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Evolution of Volcanic Clouds from the 1993 Lascar, Chile Eruption

Heather L. Shocker
Michigan Technological University

Thesis for the Degree of M.S.

  Abstract. The 19-23 April 1993 volcanic clouds from Lascar volcano, Chile were studied with the AVHRR and TOMS satellite instruments. The major ash and gas producing eruption occurred 29 hours after the onset of activity. AVHRR detected a fine silicate ash cloud for four days and TOMS detected an SO2 cloud for 3 days as they moved ESE over the South American continent and Atlantic Ocean. The maximum amount of fine silicate ash (<15 micron radius) was 4750 kt, detected 45 hours after the onset, and the mass decreased to 430 kt 24 hours later. The maximum amount of SO2 detected was 300 kt, 40 hours after the onset and this mass decreased to 125 kt 47 hours later. The rate of decrease of the sliciate ash was faster than the SO2 with ash to SO2 mass ratios of 17:1 45 hours after the onset and 3:1 69 hours after the onset. Extrapolation of the sulfur massess back to the time of emission give an estimate of 450 kt of SO2. The amounts of sulfur dioxide released by the eruption are over two orders of magnitude more than would be expected by the volume of magma emitted (0.13 km^3) and the likely pre-erupted sulfur content.

Average rates of movement of silicate ash and SO2 clouds were determined in two segments as they drifted 1) 1700 km from the vent across the continent and 2) an additioal 800 km across the ocean. The silicate ash moved at an average rate of 20 m/s decreasing to 5 m/s over the continent and later increasing to 25 m/s over the ocean. By comparing cloud trajectories and radiosonde wind data, the silicate ash was constrained to heights of 8-10 km and the SO2 to heights of 12-15 km. This indicates a separation between the ash and gas components of the volcanic cloud. It is proposed that the separation can be explained largely by gravitational settling of the silicate ash. The terminal velocities of the silicate ash particles, which have a 6-9 micron effective radius, are sufficient to explain the 2-7 km height difference between the two components. These effective radii are larger than measured particle radii from other eruptions, suggesting a faster silicate fallout rate of the Lascar volcanic cloud.