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Rheology and Flow Emplacement Processes of the 1954 Lavas, Mount Ngauruhoe

The rheology and emplacement dynamics of basaltic and rhyolitic lava flows have been studied extensively, leading to the development of numerous rheological and flow behaviour models; however, the flow dynamics of more intermediate composition lavas, particularly those emplaced on relatively steep slopes, is less well-constrained. The 1954-55 eruption of Mount Ngauruhoe, a young, composite cone of the Tongariro Volcanic Centre, generated at least 11, well-preserved, spatter-fed, basaltic andesite a'a lavas on the steep, north-west flanks of the cone. The rheological properties and flow dynamics of these lavas are quantified by incorporating morphological, petrographic and geochemical data collected from these flow deposits into a range of existing numerical models, and the results compared with documented eye-witness accounts. The lava flow deposits are typically long, narrow, discrete units characterised by comparable morphological traits on the steep slopes of the cone and varying in dimension, morphology and flow surface features on the shallower slopes. Flow surfaces are typically autobrecciated and display a large-scale, lateral trend in clast size and morphology across flow widths. The 1954 lavas are petrographically and geochemically homogenous with no apparent trends associated with successively emplaced lava flows. The rheological properties of the lavas at the time of initial flow advance are therefore assumed to be comparable for each flow. Lava viscosity was estimated at 102 to 104 Pa s for the temperature range 1150 to 950 C. Yield strength was difficult to quantify but is assumed in this study to be relatively low (~ 25 Pa). Calculated flow velocity, effusion rate and emplacement duration are not well-constrained against documented eye-witness accounts. Mean flow velocity (0.03 to 0.04 m/s) was estimated from eye-witness reports, and used to determine flow emplacement durations between ~ 2 to 48 hours, comparable with documented duration times. Effusion rates could not be definitively quantified but flow deposit morphology and documented accounts indicate that intermittent episodes of high effusion rates over a short duration were associated with the emplacement of the 1954 lavas. Three major controls on the emplacement of the 1954 lava flows have been identified. Effusion rate and duration was the primary control on the development of single unit lava flows, flow channel drainage on steep slopes and in limiting run-out distances from the vent. Low initial viscosity and yield strength promoted high flow velocities on steep slopes and low surface cooling rates. Relatively short flow emplacement duration times precluded significant downflow viscosity and yield strength increases. Slope gradient and topographic obstacles were major controls on flow emplacement processes. Slope gradient was the dominant control on flow velocity, flow width and depth and surface autobrecciation, while morphology, flow path direction and surface folding were constrained by local slope gradient variations and/or topographic obstacles.
Type of thesis
Sanders, F. (2010). Rheology and Flow Emplacement Processes of the 1954 Lavas, Mount Ngauruhoe (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/4398
The University of Waikato
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