Ground-Based Thermal Analysis of Degassing Activity at Pu'u O'o

For the last 4 years the ground-based thermal monitoring system at Pu'u O'o (Kilauea, Hawaii) has allowed a complete and continuous description of the degassing and thermal activity. The monitoring system consists on 3 Omega IR thermometers, sensitive between 8-14 microns. A 60° aperture sensor tracks a large area (FOV ∼ 150 m diameter) across the crater floor, thus providing a thermal over-view of the crater floor, while two 1° radiometers (FOV ∼ 2.5 m) allow detailed detection of degassing activity from single vents. Ground-based thermal observations reveal that degassing at Pu'u O'o consists mainly in sustained degassing, where gas is continuously emitted as intermittent gas puffs, and gas pistoning events. The continuous record points to systematic fluctuations of thermal data. Here during the period Jan 2001-Dec 2003 activity at Pu'u O'o has fluctuated between periods of high and low rates of thermal activity. This is reflected generally across the whole system, so that multiple vents show similar long-term fluctuations. At shorter time scales differences in the degassing mode between vents may be observed, with gas pistoning events concentrated mainly at one vent and sustained degassing at the others. Gas pistoning at Pu'u O'o is recorded thermally as an abrupt increase in temperature ( ∼ 50-150 ° C), which lasts ∼ 100-300 sec. The events are always preceded by a significant decrease in temperature, which points to a net drop in degassing. This is consistent with seismic data, which show a contemporary decrease in seismic energy before gas pistoning events. Statistical analysis over ∼ 5000 gas pistoning events extracted over the four year-monitoring reveals a typical recurrence of ∼ 8-9 minutes. Stacking of the events reveals two different types of thermal waveform associated with gas pistoning events: the first consists of a single thermal peak, with an amplitude of ∼ 50-100 ° C, and lasts 100-200 sec, while the second lasts longer ( ∼ 200-300 sec), and shows a precursory thermal peak immediately followed by a second major pulse ( ∼ 50-150 ° C). Such events are usually followed by periods of sustained degassing. The two different waveforms of gas pistoning appear to correlate with periods of low and high degassing respectively. Sustained degassing activity at Pu'u O'o is also recorded by the 1° FOV radiometers as intermittent, high frequency, thermal pulses, which reflect intermittent gas puffing. Here our analysis focuses mainly on puffing frequency, rather than on thermal amplitude, where the latter is strongly influenced by external conditions (visibility, targeting of the sensor, etc.), while the former is a direct expression of the source process. Our investigation points to long term fluctuations in thermal activity at Pu'u O'o, where intermittent gas puffing and occurrence of gas pistoning events are the main features of degassing activity and may reflect fluctuations in the gas and mass fluxes, or a shallow system mechanism of collection and release of gas slugs. The detailed analysis of these different degassing modes, and relationships between them, is thus critical to gain new information on the degassing processes at Pu'u O'o.