Fluctuating Eruption Style at Blue Lake Crater, Central Oregon Cascades: Insights from Deposit Granulometry and Componentry and Pyroclast Textures

Blue Lake crater in the central Oregon Cascades is one of the youngest Cascades volcanoes, erupting <3000 years ago. Based on the deep lake-filled crater excavated during the eruption, Blue Lake is commonly classified as a maar. However, little is known about this young eruption. This study focuses on mapping and detailed analyses of components and textures of the pyroclastic deposits to better understand changes in eruption style as the eruption progressed. Based on a new isopach map of the deposit thickness, the volume of erupted material is 4 x 107 m3. The deposits also suggest that the eruption was dominantly magmatic; phreatomagmatic surge deposits (<30 cm thick) occur locally at or near the base of the deposit but are overlain by much thicker scoria fall deposits. Detailed study of the pyroclasts from throughout the deposit reveal more subtle changes in the influence of external water over time. Granulometry from a complete deposit section (130 cm) reveals that the average grain size is finest immediately overlying the basal surge deposits and increases upsection. Componentry from this section shows that lithic and dense clasts are most abundant below and directly above the surges (near the deposit base) and decrease in abundance upsection, where vesicular scoria dominates. High magnification SEM BSE images of tephra clasts from throughout the deposit were obtained in order to better assess the changing role of external water during the eruption. Preliminary assessment of the images reveals that clasts from the eruption initiation have a glassier matrix compared to those upsection, which have a highly microcrystalline matrix, suggesting that early-erupted samples were likely quenched with external water. Taken together, these datasets suggest that the eruption initiated as dominantly phreatomagmatic but then rapidly transitioned to dominantly magmatic for the eruption duration. Further investigation of clast vesicularity and crystallinity will aid in understanding the changing influence of external water during the Blue Lake eruption.