Mineral changes quantify frictional heating during a large low-friction landslide

Data on thermal decomposition constrain events around the transition on 5 June 2009 from creep deformation (over many decades) with sporadic rockfalls to catastrophic collapse of Jiweishan Mountain, China, which poured millions of tons of limestone rubble from a cliff top into the valley below, causing 79 deaths. We quantified frictional resistance (mu, 0.1-0.2) and rapid frictional heating using thermogravimetry, mineral alteration close to the sliding surface, and high-speed rotary experiments. The differential thermogravimetry (DTG) trough temperature varied from 794 +/- 4 degrees C at the sliding interface to similar to 720 degrees C below 10 mm. We inferred that DTG trough changes reflected the substrate thermal history due to landslide frictional heating. X-ray diffraction (XRD) analysis showed thermal decomposition of talc (a major mineral in the shale at Jiweishan Mountain) to magnesium metasilicate (formation temperature of similar to 600 degrees C) and enstatite (formation temperature of 800-905 degrees C). The intensity of the enstatite XRD peak decreased with increasing depth below the sliding surface to disappear below 6.5 mm. Magnesium metasilicate appeared between 6.5 mm and 10.5 mm depth. We replicated the temperatures and mineral changes in high-speed rotary shear experiments. Heating above 800 degrees C at the sliding surface was confirmed by temperature measurements and thermal decomposition of dolomite to magnesium and calcium oxides in the shallowest samples (which could not have survived for 7 yr at our field site and thus were not detected in the field). We inferred that the slide surface was heated to at least 800 degrees C by frictional sliding and that the temperature below 10.5 mm depth did not rise above 600 degrees C. Evidence of dynamic localized recrystallization was found on both field and experimental sliding surfaces, which may be a further explanation for the ultra-low friction during the rapid sliding, besides the presence of a significant basal pore-gas pressure from CO2 and steam.