Ice streams are major regulators of sea level change. However, standard viscous flow simulations of their evolution have limited predictive power due to incomplete understanding of involved processes. On the Greenland ice sheet, borehole fiber-optic observations reveal a brittle deformation mode that is incompatible with viscous flow over length scales similar to the resolution of modern ice sheet models: englacial ice quake cascades that are unobservable at the surface. Nucleating near volcanism-related impurities that promote grain boundary cracking, they appear as a macroscopic form of crystal-scale wild plasticity. A conservative estimate indicates that seismic cascades are likely to produce strain rates that are comparable in amplitude to those measured geodetically, thereby providing a plausible missing link between current ice sheet models and observations.