Refining Constitutive Characterization of Geomaterials and Identifying Their Rupture Mechanisms in Natural Conditions; Montpellier, France, 7–8 September 2011 Results from rock testing and geological observations of different types of deformation bands and fractures highlight the problem of understanding the underlying formation mechanisms from granular (micro) to outcrop scale. Particularly puzzling are recently discovered compaction and dilatancy bands that are completely antipodal features. Both are narrow (several grain diameters thick), generally not planar but rather irregular and zigzagging (depending on the observation scale) bands that can form in the uniformly stressed rocks. The difference between them is that the compaction bands form at a relatively high pressure (mean stress) normal to the maximal compressive stress. The material within the compaction bands undergoes volume reduction due to the more dense grain packing and/ or crushing. Conversely, the dilatancy bands form at low pressure and normal to the least compressive stress. The material within them undergoes a volume increase due to grain rearrangements and/or formation of intragranular and intergranular microfractures. The dilatancy bands can become opened fractures with further deformation.