  The quantification of surface area between mineral and reactive fluid is
essential in environmental applications of reactive transport modelling. This
quantity evolves with microstructures and is difficult to predict because the
mechanisms for the generation and destruction of reactive surface remain
elusive. The challenge of accounting for the inherent heterogeneities of
natural porous media in numerical simulation further complicates the problem.
Here we first show a direct observation of reactive surface generation in chalk
under circumneutral to alkaline pH using in situ X-ray microtomography. The
momentary increase of reactive surface area cannot be explained by a change in
fluid accessibility or by surface roughening stemming from mineralogical
heterogeneity. We then combine greyscale nanotomography data with numerical
simulations to show that similar temporal behaviour can be observed over a wide
range of pH as porous media dissolve in imposed flow field. We attribute the
observation to the coupling between fluid flow and mineral dissolution and
argue that the extent of surface generation is strongly correlated with the
advective penetration depth of reactants. To conclude, we demonstrate the
applicability of using a macroscopic Damk\"ohler number as an indicator for the
phenomenon and discuss its environmental significance beyond geologic carbon
storage.
