Using ignimbrite morphology to assess emplacement and post-emplacement processes
Whelley, Patrick Liam
MetadataShow full item record
This study explores quantitative methods for characterizing and differentiating ignimbrite terrain using deposit morphology from which inferences can be made about parent flow emplacement conditions as well as post-depositional processes. The study has three parts: (1) A quantitative description of the morphology of the 1993 pristine pumice-flow deposit (∼0.06 km 3 : a small-volume ignimbrite) at Lascar volcano, Chile. The focus of this component of the thesis is the detailed structure of the lobate pumice flow termini emplaced during the final stage of deposition. By preferentially pushing larger clasts to the lobe fronts and margins, clast sorting by size is found to be an important process, which defines the shape and controls the propagation of these lobes. This is discussed in terms of h stop , a parameter used in the experimental granular flow community to understand flow spreading. (2) On the same deposit, a suite of subtle post-depositional changes is also studied. In particular, I investigate a fracture pattern overprinted on the Lascar pumice lobes. The fracture network is found to result from the combined effects of deaeration, which occurred during and soon after emplacement, and a decade of ongoing compaction and cooling. The identified pattern is indicative of these processes acting in an arid (Altiplano) environment. (3) The degradation of a pumice flow deposit in a temperate environment at Mount St. Helens, WA is investigated. Here, patterns in ground surface roughness based on high-resolution LiDAR data are used to classify and differentiate both primary and reworked volcanic deposits. The distribution of surface types is then used to describe the fluvio-volcanic history of the Mount St. Helens Pumice Plain . Each component of the dissertation addresses a unique aspect of the larger question: How is ignimbrite morphology related to emplacement and erosive processes? Deposit morphology is, to a great extent, found to be dependent on both size and density segregation processes during flow emplacement. Where deposits are dominated by fine ash, they have little or no relief, but where larger particles accumulate (i.e., lithic breccias, pumice blocks, and bombs), deposits produce topography that can be easily distinguished and interpreted. Furthermore, secondary processes affect ignimbrite morphology at different rates and to different degrees depending on slope, deposit temperature, and level of deposit induration as well as environmental context. Therefore, both emplacement conditions and erosive processes, affect deposit morphology in quantifiable and distinguishable ways. Results from this thesis are directly applicable to models that approximate the transport and emplacement behavior of pyroclastic density currents, eventually leading to improved hazard mitigation as well as modeling of erosional processes on new landforms. The methods developed here will also be useful for investigations of other more remote and less pristine deposits in both the terrestrial and planetary environments.