Most recent eruption of the mono craters, eastern central California

Abstract

The most recent eruption at the Mono Craters occurred in the fourteenth century A.D. Evidence for this event includes 0.2 km 3 of pyroclastic fall, flow, and surge deposits and 0.4 km3 of lava domesa nd flows. These rhyolitic deposits emanated from aligned vents at the northern end of the volcanic chain. Hence we have named this volcanic episode the North Mono eruption. Initial explosions were Plinian to sub-Plinian events whose products form overlapping blankets of air fall tephra. Pyroclastic flow and surge deposits lie upon these undisturbed fall beds within several kilometers of the source vents. Extrusion of five domes and coulees, including Northern Coulee and Panurn Dome, completed the North Mono eruption. Radiocarbon dates and dendrochronological considerations constrain the eruption to a period between A.D. 1325 and 1365. The lack of lacustrine laminae or aeolian and fluvial beds between individual pyroclastic beds suggests that the explosive phases of the eruption took place over a period of not more than several months. Within the resolution of the available radiocarbon and dendrochronologic dates, the North Mono eruption is contemporaneous with the latest eruption of the Inyo volcanic chain, about 20 km to the south. However, the Inyo tephra blanket clearly overlies, and thus postdates, all North Mono tephra. Minor disturbance of the North Mono tephra prior to deposition of the Inyo tephra indicates that the period of time between the North Mono and Inyo eruptions was probably no more than a year or two. This near contemporaneity of the two eruptions suggests a genetic relationship. Liquefaction of North Mono sands on the floor of Mono Lake occurred twice during the waning stages of the North Mono eruption and 3 times immediately before and after pulses of the Inyo eruption. This is evidence that five earthquakes of M L > 5.5 occurred during the North Mono and Inyo eruptions. The chemical and textural similarity of the erupted products and their nearly simultaneous evacuation from aligned vents indicates that the North Mono eruption resulted from intrusion of a dike beneath the northern 6 km of the volcanic chain. Several observations suggest that dike intrusion beneath the Mono Craters has replaced normal faulting as the mechanism for elastic rebound and permanent extension of the crust at this latitude. However, dike widths compatible with relief of purely tectonic strains (<3 m) are probably too narrow to have allowed the North Mono magma to erupt. Overpressurtzation of the Mono Craters magma reservoir by another mechanism, perhaps magma mixing, appears necessary as well. The gradually diminishing explosiveness of the North Mono eruption probably resulted from a decrease of water content downward in the dike. The pulsating nature of the early, explosive phase of the eruption may represent the repeated rapid drawdown of slowly rising vesiculated magma to about the saturation depth of the water within it.