that distinguishes it from other areas. The division of the earth into five climatic zones probably originated (5th cent. B.C.) with Parmenides, who recognized a torrid zone (see tropics) and north and south temperate zones and postulated north and south frigid (or arctic) zones; his classification was adopted by Aristotle and is still in use. The zones are based on latitude: the torrid zone lies between 231/2°N and 231/2°S, the temperate zones between these parallels and the polar circles (661/2° N and S), and the frigid zones from the polar circles to the poles. Later geographers, recognizing that climate is affected by such conditions as altitude, distance from water, prevailing winds, and ocean currents, have used other bases for zoning. Most geographers today recognize five major climatic groups, based mainly on the work of the German meteorologist Wladimir Köppen. Two of these groups—the rainy tropics and the dry tropics, which encompass four different climates—together correspond roughly to the former torrid zone. Two humid climate groups of the Köppen system, encompassing six climates, together correspond roughly to the former temperate zones. Köppen's two polar climates correspond roughly to the two former frigid zones. In addition to the five groups encompassing twelve climates, geographers also recognize a series of highland zones where many of the other climates of the world are duplicated. Geographic zones in which people have similar patterns of life are called culture zones or areas (see culture). An example would be the plains area of North America.

urban enterprise zone: see enterprise zone.

torrid zone: see tropics.

time zone: see standard time.

subduction zone, large-scaled narrow region in the earth's crust where, according to plate tectonics, masses of the spreading oceanic lithosphere bend downward into the earth along the leading edges of converging lithospheric plates where it slowly melts at about 400 mi (640 km) deep and becomes reabsorbed. Subduction zones are usually marked by deep ocean trenches that often exceed 6 mi (10 km) compared to the ocean's overall depth of 2 to 4 mi (3 to 5 km). A pattern of earthquakes of shallow, intermediate, and deep focus occurs along the same angle as the descending plate, which is steeply inclined (30°-60°) toward the continent behind the trench in a zone called the Benioff Zone, discovered by the U.S. seismologist Hugo Benioff. This earthquake pattern enables geophysicists to trace the descending plate to depths of 600 to 700 km (370-440 mi), where temperatures are thought to be between 1,000°C; and 2,000°C; (1,800°-3,600°F;). As the oceanic plate descends, friction between the two plates probably causes partial melting of the descending plate forming a magma of andesitic composition that rises along fractures. If the overlying crustal plate is oceanic, the magma may erupt to form volcanic island arcs, such as Japan or the Aleutians. If the overlying plate is continental, a line of batholiths and volcanoes may be created as in the Coast Ranges of Canada and the W United States. See continent; continental drift; seafloor spreading.

pelagic zone: see ocean.

littoral zone: see ocean.

frigid zone: see Arctic, the; Antarctica; zone.

euphotic zone: see ocean.

enterprise zone, designated geographical district in which resident businesses are legally entitled to receive special benefits from a government, established in economically depressed areas to encourage companies to locate there. Most states enacted enterprise zone programs during the 1980s, and changes in the federal tax code in 1993 instituted tax incentives for businesses in certain locations, sometimes referred to as empowerment zones. Typical incentives offered to attract businesses to enterprise zones include tax credits, subsidized loans, and reduced regulations. Although usually associated with urban areas, enterprise zones have also been set up in rural areas. Most enterprise zones have only been marginally successful in reviving the areas in which they are located, in part because businesses located in such zones often employ workers who live outside the area.

aphotic zone: see ocean.

Panama Canal Zone, former territory within Panama, 553 sq mi (1,432 sq km), that was administered by the United States under a 1903 treaty (with later amendments) with Panama. The zone included the Panama Canal and an area extending 5 mi (8.1 km) on each side. Under the terms of a later treaty (1977), the zone was abolished in Oct., 1979, and returned to Panamanian rule; the canal itself was ceded to Panama in 1999. Panama has begun turning former military installations into housing, civilian ports, hotels, industrial parks, universities, and sports facilities. Some bases have been converted into tourist complexes aimed at attracting cruise-ship passengers, while some undeveloped areas have become eco-tourist sites. The northern section of the former territory is now part of Colón prov., and the southern section part of Panamá prov. Cristóbal, Balboa (Caribbean and Pacific ports, respectively), and Ancon are the chief towns.

Canal Zone: see Panama Canal Zone.

Region of apparent absence of galaxies near the plane of the Milky Way Galaxy. Millions of external galaxies are in this region of the sky but are obscured by interstellar dust within the Milky Way. The zone's obscuration can be penetrated at the infrared portion of the electromagnetic spectrum. Seealso infrared astronomy.

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Any of a group of techniques used to purify an element or a compound or to control its composition by melting a short region (called a zone) and causing this liquid zone to travel slowly through a relatively long ingot, or charge, of the solid. In zone refining, the most important of the zone-melting techniques, a solid is refined by multiple molten zones being passed through it in one direction. Each zone carries a fraction of the impurities to the end of the solid charge, thereby purifying the remainder. Zone refining is particularly important as a method of purifying crystals, especially for use in semiconductor devices.

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Long, narrow, and mountainous submarine lineation that generally separates ocean-floor ridges differing in depth by as much as 1 mi (1.6 km). The largest fracture zones, in the eastern Pacific, are more than 1,000 mi (1,600 km) long and 60–125 mi (100–200 km) wide. Numerous shorter fracture zones in the Atlantic are associated with the Mid-Atlantic Ridge.

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Oceanic trench area in which, according to the theory of plate tectonics, the seafloor underthrusts an adjacent plate, dragging the accumulated trench sediments downward into the Earth's upper mantle. Seealso deep-sea trench.

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Area within which goods may be landed, handled, and re-exported freely. The purpose is to remove obstacles to trade and to permit quick turnaround of ships and planes. Only when the goods are moved to consumers within the country in which the zone is located do they become subject to tariffs and customs regulation. Free-trade zones are found around major seaports, international airports, and national frontiers; there are more than 200 such zones in the U.S. alone.

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or Panama Canal Zone

Strip of territory, a historic administrative entity in Panama over which the U.S. formerly exercised jurisdictional rights (1903–79). The zone came into being in 1904 when Panama granted the U.S., in return for annual payments, sole right to operate and control the Panama Canal, including a strip of land 10 mi (16 km) wide along the canal extending from the Atlantic to the Pacific Ocean and bisecting the Isthmus of Panama. The zone was abolished by treaty in 1979, and civil control of the territory was returned to Panama. By the same treaty a commission under joint U.S.-Panamanian ownership was established to operate the canal until the year 2000, when Panama assumed full control.

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