What is plate boundaries ?

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A plate boundary is a three-dimensional surface or zone across which there is a significant change in the velocity (speed or direction) of motion of one lithospheric plate relative to the adjacent lithospheric plate. Where subduction zones and ridge-ridge transform faults intersect the ground surface, the thickness of the shear zone that marks the plate boundary is small relative to its map length. The width of these plate boundaries might increase with depth as high-temperature deformation mechanisms become more important than the near-surface brittle mechanisms of fracturing, faulting, and cataclastic flow. We will refer to plate boundaries that are narrow at Earth's surface as discrete boundaries. A single curve on the map of a plate-boundary area is sufficient to identify where a discrete plate boundary intersects the ground surface, also known as the ground-surface trace of the boundary. In contrast to discrete boundaries, distributed boundaries are broad zones across which the motion of one lithospheric plate relative to the adjacent plate is accommodated. Distributed boundaries have also been called orogenic belts or deformation zones. The boundary between the North and South American plates, and the boundaries between the Capricorn and both the Indian and Australian plates, are possible examples of distributed boundaries. In 1970, it was recognized that the western part of the North American plate forms a broad transform boundary between the Pacific plate and the relatively rigid cratonic interior of North America, located east of the Rocky Mountains. Expanding use of GPS -based geodesy to measure crustal strain is continuing to add detail to that map. Initial ideas about triple junctions used the simplifying assumptions that plates are rigid and have definite, singular, discrete boundaries. We now know that many triple junctions involve plates whose boundary region adjacent to the triple junction undergoes measurable deformation. The simple kinematic and descriptive modeling described in this article will treat triple junctions as if they have discrete boundaries so that general kinematic statements can be made.