Persistent desiccation and contraction of muddy sediments gives rise to mud cracks. These cracks form networks of interconnected tension fractures arranged in remarkable polygonal patterns. Because tensile stress due to drying declines downwards through the sediment, mud cracks have generally been theorized to nucleate near the surface, propagate downwards, and terminate at depth. Here I trace the nucleation and growth path of natural mud cracks by analyzing in detail crack-surface morphology. The present observations show that systematic nucleation at the bottom of the polygons and upward propagation of mud cracks are much more prevalent than previously postulated. The cracks predominantly rupture the desiccated layers before they significantly extend laterally and sequentially form the polygonal patterns. The consistent location of crack origins at depth, along the bottom of the polygons, strongly suggests that stress concentration at flaw discontinuities and layer boundaries play a fundamental role during mud fracturing.
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