The Geology of the Samoan Islands

Abstract

The Samoan chain consists of high volcanic islands, atolls, and submerged reef banks near the southwest margin of the Pacific plate. The chain is unusual, particularly when compared with the Hawaiian chain, because the islands are volcanically active on both the eastern and western ends of the chain, the islands are larger westward, the easternmost edifice is an atoll not an active seamount, and the chain consists dominantly of alkali rather than tholeiitic lavas. While geological studies of the Samoan group are limited, the existing results are consistent with a hot spot origin similar to Hawaii, complicated by continued reactivation of volcanism on Savaii. The continuing volcanism on Savaii is believed to be the result of deformation of the margin due to lithospheric dilation, as the plate bends where it approaches the Tonga Trench subduction zone. The dominance of alkalic volcanism in this island chain has recently been associated with a geochemical heterogeneity in the underlying mantle.