TRANSITION FROM STROMBOLIAN TO PHREATOMAGMATIC ACTIVITY DUE TO SCORIA CONE VENT MIGRATION: COLTON CRATER, SAN FRANCISCO VOLCANIC FIELD, NORTHERN ARIZONA

The San Francisco Volcanic Field contains over 600 volcanoes, mostly basaltic scoria cones. One of these, Colton Crater, transitioned from a scoria cone to a maar volcano when magma in the conduit interacted with groundwater. The cause of this transition was investigated as part of an REU project through the analysis of facies and orientations of stratigraphic sections exposed in the walls of the crater. The degree of welding of scoria in the layers indicates proximity to the vent and can also represent the rate at which magma was ejected from the cone. Spatter and scoria layers show evidence of vent migration from the south end of the crater to the northeast edge. An ~18 m thick rootless lava flow and underlying conformable welded scoria on the west side of the crater provide evidence of the beginning stages of the eruption of Colton Crater. These beds dip to the northeast, which indicates that they were deposited on the exterior of the cone in the beginning stages of the strombolian eruption. The dense lava formed as hot spatter was deposited and flowed down the outside of the cone, suggesting that the vent was close to the area of deposition and the magma was being ejected at a high rate in a low fountain. These beds had scoria and spatter deposited on them with outward dips and facies patterns that indicate vent migration to the north. Other rootless flows located in spatter layers in the north and northwest rim of the crater suggest migration of the vent from the southwest to the northeast. This migration resulted in ascending magma coming into contact with groundwater and creating an explosion that left behind an ~1.3 km wide crater. The explosion ejected surge deposits (a matrix of hydrated basaltic ash with various rock types from underlying strata, with Kaibab limestone the most abundant) on the north, northwest, and northeast rim of the crater. The east rim is mostly composed of loose scoria and bombs that likely represent the core of the scoria cone, which was ejected in the phreatomagmatic explosions. Peridotitic xenoliths are common in surge deposits and along the south and west rims of the crater. We interpret the transition from magmatic to phreatomagmatic eruption to have been caused when the vent migrated to a new location and intersected a new water source in either the Coconino sandstone or the karstic Kaibab limestone.