The present lack of volcanic activity in the Western Aleutian Arc correlates with the lack of underthrusting at the western Aleutian trench predictable by plate tectonic theory. A simple plate model allows one to predict that another plate (Kula plate) and a spreading ridge (Kula ridge) lay south of the arc in early Tertiary time and that the Aleutian trench consumed the ridge in the middle Tertiary. Early Tertiary volcanism in the Western Aleutians may be related to underthrusting by the Kula plate. A mid-Tertiary orogeny in the Central Aleutians and a late Tertiary orogeny in Alaska may be related to the subduction of the Kula ridge.

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