EVOLUTION OF DOME GROWTH AT MOUNT ST. HELENS IN 2004-2005: AIRBORNE OPTICAL AND THERMAL-INFRARED OBSERVATIONS

Airborne optical and thermal imaging documented the explosive and effusive activity at Mount St. Helens volcano during the eruption of 2004-2005. The forward-looking infrared (FLIR) camera and paired optical video camera was mounted beneath a helicopter with a gyrostablized gimbal during flights that varied from 100-1500 meters over the vent. Quantitative observations derive from sequential DEMs at 2-4 week intervals. Through June 2005 phases of activity are: (1) 9/23-10/5/2005 unrest and vent-clearing, (2) deformation, phreatic activity, and initial dome extrusion until mid-October, (3) repeated souhward recumbent dome growth and breakup until early April 2005, and, (4) of more vertical southward extrusion across previous dome debris since April. Phase 1 comprised high seismicity, surface deformation and phreatic explosions. The relatively cold (< 175 °C) phreatic eruptions of early October indicated a subglacial vent along the south-sloping margin of the 1980s dome. Low-temperature features like fumaroles, fractures, and ground warming related to rapid uplift and deformation in the south moat of the crater characterized phase 2. The rapidly uplifted and deformed area surrounding the vent was within a few degrees of ambient temperature. FLIR images documented increased heat flow that presaged extrusion of the October lava spine. A hot basal perimeter and highest temperatures (600-750 °C; below dry-dacite solidus) indicated vertical extrusion of solid rock. Southward movement, a hot U-shaped basal perimeter to the north, and a cold shroud of fault gouge characterized phase 3 recumbent growth of the whaleback-shaped dome. The whaleback migrated along the surface of the south crater floor until it encountered the crater wall, whereupon it broke up, decoupled, and reformed. Ongoing southward dome growth pushed cold deformed rock and hot dome rubble eastward instead of westward because of oblique incidence with the crater wall and because there was more space to the east. Dome migration pushed glacier ice eastward at 5 diminishing to 1 m/day. April “whaleback “breakup and growth of a lava spine across previous dome rubble to the west heralded phase 4 activity. Continued southward growth with an increasingly vertical component and increasing incidence of large rock falls has thus far characterized phase 4.