At 15:23 on Sunday, May31, 1970, the Ancash earthquake (also known as the Great Peruvian earthquake) struck off the coast of Peru with a moment magnitude of 7.9. The shaking dislodged a slab of rock about wide from the western face of Huascarán's north peak, at an elevation of roughly . The rockfall occurred almost immediately after the shaking began, with witnesses comparing the sound to a gunshot or explosion. The fall triggered an "enormous dark cloud of dust" which rapidly blocked the view from below.