Eruptive intensity transitions from Plinian to effusive to caldera-forming: the 7.7 ka Cleetwood eruption of Mount Mazama, Oregon, USA

The 7.7 ka Cleetwood (Cltwd) eruption of Mount Mazama (Crater Lake, Oregon, USA) began with a Plinian phase emitting 0.5 km³ DRE of rhyodacitic pyroclastic fall material. Explosive activity then transitioned to an effusive stage, with no apparent break, extruding a 0.6 km³ obsidian lava flow. As opposed to other rhyolitic eruptions that exhibit the same sequence of events, the Cltwd eruption ultimately led to the climactic caldera-forming eruption of Mount Mazama. Thermal constraints on both the Cltwd flow backflowing into the caldera and the alteration of climactic pumice deposited directly on the Cltwd flow suggest that the Cltwd eruption preceded the caldera-forming eruption of Crater Lake by a period of only weeks to months. Furthermore, both eruptions produced chemically indistinguishable juvenile material inferring the tapping of the same magmatic source. By analyzing changes in particle size, shape, texture, porosity, and componentry that occur stratigraphically throughout the products of the Cltwd eruption, this study aims to link the shifts in eruption dynamics that occurred during this eruption with the initiation of the subsequent caldera-forming eruption. Preliminary results on the stratigraphy of proximal and medial deposits reveal four major eruptive stages: S1) initiation and initial vent clearing, S2) a sequence of maximum eruption intensity, S3) a pause in eruption followed by the last explosive pulse and S4) the effusion of the Cltwd flow and conduit sealing. Materials composing S1 are homogenous with a brief increased abundance in both banded pumice and obsidian pyroclasts. The layers that define S2 exhibit strong normal grading with tephra at its maximum size. S3 marks the end of the explosive phase with a layer of reverse grading and an increase in lithics, banded pumice, and obsidian pyroclasts. Detailed grain size distributions, componentry and textural analyses are currently being carried out for each stage of the eruption to better constrain and quantify any processes such as changes in conduit size, magma degassing, fragmentation depth, etc., which may cause or reflect a change in magma flow in the conduit and therefore, a shift in the type of activity.