ERUPTION DYNAMICS OF THE 7.7 KA DRIFTWOOD PUMICE-FALL, MAKUSHIN VOLCANO, ALASKA

The Driftwood Pumice deposit was produced ~7700 yr BP from a Plinian eruption of Makushin Volcano on Unalaska Island, AK. The pumice ranges in thickness from 13 cm to greater than 1.5 m over on area of greater than 140 km². It consists of vesicular, dacitic, tan-yellow pumice and dark grey-black scoria, as well as accidental ejecta. In the field, the deposit can be divided into four distinct stratigraphic horizons. The lowermost horizon (1-20 cm thick) is made up of small pumice clasts (~3 mm) and contains abundant vitric lithic fragments (22 wt%) suggesting that the eruption initiated through a pre-existing vitric capping dome. The second horizon makes up the main portion of the pumice (13-113 cm thick) and consists of coarser pumice (~11 mm) with a smaller amount of accidental ejecta (4.5 wt%), reflecting an increase in eruptive power and a relatively stable vent. The third horizon is 4-20 cm thick and contains both juvenile pumice and scoria (~ 6 mm), as well as abundant vitric and non-vitric accidental ejecta (35 wt%), which implies a significant destabilization and widening of the vent during the final stage of the eruption. A dark ash layer of lithic fragments, scoriaceous glass, and free plagioclase crystals make up the fourth horizon that caps the deposit. The deposit noticeably changes in color from tan-yellow to a distinctly darker grey-black in the upper two scoriaceous horizons.

Pumice and scoria from all four horizons contain phenocrysts and glomerocrysts of plagioclase, clinopyroxene, orthopyroxene, and titanomagnetite. The mineral populations of the pumice have the following compositions: plagioclase [An% ~50, range 41-59]; clinopyroxene [Mg# ~68, range 67-69]; orthopyroxene [Mg# ~62, range 59-65]. The pumice is dacitic in composition (~69% SiO₂) whereas the scoria is slightly more mafic (65-68% SiO₂) and mineral rich. The stratigraphic relationship between the differing types of juvenile ejecta suggests a stratified magma chamber, with buoyant siliceous magma overlying, and possibly comingling with, more cumulate-rich, mafic material. The nature of the eruption changed once the scoria began erupting, with the vent destabilizing and the ejected material being less vesicular, possibly reflecting a lower volatile content of the more mafic magma.