Investigation of spheroidal weathering and twinning

What determines the geometry of spheroidal weathering patterns? Spheroidal weathering rings appear to arise from the leaching of fresh rock, rather than from reprecipitation of dissolved minerals diffusing out of weathered rock. In previous work the authors developed a mathematical model to explain the geometry of spheroidal weathering structures, and showed how some of the features of weathering patterns, otherwise difficult to explain, were natural consequences of the model. In describing the geometrical aspect of these shapes, we hope to understand how natural forces acted upon the unweathered rock and why and under what conditions spheroidal weathering structures were formed. In this paper we review the main features of our model and examine and discuss twinning of spheroidal weathering structures, concluding that the structures were formed during a distinct period in the past. Twinning is an apparent anomaly in that its natural occurrence, as part of the weathering process, cannot be explained by our model without additional ad hoc assumptions. Careful examination of twins reveals an explanation which does not require revision of the model.