Although it is the largest present-day lithospheric plate, the Pacific Plate is shrinking because most of its margins, apart from the East Pacific Rise and the Pacific-Antarctic Ridge, are subduction zones (e.g. the Aleutian, Kuril, Japan, Izu-Bonin, and Marianas subduction zones). The Pacific Plate is the only major plate to consist of nothing but oceanic crust, parts of which date from the Triassic period.