Charring of woods by volcanic processes: An example from the Taupo ignimbrite, New Zealand

Abstract

One of the most violent and explosive eruptions in historic times was that of the Taupo volcano in New Zealand 1800 years ago. This culminated in emplacement of the Taupo ignimbrite, laid down by a single vent-generated pyroclastic density current that entombed and preserved abundant plant material. The ignimbrite consists of two layers, generated by processes within the concentrated part of the density current (flow, sensu stricto) and where the current interacted with the atmosphere and the pre-eruptive vegetative cover. Eighty-four wood samples were collected from eight localities representing both layers, at varying distances from the vent, including the previously described Pureora Fossil Forest. Samples were studied by light, scanning electron and reflectance microscopy in order to determine their systematic affinity (angiosperm or conifer), degree of charring and reflectance values. Most samples (59) were conifer, probably derived from podocarps that are known to have been common in the pre-eruptive vegetation. Samples ranged from uncharred, through incompletely charred to fully charred woods. The variation in degrees of charring and reflectance values implies that the entrained charcoal did not equilibrate with the flow temperature. Minimum charring temperatures, interpreted from the mean of the three highest maximum reflectance values, showed no major differences between ignimbrite layers and were generally low, ranging from 269 to 398 °C. Both ignimbrite layers also show a trend of increasing deposit temperature away from the vent, consistent with estimates from palaeomagnetic techniques.