THE CHARACTER OF LONG-TERM ERUPTIONS - INFERENCES FROM EPISODES 50-53OF THE PUU-OO-KUPAIANAHA ERUPTION OF KILAUEA VOLCANO

The Pu'u 'O'o-Kupaianaha eruption on the east rift zone of Kilauea beg an in January 1983. The first 9 years of the eruption were divided bet ween the Pu'u 'O'o (1983-1986) and Kupaianaha (1986-1992) vents, each characterized by regular, predictable patterns of activity that endure d for years. In 1990 a series of pauses in the activity disturbed the equilibrium of the eruption, and in 1991, the output from Kupaianaha s teadily declined and a short-lived fissure eruption broke out between Kupaianaha and Pu'u 'O'o. In February 1992 the Kupaianaha vent died, a nd, 10 days later, eruptive episode 50 began as a fissure opened on th e uprift flank of the Pu'u 'O'o cone. For the next year, the eruption was marked by instability as more vents opened on the flank of the con e and the activity was repeatedly interrupted by brief pauses in magma supply to the vents. Episodes 50-53 constructed a lava shield 60 m hi gh and 1.3 km in diameter against the steep slope of the Pu'u 'O'o con e. By 1993 the shield was pockmarked by collapse pits as vents and lav a tubes downcut as much as 29 m through the thick deposit of scoria an d spatter that veneered the cone. As the vents progressively lowered, the level of the Pu'u 'O'o pond also dropped, demonstrating the hydrau lic connection between the two. The downcutting helped to undermine th e prominent Pu'u 'O'o cone, which has diminished in size both by colla pse, as a large pit crater formed over the conduit, and by burial of i ts flanks. Intervals of eruptive instability, such as that of 1991-199 3, accelerate lateral expansion of the subaerial flow field both by pr oducing widely spaced vents and by promoting surface flow activity as lava tubes collapse and be come blocked during pauses.