INTERACTION BETWEEN HUMANS AND THE SUNSET CRATER ERUPTION, NORTHERN ARIZONA

The Sunset Crater eruption produced a >300-m-high cinder cone and basaltic lava flows that cover ~8 km². Its fallout cinder blanket covers >2000 km². Relative ceramic correlation dates the eruption at between AD 1050 and 1150. We are gathering paleomagnetic data to help constrain the length of the eruption.

The eruption heavily affected the area’s inhabitants. Field and geochemical studies of cinders in the Wupatki area north of the volcano show they are from Sunset Crater, not from local latest Quaternary cones. Data from Paricutín, Mexico, and our experiments show that cinders deeper than 25 cm severely impact agriculture, but depths of 5-10 cm are an excellent mulch. Creation of new farmlands by the cinder mulch, as well as increased precipitation at that time, led to migration to the region where cinder depths were optimal, while areas with too much cinder depth became depopulated.

In the wall of a small masonry structure 6 km west of Sunset Crater, a fresh basalt clast, geochemically correlable with Sunset Crater, contains casts of shucked ears of corn and husks. A similar clast is on display at Sunset Crater visitor center and collectors reputedly have other samples. Casts include septa 0.5 mm wide and 1.5 mm deep between kernels. The rock is several pieces of welded fusiform lava. Corn casts occur on the surface and deep into the rock. Clast interiors are vesicular, but surfaces are smooth and glazed, implying partial re-melting. The corn-impressed rocks’ texture is very similar to welded spatter found at hornitos on Sunset lava flows.

Our failed attempts to produce a cast of Hopi corn in a Hawaiian lava flow demonstrate that the Sunset Crater casts were not made by a flow over-running corn, nor by flicking lava onto corn. We suggest that corn-impressed rocks record ritual offerings at approachable hornitos. The corn probably was rapidly covered by spatter. Pressure of the overlying spatter, radiant heat from the rootless vent, and exsolving water vapor from the immature corn (lowering lava viscosity) would have aided cast formation and glazing of the rock. The makers reclaimed the pieces and carried >40 kg to the site. There, non-impressed pieces of rock were broken off, leaving a concentration of waste flakes. The quantity of debris implies that many more than the two ~1-kg corn-impressed samples were prepared at the site.