ZONATION WITHIN INCIPIENT CRACKS IN BOULDERS AND INDICATIONS FOR THEIR SUBCRITICAL PROPAGATION OVER GEOLOGIC TIMESCALES

Rock cracking is a physiochemical process that involves the breaking of chemical bonds in response to local stress loading. Whereas subaerial cracking of rocks is universally observed, the rates and mechanisms that drive such crack propagation are not fully understood. Here, we present field observations, petrographic and scanning electron microscopy (SEM) analyses of incipient cracks developed in alluvial dolomite boulders on the surface of late Quaternary (~62 and 14 ka) terraces in the hyperarid Negev desert, southern Israel. We show that crack density (number of cracks per boulder surface area) increases as a function of surface age and that cracks consist of a recurring pattern of three distinctive weathering zones that extend inward from the boulder exterior: 1) An ‘Outer Weathering Zone’, which is characterized by a well-developed weathering rind similar to that found on the boulder exterior; 2) A ‘Medial Accumulation Zone’, where aeolian particles and weathered rock fragments (i.e., dolomite rhombs) accumulate; and 3) An ‘Inner Weathering Zone’, which extends inwards from the Medial Accumulation Zone up until the crack tip. Crack intensities that increase as a function of surface age and the characteristic weathering zonation along crack inner surfaces suggest continuous piecemeal crack propagation over geologic timescales that is most consistent with long-term subcritical cracking. Evidence for dissolution processes within the Inner Weathering Zone and at the crack tip together with the penetration of allochthonous chemical elements (e.g., Al, Si, Fe) into the carbonate boulder interior along the crack plane and at its tip, support the proposed role of water in facilitating subcritical crack propagation over geologic timescales.