STRESSES DURING CINDER CONE COLLAPSE AS RECORDED BY DISPLACEMENT OF CRATER-RIM BLOCKS, STRAWBERRY CRATER, ARIZONA Courtney Pulido, Nancy Riggs: Geology Program, School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ Acknowledgements: NAU/NASA Space Grant Program, administered through the NAU Department of Physics and Astronomy by Nadine Barlow and Kathleen Stigmon, is gratefully acknowledged. CMP thanks Christopher Brailo his contributions and trips to the field, not to mention his caring and encouragement from home. Thanks to the Geological Society of America Rocky Mountain section for a travel grant. References Hardwood, R.D., 1989, Cinder cone breaching events at Strawberry and O’Neil Craters, San Francisco Volcanic Field, Arizona [M.S. Thesis]: Northern Arizona University. Head, J.E., III, and Wilson, L., 1989, Basaltic pyroclastic eruptions; influence of gas-release patterns and volume fluxes on fountain structure, and the formation of cinder cones, spatter cones, rootless flows, lava ponds and lava flows: Journal of Volcanology and Geothermal Research, v. 37, p Parfitt, E.A., and Wilson, L., 1995, Explosive volcanic eruptions; IX, The transition between Hawaiian-style lava fountaining and Strombolian explosive activity: Geophysical Journal International, v. 121, p Tanaka, K.L., Shoemaker, E.M., Ulrich, G.E., and Wolfe, E.W., 1986, Migration of volcanism in the San Francisco volcanic field, Arizona: Geological Society of America Bulletin, v. 97, p Introduction The San Francisco volcanic field (SFVF) of northern Arizona is home to over extinct 600 cinder cones, and contains lava domes and stratovolcanoes, all of which young to the northeast (Tanaka et al., 1986). Strawberry Crater is located in the northeast part of the SFVF (Fig. 1). The basal portion of the cone is composed of ~150 m of agglutinated cinders, whereas in the upper portion (up to ~ 25 m), cinders were agglutinated upon deposition. Breaching of the cone during late-stage eruption displaced the agglutinated cinder in blocks (Fig. 2), rafting some segments onto an emerging lava flow (Harwood, 1989). The agglutinate cinder along the rim of the cone records the variation in magma characteristics of the eruption during the cone building event (Fig. 3). The displacement of these blocks show how the cone was affected by the breaching event (Fig. 2, 4, 5). Conclusions Strawberry Crater was constructed during multiple stages of varied magma characteristics (Table 1). Degree of cinder agglutination depends on the accumulation rate of pyroclasts (Head and Wilson, 1989), which is in turn dominantly determined by magma-rise speed and magma- volatile content (Parfitt and Wilson, 1995). Increased magma-rise speed and/or decrease in magma- volatile content causes higher degrees of agglutination, as accumulation rates rise and pyroclast trajectories decrease, respectively. Early eruption at Strawberry Crater was Strombolian-style with low magma-rise rate and high magma-volatile content that allowed bubbles to coalesce and cause increased pyroclast trajectories, and therefore resulted in cinders that were non-agglutinated upon deposition. As magma-rise rate increased and magma-volatile content decreased during eruption cinders began to be agglutinated, enough to create some rootless flows found in the upper cone facies, typical of a more Hawaiian-style eruption (Fig. 5a). Magma-rise rate and volatile content continued to vary, causing the variance in the degree of cinder agglutination near the rim of the crater. Strawberry Crater then breached its flanks, causing significant vertical displacement of upper agglutinated cinders in blocks that experienced little to no horizontal displacement (Figs. 2, 5b, 5c). Breaching of the cone was likely a sudden event as agglutinated cinder blocks along the rim show little response to lateral migration of lava (Figs. 5b, 5c). Methods Stratigraphic analysis Description of three agglutinated cinder facies (A, B and C, from youngest to oldest) that record the original depositional history and environment of the cone, including stratigraphic sections (Table 1; Figs. 4, 6). Structural analysis Interpretation of agglutinated cinder- block displacement by correlation of stratigraphic sections and facies blocks with the crater rim (Fig. 3). Data Facies Identification Block Displacement Block displacements can be estimated by correlation of block facies with facies at the rim (Fig. 2). Based on relict part of the crater rim, original crater is nearly circular with a diameter of ~350 m (Fig. 1). Comparison to other young cone rim diameters (e.g. SP Crater, ~400 m; Sunset Crater ~475 m) shows that the relict rim has not been strongly eroded, and that displaced agglutinate blocks were derived from nearby parts of the rim. Block sizes vary from 2.5 m to 11.1 m in thickness, 1 m to 10 m in width and 6 m to 75 m in length (Fig. 4). Blocks generally thicken toward the center of the cone and to the northeast. Displacement ranges from 5 to over 25 meters, with decreasing displacement with increasing proximity to the crater rim (Fig. 4). Fig. 5 Fig Facies Degree of agglutination Color Thickness (m) ClastsCharacteristics CStrong Dark Grey 1 – 4.5 Diameter: 1-5 cm Cinder and spatter Cliff forming Competent Massive Vesicular throughout Contains rootless flows BModerate -Weak Red- Brown Diameter: 1-11 cm Broken lapilli to bombs More easily eroded than other facies Often undercuts layer above it No rootless flows AStrong Dark Grey Diameter: 1-3 cm Cinder and spatter Cliff forming Competent Massive Black vesicular clasts at some locations No rootless flows a. b. c. Fig. 1 Fig. 2 Fig. 3 Fig. 4 Table 1Fig. 6 Modified from Head and Wilson (1989) Green line shows a recognizable continuous horizon. Numbers correspond to measured sections. Width, length and thickness of blocks measured shown by W, T and L, respectively.