なぜハワイ火山では長いrift zoneが生ずるのか : 厚い海底堆積物の役割

抄録

Rift zones are characteristic features of Hawaiian volcanoes. They are long narrow zones of flank fissure eruptions but are distinct from ordinary flank eruption sites on stratovolcanoes in that eruptions, and therefore dike intrusions, occur repeatedly at the same general place for a long time and thus cause a considerable amount of lateral spreading. This spreading should somehow be accomodated. Moreover, the stress field should remain the same after accomodation in order for a new dike to intrude in the same orientation. The current spreading episode in Iceland (BJORNSSON et al., 1979) between North American and European plates revealed that the sequence of events in the spreading process is similar to that observed for Hawaiian volcanic activities. This implies that the process of plate separation and accretion is nothing but the activity of rift zones. Constructional plate boundaries may be regarded as composed of a chain of rift zones and associated feeding polygenetic centers. Room necessary for repeated dike intrusion is supplied in the case of spreading centers, by the lateral motion (separation) of lithosphere over asthenosphere. In the case of Hawaii, sliding of the volcanic edifice over a deep sea sediment layer may be the analogous mechanism such as appears to have occurred during the 1975 Kalapana earthquake, as studied by ANDO (1979) and FURUMOTO and KOVACH (1979). Kalapana earthquake had been anticipated by SWANSON et al. (1976) as one of the repeated steps as the east rift zone has continuously dilated. Thus, the primary cause for the long, well developed rift zones of Hawaiian volcanoes may be in the existence of thick enough oceanic sediments serving as a potential sliding plane beneath the volcanic edifices. Lack of rift zones in Galapagos shields which grew over the young ocean floor with rough topography is consistent with this view.