Concentration hotspots are associated with highly localized reactivity. Hotspots are small areas of the domain characterized by high concentration values, and whose dynamics controls the system behavior at the macroscale. Here we propose a rigorous definition of “hotspots”: we investigate their spatial heterogeneity as well as their temporal ephemerality in terms of relevant dimensionless numbers in a planar fracture. Finally, we construct a phase diagram in the Peclet-Strouhal space to predict hotspot dynamics and we validate it through numerical simulations.