Field Observations of Decreasing Rock Fracturing Rates Over Geologic Time

Rock fracturing regulates the topography, carbon cycle, geologic hazards, and infrastructure degradation of the Earth. Yet, there remains a paucity of constraints on long-term fracturing behavior. Here we use field measurements of 2221 clasts across a range of environments and rock types to show that the number and total length of fractures in natural surface rocks increase rapidly within the first ∼10 kyr of exposure, and then the rate of fracture growth slows exponentially over geologic time up to ∼150 ka. Similar rock breakdown deceleration trends were independently documented using a novel application of infrared photoluminescence (IRPL) dating for a visibly fractured boulder at one site. Previous work shows that increasing microcrack intensity correlates with enhanced compliance in the bulk rock over the same timescales that our bulk macroscale fracturing rates decrease. We hypothesize that enhanced compliance contributes significantly to the observed decrease in fracturing rates via increased material toughness. Our results contrast with current landscape-scale conceptual models that assume bulk fracturing rates and characteristics are invariant over time and are controlled by short term rock strength and external stress magnitudes alone. Instead, our findings indicate that, over geologic time, fracturing of rocks increases its effective toughness.