INTERPRETATION OF THE C. AD 1075 ERUPTION OF SUNSET CRATER, ARIZONA, USA

About 900 years BP, the largest recent eruption from a monogenetic basaltic volcano field in the present-day continental USA took place at Sunset Crater in San Francisco Volcanic Field, Arizona. The eruption is estimated, on volcanological evidence, to have lasted from a few weeks to perhaps 2-3 years, and most likely occurred, based on paleomagnetic evidence, between AD 1050 and 1100. Notable eruption features are: the 10-km-long eruptive fissure on which Sunset scoria cone grew; large eruptive volume (given here as DRE), apportioned between the broad (~ 2-km-wide base), 300-m-tall Sunset scoria cone (~ 0.2-0.3 km^3, depending on the topography engulfed), a 0.3-0.4 km^3 scoria fall deposit, and ~0.1 km^3 of lava (current work is re-evaluating these volumes); the widespread dispersal of some scoria fall units (up to 60 km away); the two lava flows formed, the longer of which flowed down a pre-existing valley for ~ 11 km, with both having sections with no scoria-fall on them; and evidence of disruption of a prehistoric, well-developed, agrarian society. The Sunset eruption demonstrates that societal effects of scoria cone eruptions can easily reach tens of kilometers from the vent.

This talk concentrates on interpretation of the fall deposit and eruption mechanisms of the main phase of activity. Eight scoria fall units were described in previous work by the authors. Basal fall units 1 and 2 are related to vents along the SE fissure, while the dispersal of units 3 and 4 indicate a vent at Sunset and record activity during intense periods of the main cone-building phase. They are deposited over the entire area of preserved primary fallout, with a southeasterly and almost circular (windless) dispersal, respectively. Units 5-8 are more locally dispersed and are interpreted to record later cone-building phases. The fall deposits appear typical of Strombolian fallout; lithic fragments are uncommon. Units are distinguished by abrupt grain-size changes and occasional wispy ash layers; reverse grading is dominant in widespread units 3 and 4. Units 3 and 4 have thinning and fining characteristics that place them in the violent Strombolian to sub-Plinian range on various classification scheme plots, depending on assumptions made about the units in the proximal regions, whereas earlier and later portions of the eruption had lower columns.