Geomorphic mapping and petrography of Mammoth Mountain, California
Burkett, Shannon M.
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Mammoth Mountain, a 57-110 ka rhyodacitic volcanic dome complex, is centered at 37°37'50.26"N, 119°1'57.45"W on the southwestern rim of Long Valley Caldera, California. The main edifice is composed of at least 20 overlapping lava domes and flows which were emplaced 112-57 ka (Ring, 2000). Hazard forecasting is an important part of volcanology. Concurrently, the center points of this study; a detailed geomorphic map and petrographic database, represent important (and often, overlooked) steps toward a complete hazard model for Mammoth Mountain. Morphologically, Mammoth Mountain is comprised of 18 dome and/or lava flow units. Many of the domes comprising the summit ridge and the northern flank have been oversteepened by glacial and/or tectonic processes. Nearly all of the observed rock fall, debris avalanche, debris flow, and rock slide units mapped during the course of this project are located on the northern flank, mostly where the domes have been oversteepened. The northern flank is bisected by two SE-NW trending normal faults, a primary and a smaller, antithetic one, bordering an extensional graben. The geomorphic map also detailed the locations of drainages, bodies of water, phreatic explosion craters, glacial and effusion features (e.g. crease structures), and non-glacial diamicton deposits. Petrographically, Mammoth Mountain has been divided into 15 different groups characterized by unique groundmass textures and/or phenocryst assemblages. Furthermore, this petrographic survey allowed the author to examine the degree of weathering for each lava unit comprising the mountain. Domes and flows from the northern flank were more weathered than lava units elsewhere, most of it proximal to the normal faults mentioned previously. A small, multicolored dome (RD10) at the inferred eastern extent of the graben has undergone extensive hydrothermal alteration. The northern face of this dome has collapsed leaving behind two gaping scarps and two large debris avalanche deposits. Of further note is a large pit on the northern end of the dome. This pit was observed during the summer of 2006, but was not observed during this authors 2005 field season. This feature is likely a "thaw pit" a feature generated by the localized melting of permafrost . Bacon (1982) determined that the eruptive history of basalts and rhyolites in the Coso volcanic field, a system governed by fixed rate, tensional strain, and constant magma and dike generation, would be more accurately defined by steps, or regression lines, connecting both the date of eruptive activity and the cumulative volume immediately prior. Using Mammoth Mountain as a corollary, volumes for each dome and flow comprising the mountain were calculated so that their cumulative volumes could be plotted against time. Results for this exercise are inconclusive, but seem to suggest that Mammoth Mountain is not likely to erupt in the near future. This is not a definitive conclusion as the volumes calculated only include domes material that has (1) been dated, and (2) is exposed on the surface. A pyroclastic flow deposit was observed in a manmade depression 200 m southwest of The Mill Café near the base of the Gold Rush Express chairlift. This deposit is significant because it directly underlies tephra from the ∼600 b.p. Inyo eruption. The lack of a soil or erosional horizon between these two units implies that the pyroclastic flow was emplaced shortly before, if not, contemporaneously with the ∼600 b.p. Inyo tephra. The geochemical signature of this pyroclastic flow plots outside of the Inyo tephra data on the majority of trace and major element plots. No source for the deposit was identified. The youthfulness of this deposit is significant because it suggests recent activity from an unknown source that could pose a threat to contemporary population centers.