UNDERSTANDING THE ERUPTION HISTORY OF SP CRATER, NORTHERN ARIZONA, THROUGH TEPHRA MAPPING

SP Crater, located in the San Francisco Volcanic Field 50 km north of Flagstaff, AZ, erupted at ~60 ka. Variable-depth holes dug around SP and stratigraphic columns, complemented by componentry studies, provide insight into changes throughout the eruptive process.

Variation in clasts near the cone including color, density, and vesicularity point to a change in eruption style. The possibility of a two-phase eruption, with earlier tephra more vesicular than denser later tephra, is supported by the presence of rafted bodies on the flow, and the present conical shape of the cone.

Twenty-seven holes range from 20-240 cm in depth and provide information on the dispersal area of the deposit. Tephra from the eruption covers an area of over 100 km², with two lobes to the NE and E, overlapping NE of the cone, near the cones base and extending NE ~12 km, placing the eruption between Strombolian and violent Strombolian. With tephra thickness near the cone of ≥210 cm, dissipating to ≤20 cm near the deposit edge, the total volume of tephra ejection may be greater than 0.1 km³ classifying the eruption between 3-4 on the VEI scale. Two distinct clast types were determined: one is frothy, vesicular, brittle, and brown and underlies black, less-brittle clasts. Near the cone where deposition is thicker the stratigraphy of the deposit is well preserved while bioturbation has greatly affected the stratigraphy of thinner deposits.

Variation in bedding and direction of deposition indicates a two-phase eruption, with distribution of material varying but lying generally NE-E, indicating dominant wind direction from the SW. It is hypothesized that a gaseous first episode occurred prior to flow eruption, with an additional less-gaseous eruption after the flow. The volume of ejected material as well as area of dispersal classifies the eruption as high-magnitude Strombolian.