Magma fragmentation and tephra deposition from a small-volume phreatomagmatic eruption: Blue Lake crater, Oregon, USA

Maars pose considerable hazards due to their more explosive nature (compared with more common scoria cones) and likelihood that eruptions produce pyroclastic surges. Blue Lake crater is a maar in the Oregon High Cascades that erupted within the last 3000 years, making it one of the youngest eruptions in the Oregon Cascades. Its young, unaltered deposits make it an excellent site to examine the relationship between fragmentation processes and ash characteristics. This paper presents an extensive data set of grain size, componentry, texture, particle morphology, and surface features for 23 samples from 17 layers from Blue Lake crater to better understand fragmentation style and eruptive dynamics over the course of the eruption. We present detailed stratigraphy from 22 tephra pits and analyze tephra samples following a standardized method. An improved isopach map and a new isopleth map show the extensive, ENE-trending fallout and surge deposits. Based on the tephra sheet, the eruption can be divided into three phases, starting with a phreatomagmatic phase that produced laterally extensive, lithic-rich fallout deposits and excavated the initial crater. The middle phase of the eruption produced phreatomagmatically fragmented intercalated fallout and surge deposits. The eruption closed with coarse grained fallout deposits with a declining lithic content, indicating a shift towards a hybrid or phreato-Strombolian style. This detailed examination of the deposits leads to a more nuanced explanation of the eruption and fragmentation dynamics, which contribute to a better understanding of maar eruption processes and hazards.