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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, B11201, doi:10.1029/2005JB003944, 2005

Thermal observations of gas pistoning at Kilauea Volcano

Jeffrey B. Johnson

Department of Earth Sciences, University of New Hampshire, Durham, New Hamphshire, USA


Andrew J. L. Harris

Hawaii Institute of Geophysics and Planetology, University of Hawaii, Manoa, Hawaii, USA


Richard P. Hoblitt

U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii, USA


Abstract

Data acquired by three continuously recording thermal infrared thermometers situated on the north rim of Pu`u `O`o Crater at Kilauea Volcano during 2002 revealed episodes of periodic thermal pulses originating from a degassing vent on the crater floor. These thermal pulses are interpreted as gas release (jetting events) associated with gas pistoning, a mechanism observed previously at both Mauna Ulu and Pu`u `O`o. During a 35-day-long period spanning June and July 2002, gas pistoning was frequently the dominant mode of gas release, with as many as several hundred pulses occurring in uninterrupted series. On other days, degassing alternated between periods of quasi-continuous gas jetting and intervals of gas pistoning that contained a few to a few dozen pulses. Characteristic time intervals between pistoning events ranged from 2 up to 7 min. We identify three types of pistoning. Type 1 involves emission of lava, followed by gas jetting and drain back; type 2 is the same but the elevated position of the vent does not allow postjet drain back; and type 3 involves gas jetting only with no precursory lava flow. To explain gas pistoning, we apply a model whereby a stagnant cap of degassed magma develops in the conduit below the vent. Gas bubbles rise through the magma column and collect under the cap. The collective buoyancy of these bubbles pushes the cap upward. When the cap reaches the surface, it erupts from the vent as a lava flow. Unloading of the conduit magma in this way results in an abrupt pressure drop (i.e., the overburden felt by the bubbles is reduced), causing explosive gas expansion in the form of gas jetting from the vent. This terminates the event and lava drains back into the conduit to start the cycle anew. In the case where there is no surface lava emission or drain back, the cap instead pushes into and spreads out within a subsurface cavity. Again, this unloads the conduit magma and terminates in explosive gas release. Once gas is expelled, lava in the cavity is free to drain back. We hypothesize that pistoning is a stable mode of degassing for low-viscosity basaltic magmas with appropriate conduit geometries and volatile supply rates.

Received 15 July 2005; accepted 29 July 2005; published 3 November 2005.

Keywords: gas pistoning; conduit processes; thermal monitoring; Kilauea Volcano; Pu`u `O`o.

Index Terms: 8414 Volcanology: Eruption mechanisms and flow emplacement; 8494 Volcanology: Instruments and techniques.

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Citation: Johnson, J. B., A. J. L. Harris, and R. P. Hoblitt (2005), Thermal observations of gas pistoning at Kilauea Volcano, J. Geophys. Res., 110, B11201, doi:10.1029/2005JB003944.