Methods of identifying late Quaternary rhyolitic tephras on the ring plains of Ruapehu and Tongariro volcanoes, New Zealand

Abstract

On the ring plains of Ruapehu and Tongariro volcanoes, distal rhyolitic marker tephras provide a valuable stratigraphic framework. However, identification of many of these tephras has been imprecise. Here we provide a quantitative approach for identifying tephras within the ring‐plain sequences. We extend from simple canonical discriminant function models of glass chemistry to show how these, in conjunction with other geological information, can be used in a practical field‐based study. In two stratigraphically distinguishable groups (10–22 ka and 22–65 ka), we established discriminant models for possible tephra correlatives from standard glass analyses. Testing analyses from unknown tephras against the models classified 34 of the 41 samples with probabilities >0.75 to tephras that were consistent with mineralogical and stratigraphic evidence. Unknowns with lower probabilities of classification had several possible correlatives. Some of these were improved when the tephras classified with >0.75 probability, and which were consistent with stratigraphic and other evidence, were added to the discriminant models. The classifications were improved because of an increased number of samples for each tephra and also because the added analyses were produced by the same EMP operator under the same instrument conditions. Classifications of other unknowns were improved by considering them as mixed tephras. In addition to more rigorously correlating several tephras previously identified in this area, we have identified four tephras in the area for the first time—the Okaia, Omataroa, and Hauparu Tephras and the Rotoehu Ash. These occur as microscopic accumulations of rhyolitic glass shards within weathered andesitic tephra deposits.