DRAFT Summary -- Collaborative Research: Understanding the causes of highly explosive basaltic eruptions: the Sunset Crater case study Intellectual Merit: Eruption of low-viscosity basalt dominates volcanism on Earth, Mars, the Moon, and Venus, and occurs on Earth in a variety of tectonic settings. A common misconception is that, compared to more viscous magmas, basalts produce only mild eruptions affecting small geographical areas. Basaltic eruptions, however, display large variability, producing a wide range of eruption scales and styles, from quiet lava flows and mild explosions to large sustained explosive eruptions that form 10- 25-km-high plumes that disperse ash up to several hundreds of kms. The exact causes of these upper end-member explosions are debated, and are the focus of this proposal. Hypotheses - The work proposed here will examine the well-preserved deposits of the AD 1085 Sunset Crater eruption in order to test hypotheses of the causes of large and violent basaltic eruptions. The leading hypotheses and corresponding testing techniques are: a) The magma contains unusually high abundances of volatiles, such as H2O and CO2. We will test this hypothesis primarily by interrogating melt inclusions hosted in olivine crystals. b) Gas escape is inhibited, resulting in coupled gas-magma ascent and closed-system degassing, typical of more evolved magmas. This process may be encouraged by a number of factors, especially rapid ascent causing sudden, shallow onset of bubble nucleation and abrupt viscosity increases in the shallow sub-surface due to syn-degassing crystallization. We will test this hypothesis by quantifying bubble and groundmass glass and crystal characteristics in the scoria, to provide constraints on late-stage subsurface processes, including decompression and nucleation rates, degree of bubble coalescence, corresponding degassing style, and magma viscosity at the time of quench (eruption). Case Study: Sunset Crater, in the San Francisco Volcanic Field (SFVF) about 25 km NE of Flagstaff, AZ, represents the largest (0.9 km3) recent monogenetic basaltic scoria-cone-forming eruption in the USA. The eruption produced lava flows, but most of the erupted volume was ejected during eight explosive phases. Previous work by others and ourselves shows that eruption plumes may have reached 25 km in height and can be classified as subplinian, with some phases transitional to plinian. Refinement of the stratigraphic sequence and column heights are needed and included in the work proposed here. Sunset Crater erupted more magma than the Mount St. Helens 1980 eruption and was four orders of magnitude larger than recent paroxysms at Stromboli volcano. No modern basaltic eruptions compare to Sunset in magnitude, explosivity, and style, making it an important example of upper end-member explosive basaltic eruptions. Broader Impacts: This project supports a new collaboration between two US institutions, includes early-career researchers, and trains a post-doc, a PhD student, an MS student, and many undergraduates. The site is a teaching site for both Ort and Clarke. Ort is a mentor in an REU project in which undergrad students will conduct fieldwork in the SFVF, and some undergrads may become involved in this project. The eruption strongly affected local populations, and archaeologists desire better constraints on eruption column heights and durations in order to evaluate the impacts. The eruption is important to seven tribes in the area and figures in oral traditions. Dr. Steven Semken (ASU) has extensive experience teaching within a Native American pedagogic reference frame and will, along with Ort, guide the students in conveying our results to native teachers and classrooms within this reference frame. We will attempt to involve native undergraduate students in studying and interpreting the ethnogeological aspects of the eruption with funding from NASAs Space Grant Program. Because of its youth and location, Sunset Crater is important in characterizing modern volcanic hazards in the SW US. Participants in a recent international workshop in Flagstaff, hosted by the US Geological Survey, recommended detailed study of Sunset Crater as it represents an upper end-member basaltic eruption that could significantly affect the region and disrupt air traffic to major hubs.
|Effective start/end date||7/1/13 → 6/30/18|
- National Science Foundation (NSF): $297,562.00
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