Origin of Agglutinate Material on the SP Crater Lava Flow and Implications for Eruptive History of a “Simple” Cinder Cone, San Francisco Volcanic Field, Northern Arizona

SP Crater, located in the San Francisco Volcanic Field 55 km north of Flagstaff, is a simple cone and flow as seen in satellite imagery. Near-vent agglutinate and welded-cinder facies are found not only at the rim and inner crater of SP, but also on the associated lava flow. Rafted material on the flow provides evidence of a complex eruptive history.

The flow is ~7 km long and agglutinate outcrops are found along its edges, following all of the curves. The largest outcrop on the lava flow is 200 m in length and 20 m tall. This outcrop includes breadcrust and ribbon bomb fragments as large as 0.5 m in length that reconstruct up to 2 m long. Fragments of bombs up to a meter in length are found from outcrops near the vent to outcrops 350 m from the end of the flow. The size of outcrops increase with distance from the vent. Analysis of thin sections shows that the agglutinate at the rim of SP Crater and on the basaltic flow, as well as lava of the SP flow itself, contain an identical mineralogy of quartz xenocrysts and euhedral olivine.

SP Crater was initially built through strombolian eruptions. Agglutinate was rafted during the extrusion of the first lava flow and the cone may have partially collapsed. The initial extrusion was vigorous, breaking off large blocks of agglutinate that were carried far out on the flow. The mass flux then decreased, as suggested by a central area along the flow relatively free of mounds. In the final stage of the early flow, flux decreased, and smaller sections of agglutinate were rafted. Renewed strombolian activity rebuilt the cone completely. The final stage involved the extrusion of the second lava flow from the base of the cone without breaching it.