NEW EVIDENCE OF A LARGE ERUPTION IN THE ISLANDS OF THE FOUR MOUNTAINS, AK

This study is the first documentation of a young (Pleistocene to Holocene) welded pyroclastic density current (PDC) deposit identified in the Islands of the Four Mountains (IFM). The IFM make up a cluster of small stratocones [5-10 km diameters, with elevations 1300-1700 m] in the east-central Aleutian volcanic arc, comprising Tana, Cleveland, Carlisle, and Herbert volcanoes. Despite the frequent activity of Mt. Cleveland, there is little published geological data on the IFM, and until 2014 all volcanic monitoring was conducted remotely. Without knowledge of the geologic nature of these islands, it is difficult to constrain the potential scale of future eruptions. A widespread welded PDC deposit on the northern flank of Tana is evidence of a substantial previous eruption. The deposit is ~5-10 m thick and densely welded with large [5-15 cm] misshapen fiamme and flattened clasts throughout. It has a bimodal composition with basaltic andesite to andesite clasts [56 wt. % SiO₂, 4.2 wt. % MgO] and dacite matrix [67.6 wt. % SiO₂, 1.25 wt. % MgO]. Minerals in the matrix show various disequilibrium textures in thin section, possibly implying some extent of mixing or magma recharge prior to eruption. The matrix has glassy groundmass with ~15% plagioclase phenocrysts and <1% biotite. The clasts are coarse grained with ~70% plagioclase phenocrysts, and ~2-3% clynopyroxene. At present, the source of this deposit is unknown, but the large size of the clasts [2-15 cm] suggests that the sampled outcrop is very near the vent.