Use of thermal infrared imaging for monitoring renewed dome growth at Mount St. Helens, 2004

A helicopter-mounted thermal imaging radiometer documented the explosive vent-clearing and effusive phases of the
eruption of Mount St. Helens in 2004. A gyrostabilized gimbal controlled by a crew member housed the radiometer and
an optical video camera attached to the nose of the helicopter. Since October 1, 2004, the system has provided thermal
and video observations of dome growth. Flights conducted
as frequently as twice daily during the initial month of the
eruption monitored rapid changes in the crater and 1980-86
lava dome. Thermal monitoring decreased to several times
per week once dome extrusion began. The thermal imaging
system provided unique observations, including timely recognition that the early explosive phase was phreatic, location
of structures controlling thermal emissions and active faults,
detection of increased heat flow prior to the extrusion of lava,
and recognition of new lava extrusion. The first spines, 1 and
2, were hotter when they emerged (maximum temperature
700-730°C) than subsequent spines insulated by as much as
several meters of fault gouge. Temperature of gouge-covered
spines was about 200°C where they emerged from the vent,
and it decreased rapidly with distance from the vent. The
hottest parts of these spines were as high as 500-730°C in
fractured and broken-up regions. Such temperature variation
needs to be accounted for in the retrieval of eruption parameters using satellite-based techniques, as such features are
smaller than pixels in satellite images.