DYNAMICS OF CO-IGNIMBRITE PLUMES GENERATED FROM PYROCLASTIC FLOWS OF MOUNT ST-HELENS (7 AUGUST 1980)

Four co-ignimbrite plumes were generated along the flow path of the py roclastic flow of 7 August 1980 at Mount St. Helens. Three of the Flum es were generated in discrete pulses which can be linked to changes in slope along the channel. One plume was generated at the mouth of the channel where the flow decelerated markedly as it moved onto the lower slopes of the pumice plain. Plume generation here may be triggered by enhanced mixing due to a hydraulic jump associated with an abrupt slo pe change, Measurements of plume ascent velocity and width show that t he co-ignimbrite plumes increased in velocity with height, The plumes have initial velocities of 1-2 m/s. Two of the plumes reached a veloci ty maximum (4.6 and 8.8 m/s, respectively, at heights of 270 and 315 m above the flow) and thereafter decelerated. The other plumes reached velocities of 6.2 and 13 m/s. The four plumes become systematically le ss energetic downstream as measured by their ascent rates, which can b e interpreted as a consequence of decreasing interaction of the pyrocl astic flow front with the atmosphere. Theoretical models of both co-ig nimbrite plumes and discrete co-ignimbrite clouds assume that there is no initial momentum, and both are able to predict the observed accele ration stage. The rising plumes mix with and heat air and sediment out par-tides causing their buoyancy to increase. Theoretical models agre e well with observations and suggest that the initial motion of the as cending material is best described as a discrete thermal cloud which e xpands as it entrains air, whereas the subsequent motion of the head m ay become influenced by material supplied from the following plume. Th e models agree well with observations for an initial temperature of th e ash and air mixture in the range of 500-600 K, which is in turn cons istent with the measured initial ash temperature of around 920 K. Ash masses of 3.4 x 10(5) to 1.8 x 10(6) kg are estimated.