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  1. A zone within earth's mantle where plastic movements occur to permit isostatic adjustments. Begins 50 to 100 km below surface and extends perhaps to 500 km.
    Source: Leet, L. Don. 1982. Physical Geology, 6th Edition. Englewood Cliffs, NJ: Prentice-Hall
  2. The layer or shell of the Earth below the lithosphere, which has reduced yield strength, permitting viscous or plastic flow under relatively small stresses; it is a zone in which isostatic adjustments take place, magmas may be generated, and seismic waves are strongly attenuated. It is a part of the upper mantle. Syn: zone of mobility AGI
    Source: Dictionary of Mining, Mineral, and Related Terms

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