Abstract
Large mounds of welded pyroclasts and rootless flows on top of mafic lava flows associated with symmetrical scoria cones may be evidence for breaching and subsequent restoration of the cones. Textures and bedding structures of the agglutinate mounds are distinctly different from those of hornitos and indicate deposition proximal to a major explosive vent. Breaching of a cone armored by an agglutinate carapace may cause the welded deposits to collapse onto the emerging lava, which then rafts them away. Renewed or continued Strombolian eruptions from the vent can restore the symmetry of the cone and conceal the disrupted structure. Rafted agglutinate mounds on the Bonito lava flow at Sunset Crater, a symmetrical Holocene scoria cone in northern Arizona, extend in a belt from near the base of the cone toward the northwest, the direction the lava flowed. Eye-witness accounts of the eruption at Paricutín, Mexico, in 1943 record breaching and restoration events similar to the one inferred to have occurred at Sunset Crater and demonstrate that symmetrical monogenetic cones may have complicated histories.
