Weathering zonation within cracks in desert boulders reveals piecemeal crack propagation over geologic timescales

Rock fracturing can be slow and steady, comprising physiochemical processes that involve the chemical breaking of bonds that are weakened in response to local stress loading. Whereas subaerial cracking of surface boulders is universally observed in desert environments, the rates and specific mechanisms that drive crack propagation in such conditions are yet to be completely understood.

Here, we present new field and petrographic observations from mode-1 (tensional) incipient (rocks are not yet split) fractures in alluvial boulders from the hyperarid southern Negev desert (Israel). Over 100 carbonate boulders embedded in a well-developed, 70 ka desert pavement that held visible fractures were forced apart along the incipient cracks. Doing so revealed a systematic recurring tri-zone pattern in crack morphology whose boundaries consistently paralleled the crack propagation front: Zone 1 – A weathered (as evidenced by incipient patina) zone proximal to the boulder surface; Zone 2 – A relatively fresh crack zone partly filled with aeolian particles and salts medial from the boulder up-facing surface;  Zone 3 – A chemically altered (as evidenced by petrographic analyses) zone of otherwise intact rock at the crack tip. The occurrence of such micro-morphological crack zonation suggests slow sub-critical crack propagation at sufficiently long geologic timescales that support development of differential weathering within the crack. The petrographic analyses of sections perpendicular to the plane of the crack indicate chemical alteration that precedes the crack propagation in both space and time (i.e., extends in front of the crack tip), also indicates slow piecemeal propagation of the crack. This linkage between chemical weathering processes at the crack tip and slow subcritical propagation of the crack into the boulder provides additional support for first-order control of environmental and climatic conditions on boulder cracking rates, regardless of the physical stress-loading mechanism.