Giant rockslides from the inside

The growing body of research on large-scale mass wasting events so far has only scarcely investigated the sedimentology of chaotic deposits from non-volcanic terrestrial landslides such that any overarching and systematic terminological framework remains elusive. Yet recent work has emphasized the need for better understanding the internal structure and composition of rockslide deposits as a means to characterise the mechanics during the final stages of runout and emplacement. We offer a comprehensive overview on the occurrence of rock fragmentation and frictional melt both at different geographic locations, and different sections within large ( > 10 6 m 3 ) rockslide masses. We argue that exposures of pervasively fragmented and interlocked jigsaw-cracked rock masses; basal mélange containing rip-up clasts and phantom blocks; micro-breccia; and thin bands of basal frictionite are indispensable clues for identifying deposits from giant rockslides that may remain morphologically inconspicuous otherwise. These sedimentary assemblages are diagnostic tools for distinguishing large rockslide debris from macro- and microscopically similar glacial deposits, tectonic fault-zone breccias, and impact breccias, and thus help avoid palaeoclimatic and tectonic misinterpretations, let alone misestimates of the hazard from giant rockslides. Moreover, experimental results from Mössbauer spectroscopy of frictionite samples support visual interpretations of thin sections, and demonstrate that short-lived (<10 s) friction-induced partial melting at temperatures >1500 °C in the absence of water occurred at the base of several giant moving rockslides. This finding supports previous theories of dry excess runout accompanied by comminution of rock masses down to μm-scale, and indicates that catastrophic motion of large fragmenting rock masses does not require water as a potential lubricant.