Dome collapse and block-and-ash flow emplacement, Mono Craters, California
Dennen, Robert Llewellyn
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The ∼1340 A.D., rhyolitic North Mono eruption, Mono-Inyo Craters, CA, included the extrusion and destruction of an earlier dome in Panum Crater and its associated clastic deposits. One of these deposits, the Panum block-and-ash flow (BAF) deposit, consists of three main facies; (a) a debris avalanche facies, (b) a block-and-ash flow facies, and (c) a proximal concentration of rhyolitic reticulite. The debris avalanche facies is surrounded by the more widespread block-and-ash flow facies and is composed mainly of jigsaw fractured blocks up to more than a few meters in size. One block sits upstream of a long, thinning pile of block-and-ash flow facies debris; this "flow shadow" indicates that the block remained relatively stationary while the BAF propagated around it. Blocks of the block-and-ash flow facies show white, powdery marks interpreted to result from collisional impacts and frictional contacts among clasts. A proximal facies contains rhyolitic reticulite and bread-crusted obsidians with pressed-in clasts. Impact marks on block-and-ash flow facies blocks reveal that the majority of particle-particle impacts were of low energy, and had a larger collisional component; the flow head of the BAF shows an increase in impacts with a larger frictional component. A general decrease in the energy dissipated during particle-particle impacts occurred with increasing runout. Ground Penetrating Radar revealed intermingling units at distal locations within 1 km of Mono Lake, that, along with field observations, indicate that the distal reaches of the deposit were emplaced into Mono Lake, which was at a higher level than today. The previous dome to occupy Panum Crater is concluded to have been of similar size to the current Panum Dome. A conceptual model is presented for the destruction of the previous dome, including three separate, sequential events. An initial debris avalanche consisting of older, cooled dome material, a BAF, which was likely forced by underlying rhyolitic magma foam, and a "lateral expansion" of the molten rhyolitic foam that was exposed by the earlier events.