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plate margin (plate boundary) The boundary of one of the plates that form the upper layer (the lithosphere) and together cover the surface of the Earth. Plate margins are characterized by a combination of tectonic and topographic features: oceanic ridges, Benioff zones, young fold mountains, and transform faults. Plate margins are of three main types: (a) constructive margins where newly created lithosphere is being added to plates which are moving apart at oceanic ridges; (b) convergent margins which can be either destructive margins, where one plate is carried down into the mantle, beneath the bordering plate, at a subduction zone, or a collision zone, where two island arcs or continents, or an arc and a continent, are colliding; or (c) conservative margins, where two plates are moving in opposite directions to each other along a transform fault. All three margins are seismically active, with volcanic activity at constructive and destructive margins. Some plate margins exhibit features of more than one of the three main types and are known as combined plate margins. See PLATE TECTONICS; and SEA-FLOOR SPREADING.