COMPLEX CINDER CONE ERUPTIVE SEQUENCE AT O'Neill CRATER, SAN FRANCISCO VOLCANIC FIELD, ARIZONA

The San Francisco Volcanic Field (SFVF) in northern Arizona covers the 4,700 km², and has been active for six million years. In that time over 600 volcanoes have formed, dominantly as basaltic cinder cones; volcanism has moved from west to east. Ongoing field work indicates that few of these cones erupted in cone-to-flow progression of a classic cinder cone model, and that cone breaching may be a common characteristic of SFVF eruptions. In many cases the cone was rebuilt after breaching, but at O’Neill and other craters, rebuilding did not occur. These cones provide insight into complex processes that occur at cinder cones.

Previous studies of O’Neill Crater indicate that three magma compositions make up the cinder cone, plug, and lava extrusions. These include a dacitic dome, basaltic-andesite, and a more mafic clinopyroxene-rich basalt. Further, it has been suggested that a cone-building eruptive phase was followed by a basaltic-andesite lava flow that breached the cone, with dacitic dome extrusion being the last stage of eruption to occur and without cone rebuilding around the dacite dome. However, evidence for further eruptive events has been found as spatter and cinder welded to dacitic dome fragments suggesting that a change in conduit regime prevented re-growth of the cone.

Basaltic-andesite inclusions are found within the dacite suggesting intermingling of the dacite and basaltic-andesite magmas within the conduit. Welded basaltic-andesite cinder and spatter is found on the interior slope of part of the crater. In addition, basaltic-andesite cinder welded to fragments of dacite has been found locally along the top suggesting that pulsating lava eruption occurred after the extrusion of the dome.

We propose that at O’Neill crater, the more viscous dacite trapped within the conduit erupted with more fluidal basaltic-andesite that did not have sufficient time for degassing to occur and allow a further cone-building eruptive stage. Instead, insufficiently degassed lava erupted as a more homogenous-phase flow in which magma coupled with gas erupted in a low fountain and deposited only locally within the breached crater. In addition, an inadequate volume of magma in this final eruptive stage prevented the re-growth of the crater.