Subplinian monogenetic basaltic eruption of Sunset Crater, Arizona, USA
Sunset Crater volcano, located ~25 km northeast of downtown Flagstaff (Arizona, USA) in the San Francisco volcanic field, represents an interesting case of highly explosive monogenetic volcanism. Sunset Crater is a young (ca. 1085 CE) scoria cone, ~290 m high, that produced 0.52 km3 dense rock equivalent (DRE) of basaltic magma (0.16, 0.12, and 0.24 km3 DRE for the scoria cone, lava flows, and tephra deposit, respectively). The activity developed in three distinct phases: an initial fissure phase, followed by a highly explosive phase, and a final low-explosivity waning phase. The first phase was characterized by the opening of a 10-kmlong fissure, which produced the Gyp Crater spatter mounds and the Vent 512 lava flow ~10 km SE of the main scoria cone, followed by lava fountaining activity that produced the first tephra layer (unit 1, 0.01 km3 DRE). During the second highly explosive phase, the activity migrated to the northwest to evolve into a single-vent eruption, which formed the main scoria cone. The central vent activity was initially characterized by variable eruptive styles, which started the process of cone building, including deposition of a second tephra layer (unit 2, 0.01 km3 DRE), and the effusion of the two main lava flows (Bonito and Kana’a). Following the initiation of effusive activity, the eruption increased in explosivity to produce three subplinian tephra layers from the central vent (units 3, 4, and 5), which emitted more than 0.22 km3 DRE of basalt, with associated eruption columns >20 km high. By the end of this phase, the Kana’a lava flow was almost completely emplaced, but the Bonito flow continued to grow, forming a 3-km-diameter flow field. A final waning phase produced several tephra units, commonly discontinuous and reworked, with <0.01 km3 DRE cumulative volume. The comparatively large volume of tephra, the high eruptive columns, and high mass eruption rates make Sunset Crater the most explosive monogenetic eruption studied to date (volcano explosivity index [VEI] 3–4). Sunset Crater volcano represents an interesting case of monogenetic volcanism in that its activity was characterized by highly explosive eruptive phases up to subplinian in scale and intermittent episodes of lava effusion that heavily affected prehistoric communities in the area. Explosive basaltic volcanism should be considered when assessing volcanic hazards in continental settings, such as in the western United States.