Seismic origin of the Atacama Desert boulder fields

• Fields of boulders are common around bases of hills in the Atacama Desert.
• Granitoid boulders emerge as corestones at an erosion rate of 0.1–1 m Ma− 1.
• Physical and isotopic data suggest that boulders slide rather than roll, down hill.
• Boulders rub during earthquakes, producing smoothing of mid-sections and moats.
• Boulders may be exposed for > 12 Ma while in the flats.
• The role of seismicity in landscape shaping of the Atacama Desert is emphasized.

Extensive fields of large boulders are common around the base of hills in the Atacama Desert. How these boulders are transported from nearby hillslopes is unclear given the lack of rainfall of the region. Here we document the central role of seismicity, not runoff, in transporting and smoothing > 1 ton boulders all across the hyperarid core of the Atacama Desert. The generally granitoid boulders emerge as corestones on hillslopes at an erosion rate of 0.1–1 m Ma− 1. Thereafter, physical and cosmogenic isotopic evidence suggests that boulders slide and bounce rather than roll down hills and onto adjacent flats. In the transport process, the largest boulders are split and the smaller ones are weathered to grus, narrowing average boulder mass to ~ 2 tons (< 1 m3). At the base of hills, the boulders bunch together and rub during the frequent earthquakes in the region, producing distinctive smoothing around boulder mid-sections, and silt moats around the boulder bases. Our measurements show a strong correlation between boulder field density and rubbing, and only when the density exceeds 60–70% does rubbing become common. Except for slow removal by rubbing, the boulders seem to undergo no further erosion while in the flats. Exposure times for some boulders are > 12 Ma, making them among the oldest continuously exposed features on the Earth. Boulder rubbing is just one geologic feature among many in the Atacama that underscore the role that seismicity probably plays in shaping landscapes of the waterless worlds of the solar system.