The 1976 1982 Strombolian and phreatomagmatic eruptions of White Island, New Zealand: eruptive and depositional mechanisms at a `wet' volcano

摘要: White Island is an active andesitic-dacitic composite volcano surrounded by sea, yet isolated from sea water chemically sealed zones that confine a long-lived acidic hydrothermal system, within thick sequence of fine-grained volcaniclastic sediment and ash. The rise at least 106 m3 basic andesite magma to shallow levels its interaction with the system resulted in longest historical eruption 1976–1982. About 107 mixed lithic juvenile ejecta was erupted, accompanied collapse form two coalescing maar-like craters. Vent position craters changed 5 times during eruption, but vents were repeatedly re-established along line linking pre-1976 vents. consisted seven alternating phases phreatomagmatic Strombolian volcanism. eruptions preceded followed mildly explosive degassing production incandescent, blocky ash margins body. Phreatomagmatic contained styles activity: (a) near-continuous emission gas (b) discrete explosions prolonged quiescence. activity reculted streaming magmatic volatiles phreatic steam through open conduits, frittering juvennile shards eroding loose particles unconsolidated wall rock. larger produced ballistic block aprons, downwind lobes fall tephra, cohesive ‘wet’ surge deposits confined main crater. key features their focus, random occurrence lack precursors, thermal heterogeneity ejecta. This unusual because low discharge rate over extended time period influence unique physical hydrological setting. led very effective separation high fluxes even eruption. While true did occur, more frequently decoupled rose interact conduit walls producing eruptions. these governed delicate balance between erosion weak walls. If relatively stable, fine slowly eroded erupted weak, near-continous events. When unstable, triggered explosions.