Paper No. 176-4
Presentation Time: 9:00 AM-6:30 PM
CHANGING ERUPTION STYLES DURING THE ~1085 CE SUNSET CRATER ERUPTION, NORTHERN ARIZONA
Sunset Crater erupted in northern Arizona around 1085 CE, causing changes in the culture of the local Hisatsinom people. The eruption began with a discontinuous 11-km-long fissure that then reduced to 2–3 vents, with Sunset Crater at the NW end of the fissure and Vent 512 at the SE end. This fissure deposited two fallout beds. Vent 512 produced a 4-km-long, thin, spatter-fed lava flow. Activity then centered on Sunset Crater, which produced three separate widely distributed fallout deposits. Each has an ash layer at its base, but the rest of each deposit is poor in ash. All five widespread tephra layers are coarse and came from plumes modeled to be Violent Strombolian to Subplinian to possibly even Plinian (>20 km) in height. The ashes at the base of units III–V only occur in proximal to medial areas and are interpreted to be from Strombolian activity between the paroxysmal events. Two lava flows were emitted during the time when units III–V were erupted (the flow-top tephra is oxidized), but the exact timing (did they flow during, or between, the paroxysmal events?) is not known. After unit V, more lava was erupted through both lava flow systems, inflating the previous flows and breakouts fed flows farther from the vent. Relative timing of these is known by the amount of Strombolian black ash on them. Strombolian activity continued at Sunset Crater during this time, and the scoria and bombs on the cone rim are oxidized. The eruption ended with an explosion, creating a crater nested in the eastern main crater. The ejecta were black basalt blocks and bombs, some cored with limestone, and some limestone blocks. We interpret this to record a phreatic explosion when water from fractures in the Kaibab limestone entered the hot dike as magma receded, after most oxidation at the crater rim was finished. Dates from tree-rings, archaeological correlation, and paleomagnetism point to a 1085 CE eruption. Tephra distribution patterns and corn imprints in lava imply an eruption primarily between late August and March. The late-stage Strombolian activity and explosion may have ended a few months later. Hisatsinom people were forced to leave because of tephra fall and subsequent long-lived eolian reworking. They migrated to lower areas, now arable due to the tephra mulch, and developed new architectural styles, larger sites, and a more complex social system.