The spectrum of basaltic feeder systems from effusive lava eruption to explosive eruption at Miyakejima volcano, Japan

Basaltic feeder systems exposed in the caldera wall of Miyakejima volcano are classified into three groups: (1) effusive feeders, (2) moderately explosive feeders, and (3) highly explosive feeders. The surface deposits and feeder systems reveal a wide variation in the explosivity of the eruptions that produced them, ranging from non-explosive lava effusions to violent explosive eruptions, despite the apparent lack of influence of external water. Effusive feeders are filled with coherent (non-fragmented) intrusive rock, indicating no significant fragmentation in the feeder system. The other two types of feeder systems consist of a coherent dike in their deeper part and a pyroclastic fill in their uppermost part. Their uppermost parts show an upward-flaring shape. The transition from coherent intrusion to pyroclastic fill in the feeder systems suggests underground fragmentation of the rising magma. The depth of the coherent–pyroclastic transition is deeper (20–150 m) in highly explosive feeders than in the moderately explosive feeders (<20 m), and coincides with the depth at which the system flares upwards. Presence of lithic fragments derived from the host rock in the products of the highly explosive feeder systems indicates the removal of the wall rock by explosive activity. This observation suggests that the fragmentation of rising magma promoted the enlargement of the feeder systems to form their upward-flaring shapes, by mechanical erosion and wall collapse.