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