Internal flow variations and diachronous sedimentation within extensive, sustained, density-stratified pyroclastic density currents flowing down gentle slopes, as revealed by the internal architectures of ignimbrites on Tenerife

During a protracted explosive eruption, at least four laterally extensive and sustained pyroclastic density currents radiated across the flanks of Las Canadas volcano, Tenerife. Each pyroclastic current developed marked local and regional spatial variations in response to the incised, gently concave substrate topography. The locations of these variations shifted in space as rapid sedimentation from the current progressively buried and modified the topography. This complex, shifting response of the density currents to minor topographic variations has been reconstructed in high-resolution over a wide area (>500 km(2)) using the internal architecture of cryptic time-surfaces (entrachrons) marked by compositional zoning in the deposit, including variations in clast types. Valley-side field relations reveal that the currents were density-stratified. Yet, at a single instant in time, the lower levels of each current comprised a granular-fluid at some locations but were fully dilute and turbulent at others. Moreover, the locations of these variations shifted geographically as the topography changed during the eruption. The variations within the current are recorded by numerous superbly exposed gradational transitions from various stratified to massive lithofacies, both laterally and in the downcurrent direction. Individual currents were regionally widespread and travelled >15 km, but deposited only in longitudinally restricted, localised zones that spanned several small valleys and interfluves. The currents bypassed slopes up- and downcurrent of the restricted depositional zones, without depositing. The locations of deposition then gradually shifted with time, such that the extensive deposit sheet was gradually assembled beneath the sustained current in a diachronous fashion. Onlap relationships of internal entrachrons reveal that the base of the ignimbrite sheet and even the bases of individual flow-units are markedly diachronous. Deposition of a flow-unit commenced and ceased at different times in different places. This study suggests that in hazard assessments: (a) models of density currents that incorporate only pre-existing topography (e. g. from DEMs) may give misleading results in the case of sustained currents because sedimentation from these significantly modifies the topography during emplacement, altering flow paths; (b) frequencies and scales of previous pyroclastic currents determined from pyroclastic successions are likely to be significantly under-estimated because currents commonly bypass without leaving a deposit record; and (c) even where preservation appears to be complete, an ignimbrite at a single exposure commonly will not record the current's entire flow history at that site.