Phreatoplinianische Eruptionen: Vergleichende Fallstudien zur Herleitung ihrer Mechanismen
Zusammenfassung der Projektergebnisse
The objects of study were three widespread tephras in Central America: the c.84 ka Los Chcoyos Tephra (LCY) from Atitlan Caldera, Guatemala, and the 53 ka Congo Tephra (CGT) and 51 ka Conacaste Tephra (CCT) from the Coatepeque Caldera in El Salvador. We have selected three deposits from eruptions through preexisting calderas that most probably hosted a substantial lake. All three eruptions were driven by water-oversaturated magma expelled from uppercrustal, voluminous reservoirs. The two eruptions from Coatepeque caldera (CGT and CCT), separated by an about 2 ka time gap, produced very similar tephra successions. Prior to the terminal phases of highly unstable eruption producing various density-current deposits, the phreatoplinian eruption phase with the maximum affect of surface water on fragmentation intensity and moisture-flushed sedimentation occurred during a period of somewhat relaxed mass eruption rate but still high eruption velocity. The bracketing phases of higher mass eruption rates involved strong lithic entrainment by conduit erosion but the effect of entrained surface water on sedimentation was greatly reduced, approximating conditions for dry Plinian fallout. The typical accretionary-lapilli-rich phreatoplinian ash deposits of CGT and CCT are missing in the Los Chocoyos Tephra (LCY) from Atitlan caldera. The pumice lapilli fallout of LCY is relatively fine grained but well sorted with no indication of moisture-flushed sedimentation. The abundant fine ash of that eruption is only evident by the very widely dispersed distal fallout ash beds. There is no evidence for magma-water interaction in the LCY fallout but obviously there was very intense fine fragmentation. We attribute this fine fragmentation to the extremely highly vesicular, fluffy nature of the magma foam evidenced by very light pumices, to the high elongation rates during conduit flow shown by strongly stretched vesicles, and to the fine brittle fragmentation of vesicle walls during massive bubble coalescence as observed by micro-tomography.
