TEPHRA-FALL DEPOSITS FROM THE 1992 ERUPTION OF CRATER PEAK, ALASKA - IMPLICATIONS OF CLAST TEXTURES FOR ERUPTIVE PROCESSES

The 1992 eruption of Crater Peak, Mount Spurr, Alaska, involved three subplinian tephra-producing events of similar volume and duration. The tephra consists of two dense juvenile clast types that are identified by color, one tan and one gray, of similar chemistry, mineral assembl age, and glass composition. In two of the eruptive events, the clast t ypes are strongly stratified with tan clasts dominating the basal two thirds of the deposits and gray clasts the upper one third. Tan clasts have average densities between 1.5 and 1.7 g/cc and vesicularities (p henocryst free) of approximately 42%. Gray clasts have average densiti es between 2.1 and 2.3 g/cc, and vesicularities of approximately 20%; both contain abundant microlites. Aver age maximum plagioclase microli te lengths (13-15 mu m) in gray clasts in the upper layer are similar regardless of eruptive event (and therefore the repose time between th em) and are larger than average maximum plagioclase microlite lengths (9-11 mu m) in the tan clasts in the lower layer. This suggests that m icrolite growth is a response to eruptive processes and not to magma r eservoir heterogeneity or dynamics. Furthermore, we suggest that the l ow vesicularities of the clasts are due to syneruptive magmatic degass ing resulting in microlitic growth prior to fragmentation and not to q uenching of clasts by external groundwater.