Energy budget for a rock avalanche: fate of fracture-surface energy

A detailed energy budget of a rock avalanche at Lake Coleridge, New Zealand, included fragments as small as 70 nm in diameter in the debris particle-size distribution, and used ultrasonic disaggregation of agglomerates formed during emplacement of the deposit to properly sample the fine fragments created during the rock avalanche. Using the maximum likely value of potential energy released during the debris fall and runout, and minimum values of energy lost to friction and of energy used in creating new rock surface area by fragmentation, the energy budget showed a substantial energy deficit; the available potential energy almost matched the energy lost to friction, leaving very little energy available for creating the new surface. This deficit was much less prominent if sub-micron fragments were ignored as in earlier energy budgets, because about 90% of total fragment surface area occurred on sub-micron fragments. Close examination of possible sources of error in the calculated budget leads to the conclusion, supported by published data, that only a small proportion of the energy used to create new rock surface transforms to surface energy, while a large proportion of it remains available in the form of elastic body-wave energy.