Abstract:
The 1967-1968 eruption of Metis Shoal, Tonga, was evidentially similar to the frequent shallow submarine eruptions of the inner island arc of Tonga. The eruption began about 10 December 1967, and an island eventually emerged; by 19 February 1968, the island had been eroded to beneath wave base. The eruptions were characterized by explosions of steam and ash which hurled bombs a few to several hundred feet into the air. The rocks ejected are pumiceous dacites which, for their silica content, have unusually low alkali contents and rare earth-element contents. The chemical characteristics of the dacite are hard to account for by partial melting of an ocean-ridge basalt parent. The peculiar properties of the dacite appear to characterize other Tongan lavas and support the idea that Tonga is part of a distinct petrographic province.
