Small-volume pyroclastic density currents (PDCs) are generated frequently during explosive eruptions with little warning. Assessing their hazard requires a physical understanding of their transport and sedimentation processes which is best achieved by the testing of experimental and numerical models of geophysical mass flows against natural flows and/or deposits. To this end we report on one of the most detailed sedimentological studies ever carried out on a series of pristine small-volume PDC deposits from the 1975 eruption of Ngauruhoe volcano, whose emplacement were also witnessed during eruption. Using high-resolution GPS surveys, a series of lateral excavations across the deposits, and bulk sedimentological analysis we constrained the geomorphology, internal structure and texture of the deposits with respect to laterally varying modes of deposition.
Deposition from these PDCs began only on slopes at or around the material's angle of repose (c. 30°). In unconfined settings, the granular PDCs are interpreted to have been quasi-steady, forming sheets and lobes around the angle of repose. Where flows were confined, sheet-like proximal facies made up around 10% of the deposit volume at the angle of repose, but 90% of the material was deposited from apparently unsteady inertial granular PDCs as a distal levée-and-channel facies on slopes well below the repose angle. Hence, confined PDCs were able to travel up to 50% farther than unconfined flows. In the distal facies the deposit width is inversely correlated to the local slope, and the height of the levées (above the deposit centreline) is positively correlated with slope. Internally the deposits comprise three parts, a coarse-grained fines-free sole layer that laterally connects to levées (Zone I), an ashy matrix-supported central body (Zone II) and an overlying coarse plaster of clasts (Zone II). Trends in grain-size data suggests these zones derive from a continuous un-mixing of coarse particles from the initial bulk material by granular segregation that preferentially drives large particles to the upper free surface of the flow where they are concentrated at the front of flow before being deposited and overrun. By comparison to analogue experiments, we suggest a model of flow and deposition where the temporally and spatially varying mode of deposition is determined by the flow velocity, the local slope, the vertical velocity gradient, the velocity gradient at the free surface and the vertical deposition rate. Using this model, estimated vertical deposition rates of c. 5 cm s− 1 from the Ngauruhoe PDCs agree with those determined in laboratory experiments on inertial granular flows.