COLLAPSE-INDUCED EXPLOSIONS DURING THE 2008-2009 SUMMIT ERUPTION OF KILAUEA VOLCANO, HAWAI`I

After a 26-year hiatus in eruptive activity at the summit of Kilauea Volcano, a new eruption began with the explosive widening of a small gas vent in the Halema`uma`u pit crater in March 2008. The eruptive vent, initially about 30 m in diameter, grew to more than 130 m across by August 2009 and has hosted seven additional small explosions that deposited ash- to block-sized lithic and juvenile tephra in the summit area. Views of an actively overturning lava surface about 200 m below the vent rim have been sporadically visible through thick fume since September 2008, but bright glow, the production of juvenile ash, and vent noises consistent with spattering imply that the lava column has been present at relatively shallow levels within the vent throughout much of the eruption. Physical observations suggest that the explosive events were caused by vent rim and wall collapses into the top of this lava column. Since the opening of the vent, video, time-lapse imagery, field observations, and seismic data have shown that vent expansion has been the result of hundreds of discrete collapses of the vent walls and rim. While most collapses did little more than produce an ashy plume, five of the seven post-eruption-onset explosions were immediately preceded by vent-rim collapses, and all were preceded by high-frequency seismicity consistent with a rock-fall source. Seismic trigger counts, daily ash production, and the number of ash-producing collapse events ramped-up over a period of a few days preceding at least some of the explosive eruptions, suggestive of a slowly escalating process of stoping within the vent that culminated in the collapse of a large section of the upper vent walls and rim. Audible rock falls have been directly correlated with degassing events and ash emissions at Kilauea's summit. Furthermore, video captured looking into the summit vent clearly shows that minor degassing and spattering events can occur when rocks impact the top of the lava column. Observations of similar processes have been made at the Mauna Ulu and Pu`u `O`o vents, on Kilauea's east rift zone, in the 1970's and 1990's respectively. There, collapse of the vent walls into a lava pond resulted in vigorous degassing and spattering events. Thus, initiation of degassing events and, potentially, small explosive eruptions, may require little more than dropping rocks into a gas-charged lava column.