Ignimbrite morphology and the effects of erosion: a New Zealand case study

Abstract

Ignimbrite morphology, previously generalised using aspect ratios, is here quantified as the relationships between the various thicknesses of material forming an ignimbrite and the areas and volumes represented by those thicknesses. The morphology can be measured for the deposit in its present-day, eroded condition, or reconstructed for the original deposit. The reconstructed morphology of the 22 500 year BP, ca. 11 500 km², ca. 300 km³ Oruanui ignimbrite in New Zealand is documented to illustrate the latter approach. The Oruanui ignimbrite is an intermediate aspect ratio deposit and shows broadly linear relationships between (1) In thickness and the cumulative area occupied by that thickness or less of material and (2) thickness and the volume represented by that thickness or less of material. Two theoretical morphologies, one where thicknesses exponentially decay with distance from a maximum and the other of uniform thickness (“slab”), are compared with the Oruanui data. Limited comparative data suggest that low aspect ratio (violently emplaced) ignimbrites will show upward-concave curves (at one extreme following the “exponential decay” model) and high aspect ratio (gently emplaced) ignimbrites downward-concave curves (with the “slab” model as an extreme) when plotted on diagrams where the Oruanui data show linear trends. The effects of erosion on Oruanui and model ignimbrite morphologies are modelled using two theoretical erosion scenarios: (1) material is evenly removed from the land surface, and (2) thinner, non-welded material is preferentially removed. For the Oruanui ignimbrite data, area is lost much more rapidly in the first instance than volume; for example, 5 m of erosion is sufficient to remove 50 area %, whereas 40 m (scenario 1) or 120 m (scenario 2) of erosion is required to remove 50 volume %. In old ignimbrites, volume estimates may be reasonably accurate even after strong erosion, provided the original thicknesses of ponded/landscape-forming material can be inferred, but estimates of original area and aspect ratio will be inaccurate. An envelope enclosing all known outcrops of an ignimbrite will give a better estimate of original area and aspect ratio than simply summing the areas of known outcrops.