The structures of geological formations, as well as flow and chemical transport patterns within them, are profoundly affected by chemical dissolution and precipitation processes (i.e., the interactions among flow, chemical transport, buoyancy, and dissolution and precipitation reactions). These processes are intrinsically hard to measure, and therefore are not well understood. Nuclear magnetic resonance imaging is applied to study the dynamic behavior of coupled flow and dissolution in natural rock fractures. Our findings reveal that flow and transport in evolving fractures are far more unpredictable than commonly assumed, due to complex interactions among fracture morphology, flow, dissolution, and buoyancy. This can explain physical processes causing catastrophic collapse and subsurface structural instabilities, such as sinkholes and land subsidence.