Morphology and Formation of Glassy Volcanic Ash from the August 12-15,1991.

SCASSO, R.A.; CAREY, S.

Pucón, Chile

Congreso; IAVCEI General Assembly 2004; 2004

IAVCEI (Interantional Association of Volcanologists ....)

The 1991 explosive eruption of Hudson volcano in Chile ejected about 2.7 km3 (DRE) of basalt and trachyandesite magma as tephra fall. A majority of the fallout occurred from a plinian eruption during the period August 12-15, 1991, producing an extensive deposit to the east of the volcano in Chile and Argentina. Glassy tephra exhibit a remarkable variety in particle morphology and color. However, microprobe analyses demonstrate that the variability in color and texture is not related to magma composition. Fractal analysis of glassy particle outlines show that at least four different types can be discriminated: blocky, poorly-vesicular, highly vesicular with spherical vesicles, and pipe vesicular. A comparison of particle morphologies using factor analysis of fractal spectrums with other tephra produced by eruptions with known fragmentation mechanisms, suggests that production of the Hudson tephra involved both interaction with external water and exsolution of dissolved volatiles. Microprobe analytical totals of glassy Hudson tephra are positively correlated with degree of vesiculation suggesting that particle morphology was not controlled by variations in dissolved volatile components. Blocky particles with low analytical totals likely represent magma that was fragmented by magma-water interactions before exsolution of all dissolved volatiles could occur. The great diversity of particle types in the August 12-15, 1991 Hudson tephra fall can be attributed to eruption of volatile-rich trachyandesite magma through an ice-filled caldera where subglacial melting led to a supply of external water that interacted intermittently with the discharge of magma driven primarily by volatile