Data Supplement

Data Supplement

Small-scale experiment with the constrained rod showing helical configuration - short video of the small-scale experiment with the constrained rod showing helical configuration. An electric DC motor inputs rotational motion from the right, and at the left the rod is attached to a metallic disc which rotates in contact with a rock sample. An axial compressive load is imposed through the rock, generating frictional torque at the disc-to-rock interface. The dynamical helical configuration is observed during the brief �stick phase�.