Physical modelling of part of prehistoric Waikaremoana landslide shows that the blockslide must have hit the valley wall at c. 40 m/s, after sliding 2 km on a 5.5–8° slope, in order to form the 150-m high mound of debris known as Raekahu. Both the blockslide and a distal rock avalanche were in simultaneous motion when the impact occurred. Finely ground rock on the slide plane suggests that a mechanism of dynamic rock fragmentation may explain the low friction necessary for acceleration to 40 m/s. When a rock particle fractures in a confined space, an isotropic dispersive pressure equal to the rock's Hugoniot elastic limit (in the GPa range) at the ambient pressure and strain rate may be exerted on its surroundings. Beneath the 275-m thick block, about one particle in 15–30 or so fragmenting at any instant (with lower density for higher rock strength at higher strain rate), could completely support the weight of the block by fragmentation pressure; but then there would be no frictional resistance (and hence no further fragmentation). Self-regulation of the process may explain the apparent coefficient of friction of c. 0.1 in the blockslide. Low friction through dynamic fragmentation may apply widely to blockslides with a basal layer of comminuted rock.