| Summary: | International Symposium on the Activity of Oceanic Volcanoes. Ponta Delgada, 4-9 August 1980. Rift zones are one of the characteristic features of Hawaiian volcanoes. They are long narrow zones of flank fissure eruptions but are distinct from ordinary flank eruption sites on po1ygenetic volcanoes in that eruptions, and therefore dike intrusions, occur repeatedly at the same general place for a long time and thus cause a considerable distance of horizontal spreading. This spreading should somehow be accommodated and the direction of the minimum compressive stress should remain the same after accommodation in order for a new dike to intrude in the same orientation. The Krafla spreading events in Iceland between North American and European plates revealed that the process of the lithospheric spreading is similar to that observed for Hawaiian volcanic activities, including rift zone eruptions. Accreting plate boundaries may be understood as consisting of chain of linear rift zones and their source polygenetic centers where the magma supplied from the asthenosphere is temporarily .stored. Horizontal spreading caused by repeated dike intrusions has been accommodated in the case of the accreting plate boundaries by the lateral separation of lithosphere over astenosphere. In the case of Hawaii sliding of the volcanic edifice over deep sea sediments may be the analogous mechanism such as appears to have occurred during the 1975 Kalapana earthquake which was anticipated by SWANSON et al. (1976) as one of the repeated events as the east rift zone has continuously dilated. Lack of rift zones in otherwise similar Galapagos shields which sit over the young ocean floor with higher relief is consistent with this view. info:eu-repo/semantics/publishedVersion
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