Observations on the New Plymouth Sugar Loaf Islands and their framework indicate that the dacitic intrusions “protruded” to the surface in a nearly solid state. They are on the summit of a much larger substructure with its greatest dimension in a north-east-south-west direction. Huge blocks of sedimentary sandy mudstone and pyroclastics were brought upwards on the flanks of rising plugs. Subsequently, these blocks slumped down the sides of the composite volcano, in all likelihood sub-aqueously. The protrusion of the Sugar Loaf plugdomes marks the latest phase of igneous activity, and post-dates the deposition of the Lower Pouakai Agglomerate. The movement terminated before the end of the deposition of the Upper Pouakai Sands.