Ground-Based Thermal Infrared Monitoring of Dome Growth at Soufrière Hills Volcano, Montserrat

Activity at Soufrière Hills Volcano, Montserrat, has included the growth and partial collapse of a lava dome or dome lobe complex. During the latest period of magma eruption (Aug 2005-Apr 2007), thermal infrared monitoring was conducted from various ground and aerial positions using a hand-held Mikron Model TH7515 forward-looking infrared (FLIR) thermal imaging camera. The camera monitors at 8.0-14.0 μm, yielding apparent temperatures of <500C. Each pixel typically represented <1 m to 3 m resolution depending on observation position. Thermal observations were directed at the dome lobe complex, associated pyroclastic flow and rockfall deposits, and ash and gas vents. Early-stage dome growth showed pancake morphology with rubbly top surfaces at near-ambient to 100C apparent temperatures (T, uncorrected for emissivity and atmospheric conditions) and flanks at 150-300C. Dome morphology evolved after 1-3 months into shear lobes, including short-lived spines, fins, and rare whalebacks, and unstable oversteepened dome faces. Most surfaces of active dome lobes were relatively cool (T=35-100C). Isolated hot areas of 200-450C marked spall zones and expansion cracks in early stages, or pfs, rockfalls, and cracks in later stages. Active spines and fins typically had surfaces of 70-250C that reached 300C where cracked or spalling. Hotter (+25C to +75C) subhorizontal zones 2-10 m thick developed in the lower portion of the active shear lobe face in early 2006, perhaps as expansion cracks during endogeneous growth partly unconfined by crater walls. Overall, most thermal flux was not from the dome itself, but rather from rockfall and pf deposits at T>200C, and max T=300-400C. The major dome collapse on 20 May 2006 was preceded by a significant qualitative decrease in thermal flux in the preceding 2 weeks, both from the dome and surrounding rockfall and pf apron. The data suggest collapse triggered by increased internal dome pressure following a switch to more endogenous growth.