We demonstrate the use of Geostationary Operational Environmental Satellite (GOES) images of the May 18, 1980, Mount St. Helens volcanic plume in providing details of the dynamics and changing character of this major explosive eruption. Visible and thermal infrared (IR) data from a sequence of images at 30-min intervals from 0850 to 1720 Local Time (LT) give information on dispersal and plume top temperature. Initial visible and IR images at 0850 show the top of a spreading co-ignimbrite-like umbrella plume and an overshooting column emerging from it, both rising off the ground-hugging pyroclastic gravity flow generated by the opening directed blast. The overshooting column had a minimum temperature significantly colder than local ambient atmosphere, indicating substantial undercooling, and a maximum altitude of 31¿2 km at 0920. This large plume system then formed a high-velocity, radially spreading, gravitationally driven current before becoming advected in the wind field at an average downwind velocity of 29 m/s. Within 30 to 60 min the undercooled plume adjusted to ambient temperatures. After this initial event, the eruption column fluctuated over the next 8 hours between stable Plinian column rise and unstable conditions favoring column collapse, leading to pyroclastic flows and further development of co-ignimbrite ash columns. Over this period of observation, profiles of plume top temperatures derived from IR data versus distance along the downwind axis have distinctive patterns, with steep slopes associated with co-ignimbrite plumes and shallower slopes associated with Plinian plumes. Moreover, reflectance values from visible GOES data change from lower to higher during periods of transition from darker toned Plinian to lighter toned co-ignimbrite plumes indicating that in this case satellite data resolved changes in eruptive style from plumes with a coarser to a finer dominant particle size. ¿ American Geophysical Union 1995