Abstract Volume increasing processes in rocks can induce various types of fractures, such as hierarchical domain division of the unreacted material and spalling of the reacted material. The mechanism responsible for the selection of a particular fracture type is generally not known. We study such deformations and fracture mechanisms in a simplified system with a range of numerical techniques, and find that sharpness of reaction fronts and magnitude of volume increase are the main parameters controlling which fracture type forms. Spalling dominates when there is sharp reaction fronts and large magnitude of volume change, otherwise, we get mainly domain dividing fractures. Increasing reaction front sharpness and magnitude of volume change also increases the fracture density, and we argue that the change in fracture density can be important for the viability of in situ CO2 sequestration in ultramafic rock.