The primary tool in reconstructing cinder cone breaching events is the location of the rafted mounds of cone material on the surface of the breaching lava flow. This indicates: 1) the relative timing of the breach in relation to the lava extrusion event, 2) the initial stratigraphic position of the breach, and 3) the number of flow units. Detailed mapping of the volcanic rocks at Strawberry Crater and O'Neill Crater in the San Francisco volcanic field, northcentral Arizona, has revealed unique eruptive and breaching histories for the two cinder cones. The sequence of events at Strawberry Crater are: 1) building of the basaltic andesite cinder cone, 2) intrusion of a magma body into the cone structure, resulting in the initial breaching of the cone, 3) extrusion of a basaltic andesite block lava flow, and 4) extrusion of a dacite vitrophyre plug located in the breach.
The sequence of events at O'Neill Crater are: 1) Building of the basaltic andesite cone, 2) extrusion of the first, non-breaching lava flow, 3) extrusion of the second andesitic block lava flow, which breached the cone, and 4) extrusion of a dacite vitrophyre dome in the breach and two small plug.
Statistical analysis of breach azimuths for 36 cinder cones in the San Francisco volcanic field results in a bimodal distribution with mean vectors of 122.3 degrees and 292.1 degrees. These directions are roughly perpendicular to the inferred least principal horizontal stress direction (50 degrees) for the volcanic field. Five local controls on breaching mechanisms appear to operate in determining the direction of breach for cinder cones: 1) fault/joint systems, 2) vent location of breaching lava, 3) substrate buttress, 4) wind direction during eruption, and 5) local topographic stress regimes. The breaching control at Strawberry Crater appears to be a combination of substrate buttressing and cone strength, while at O'Neill Crater it appears to be the local fault system and the vent location of the breaching lava.
Strawberry Crater (top) and O'Neill Crater (bottom) in the San Francisco Volcanic Field, Arizona. (Click on the image for a large view)