Small-volume basaltic volcanoes: Eruptive products and processes, and posteruptive geomorphic evolution in Crater Flat (Pleistocene), southern Nevada

Abstract

Five Pleistocene basaltic volcanoes in Crater Flat (southern Nevada) demonstrate the complexity of eruption processes associated with small-volume basalts, and the effects of initial emplacement characteristics on posteruptive geomorphic evolution of the volcanic surfaces. The volcanoes record explosive processes ranging from classical Strombolian mechanisms to, potentially, violent Strombolian mechanisms. Cone growth was accompanied and sometimes disrupted by effusion of lavas from the bases of cones. Pyroclastic cones were built upon a gently southward-sloping surface and were prone to failure of their downslope (southern) flanks. Early lavas flowed primarily southward and, at Red and Black Cone volcanoes, carried abundant rafts of cone material on the tops of the flows. These resulting early lava fields eventually built platforms such that later flows erupted from the eastern (at Red Cone) and northern (at Black Cone) bases of the cones. Three major surface features¯scoria cones, lava fields with abundant rafts of pyroclastic material, and lava fields with little or no pyroclastic material-experienced different posteruptive surficial processes. Failure to account for the different initial surface features can lead to erroneous conclusions about the relative ages of the surfaces based upon modern morphology. Conversely, the recognition of different volcanic surfaces provides the opportunity to study the effects of different initial properties upon posteruptive geomorphic processes. Contrary to previous interpretations that the individual volcanoes were emplaced by polycyclic eruptions (separated by thousands or tens of thousands of years), we argue that the Pleistocene Crater Flat volcanoes are monogenetic, each having formed in a single eruptive episode lasting up to a few years. We show that all eruptive products emanated from the areas of the volcanoes' main cones rather than from scattered vents, as inferred by previous workers. These conclusions provide insight into the processes associated with continental basaltic magmatism and information related to the geometry and timing of volcanic events, which is useful in risk assessments.