What doesn’t break rock makes it tougher

Abstract Rock fracturing drives and limits the evolution of Earth’s topography and life, the global carbon cycle, geologic hazards, and infrastructure stability. Yet there remains a paucity of constraints on long-term fracturing behavior. Here we use field observations to show that fracturing rates in natural surface rocks decrease exponentially over time. We present lab measurements showing that these results are consistent with rock mechanics experimental data and theory linking progressive fracturing to decreasing brittle strain response to stress. We characterize fracture evolution over periods of 1 – 100 kyr for three different field sites and three rock types. For rocks with less than ~5 kyr of exposure, fractures grow at rates on the order of 10 – 100 mm/kyr, then after ~10 – 15 kyr, fractures grow by less than 2 mm/kyr. Over similar timescales, the ‘appearance’ of visible fractures also decreases, dropping from 36 new fractures per m^2/kyr to <2 per m^2/kyr. We independently document similar fracturing deceleration trends using microfracture analyses, plus a novel application of infrared photoluminescence (IRPL) dating for three in situ fractures in a single clast. Our results contrast significantly with current landscape-scale conceptual models that assume 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, rock’s fracturing increases its toughness.