‘A’ā lava emplacement and the significance of rafted pyroclastic material: Marcath volcano (Nevada, USA)

Lava emplacement at monogenetic volcanoes has been seldom documented when compared to persistently active volcanoes such as Mt. Etna or Kilauea. To expand the scientific understanding of lava emplacement, and to help assess the hazards posed by monogenetic systems and of lava flows in general, we present an investigation of the lava field at Marcath volcano (~ 35 ka; Lunar Crater volcanic field, central Nevada, USA). Key data include lava surface characteristics as well as size and abundance data of rafted pyroclastic material derived from the source cone. Upon exiting the source crater, lava initially ponded in an upland saddle between Marcath and an older cone before breaking out to feed three lava lobes that spread out onto a flat valley floor below. Quantitative estimates based upon morphological parameters suggest time-averaged discharge rates of 40–100 m³ s⁻¹, emplacement duration of 5–20 days, and bulk lava viscosities on the order of 10⁶ to 10⁷ Pa s in the main lobes. Sizes and distributions of 1021 mapped pyroclastic rafts suggest that the early stages of the eruption produced much of the total raft volume. The source of most of the raft material was likely the now-missing western portion of an early Marcath cone. After the initial breaching of the cone, the raft production rate likely kept pace with cone building over the eruption’s duration such that once the cone was breached on one side, it did not heal. Rafts were not always passive passengers on the lava: they had an important impact on lava morphology in specific locations, as they caused bifurcation of entire flow lobes and produced other flow features. The common occurrence of rafts in monogenetic lava fields suggests that incorporation of raft-lava interactions may be important in lava propagation models.