Experimental simulation of joint morphology

Desiccation experiments with starch–water mixtures whose dimensions and thickness are several centimeters produce a broad spectrum of tensile cracks. Their properties are remarkably similar to those of joints in rocks. This note concentrates on crack morphology in the form of plumose structures and fringe zones. Moreover, rupture velocity is determined from videos. After an initial, dynamic rupture phase with velocities of about 100 mm/s, which is restricted to a thin, strongly dried surface layer, the main rupture through the specimen is quasi-static with velocities of 10 mm/min and less. This phase is driven by the low supply of strain energy due to the ongoing desiccation. Quasi-static rupture produces plumose topographies, which range from simple to complex. From the similarity of plumose topography on joints in marine sedimentary rocks, it is hypothesized that these joints are generated by quasi-static rupture in an early stage of subsidence, compaction and diagenesis of water-bearing sediments. Rupture propagation in the fringe zone of cracks in starch, where topography is stronger than on the main crack, was slower than over the main crack. The same applies probably to fringe zones along bedding planes in sediments.