Plastic faulting is a brittle‐like failure phenomenon exhibited by water ice and several other rock types under confinement. It is suspected to be the mechanism of deep earthquakes and extreme cases of shear localization in shallow rocks. Unlike ordinary Coulombic failure, plastic faulting is characterized by a pressure‐independent failure strength and fault plane oriented 45° to maximum principal stress. To research the question of how the instability initiates, we conducted over 50 constant‐displacement‐rate experiments on polycrystalline ice (phases Ih and II) near the brittle‐to‐ductile (B‐D) transition, at confining pressures P = 0–300 MPa, applied strain rates = 5 × 10−5 – 7 × 10−3 s−1, temperatures T = 105–233 K, and mean grain sizes d = 0.25–1.18 mm. We find that (1) the width of the B‐D transition in variable space is vanishingly narrow, to the point of appearing as a crossover, (2) a plastic fault plane, once formed, is not a zone of subsequent weakness, (3) distributed ice I→II phase transformation in small amounts (
