Popups on Moon Rock, Augrabies Falls National Park

Abstract

Naturally occuring popups (or A-tents) occur mainly on outcrops of horizontally compressed, sheeted granite. They consist of two slabs of surface rock pushed up to form a tent-like structure, or a surface sheet pushed up into an arc. Although popups have been studied in several countries, none have so far been described in South Africa. A survey of Moon Rock, a granite dome in the Augrabies Falls National Park, led to the discovery of 14 of these structures, including both angular and arched forms. The dimensions and orientations of the high, angular forms support the hypothesis that they constitute a stress release phenomenon, but the characteristics of the low, thin plated forms suggest that these are erosional features. The structures are eventually destroyed by the movements, as yet largely unexplained, of surrounding rock slabs, and by the gradual abrasion of the edges of the popped up slabs as a result of slight movements caused by daily heating and cooling of the surface sheet.