Eruption of low-viscosity, dark magma called basalt dominates volcanism on Earth, Mars, the Moon, and Venus. Compared to more viscous and more volatile-rich magmas, basalts are generally considered to have lower explosive potential, producing mild eruptions affecting relatively small geographical areas. Basaltic eruptions, however, display large variability and range from quiet lava flows and mild explosions to less common large explosions forming >30 km-high plumes that disperse fine ash up to several hundreds of kilometers. The exact causes of these upper end-member explosions are debated in the literature. The largest recent eruption of a monogenetic basaltic scoria-cone volcano in the present-day contiguous USA took place ~900 years ago at Sunset Crater, about 25 km NE of Flagstaff, AZ. The eruption produced lava flows, but most of the erupted volume was ejected during at least 8 explosive phases that produced eruption plumes from 7 km to 25 km high and deposited scoria and ash blankets over the surrounding area. Scoria and ash thicknesses greater than 1 cm cover approximately 2,300 km2 and impacted prehistoric inhabitants, leading to population movement and dramatically altered settlement, subsistence, economic, and ritual systems. The eruption figures in oral traditions of seven tribes in the area. A similar modern eruption would significantly affect the Southwest US, causing major disruption to commerce and air traffic.
This project will examine the Sunset Crater eruption in order to test hypotheses explaining the causes of unusually large and violent basaltic eruptions. The leading hypotheses are a) the magma is especially rich in volatiles, such as water vapor and carbon dioxide, which provide the energy required to drive explosive eruptions and b) the magma viscosity abruptly increases in the shallow sub-surface due to magma outgassing and rapid crystallization, causing the basalt to hold its gases tightly, only to release them in a violent and explosive way upon reaching the surface. To help determine the importance of these processes at Sunset Crater, we will first determine the relative timing of the lava flow and eruption plume eruptions through stratigraphic correlation of the deposits. Scoria textures (e.g. bubbles, crystals) will help reveal magma viscosity and provide information on late-stage eruption processes, whereas analysis of melt inclusions trapped in crystals that formed at greater depths will provide information on initial magma volatile contents.
