Cordilleran Section - 113th Annual Meeting - 2017

Paper No. 52-12
Presentation Time: 8:30 AM-5:00 PM


VAN GUNDY, Daniel N.1, RIGGS, Nancy2 and STEVENS, Laney E.2, (1)Northern Arizona University, Flagstaff, AZ 86001; School of Earth Sciences and Environmental Sustainability, Northern Arizona University, PO Box 4099, Flagstaff, AZ 86011-4099, (2)School of Earth Sciences and Environmental Sustainability, Northern Arizona University, PO Box 4099, Flagstaff, AZ 86011-4099,

The San Francisco Volcanic Field (SFVF) lies on the southwestern edge of the Colorado Plateau in northern Arizona. It contains approximately 600 scoria cone volcanoes and fewer than 20 felsic domes and stratovolcanoes. The SFVF overlies a thick sequence of subhorizontal Paleozoic strata. Ages of volcanism in the SFVF range from late Miocene to Holocene.

Red Mountain, a late Pleistocene scoria cone, located approximately 60 km northwest of Flagstaff, AZ is distinct from other scoria cones in the SFVF. The northeastern portion of the cone, commonly referred to as the amphitheatre, is an area of narrow and steep relief, which has exposed the internal structure of the cone as high as 150 m from the base of the amphitheatre. This area is separate from the rafted vent, located on the western flank of the volcano.

The amphitheatre is extensively cemented with calcite, iron oxides, and several types of zeolites. Cementation occurred as hydrothermal fluids altered minerals in the scoria, creating a pressurized system. As groundwater interacted with residual heat beneath the cone, an increase in pressure eventually induced an explosion, ejecting approximately 750,000 m3of material. This blast was localized to the area cemented by fluids and did not otherwise alter morphology of the volcano.

Mapping in the amphitheatre based on change in color of altered scoria reveals the extent of hydrothermal alteration. As fluids permeated the cone, changing temperature and chemistry precipitated different minerals, creating distinct zones of alteration. The apparent movement of hydrothermal fluids began in the western portion of the amphitheatre. Fluids travelled in a radial pattern away from the vent to the north and northeast. Fluids stopped in the northeast drainage of amphitheatre.

Iron oxide was the most widespread cementing agent, accompanied by calcite and zeolites. The western portion of the amphitheatre, nearest the rafted vent, contains little to no zeolites nor calcite but are cemented heavily with iron oxide. In areas of lower relief within the amphitheatre, calcite and various zeolites are prominent, along with iron oxides. Black scoria cemented with zeolites underlies unaltered, non-cemented black scoria in drainages at the lowest reliefs, indicating the final extent of hydrothermal fluids.