EVOLUTION OF A COMPLEX CINDER CONE, RED MOUNTAIN, ARIZONA

Red Mountain (RM) is an ~740 ka cinder cone located near the NE edge of the San Francisco volcanic field in northern AZ. RM underwent phases of both fountaining and cinder eruption followed by a rafting event that lead to a broad breach and crescent shape to the west side of the volcano. Modification, possibly through structurally controlled hydrothermal circulation and/or phreatic explosions, created a deep amphitheater on the northeastern flank of the mountain. This complex cinder cone gives insight into variations that may occur during monogenetic eruptive episodes.

Volcanic facies are present around the cone in mounds (W - SW) or as a broad plateau (NW - N). Agglutinate and cinder clasts ≤30 cm in diameter are present up to 4 km from their original position at the vent. The plateau is bordered by agglutinate as much as 30 m thick, with only minor poorly welded cinder horizons. Lava flow occurs only rarely at the base of rafted material.

The earliest eruptions of RM were fluidal and explosive, yielding a spatter cone perhaps 30 m high. Prolonged cinder eruption then produced the cone that at present rises ~375 m from the surrounding plain. Lava spilled out to the west, rafting ~ 16% of the cone away and flowing in an arc nearly 270^o around the cone. Much of the rafted material mixed as it was rafted, but the relative proportions of cinder and agglutinate indicate that the outer, cinder-rich part of the cone moved W and SW, while the inner, agglutinate-rich portion was carried more to the NW and N. The absence of lava flow under agglutinate may be due to their similar densities, causing the agglutinate to sink into the flow and be pushed as well as carried along.

Timing and cause of alteration/lithification of material in the northeast amphitheater are uncertain. An apparently rafted section of similarly hued and cemented cinder ~ 4 km from source suggests that these processes occurred early in the cone’s evolution; in this case we speculate that the presence of strongly cemented material directed the lava flow toward the west. Alternatively, ground water along the regional Mesa Butte structure may have entered the cone through capillary action after eruption had ceased, causing cementation and phreatic explosions.

Students M Begay, B Ely, J Finger, J Gorney, J Hein, D Leppke, L Ludwig, and T Newkirk also contributed to this study.