The use of physical and chemical techniques in the identification of Tephera (volcanic ash) in the North Island, New Zealand
type: CreativeWork
description: Field evidence is no longer adequate to resolve the problems of tephrostratigraphy, particularly in areas far from the likely volcanic source. In such areas the deposits are thin, and consequently more strongly weathered, as they are more or less continually involved in soil development processes. Under such conditions lithologic resemblance of a given ash to the same material closer to source may be lacking. The identification and correlation of tephra in the Waikato Basin of the North Island of New Zealand is one such example. These tephra form the parent materials of the agricultural soils of the region, and are at considerable distances from the likely sources, these being the Egmont volcanic centre in Taranaki, the Okataina volcanic centre in the Bay of Plenty, and the Tongariro and Taupo volcanic centres in the Central North Island. In particular, this thesis sought to resolve the problem of the identity of the Tirau and Mairoa Ash beds; whether they merely represent the products of pedogenesis of the same parent materials under differing weathering conditions. In the resolution of this problem, many laboratory techniques were exhaustively and systematically investigated, the methods being reviewed and extended in their applicability and their sensitivity being improved where possible. Physical properties of the rhyolitic glass associated with the samples that were found to provide useful data were refractive index, density, and magnetic properties. (a) Refractive index: This classical method of determination of a parameter indicative of bulk composition was improved an order of magnitude over that typical for petrologic investigation by the development of a thermal variation method. The increased sensitivity so obtained enabled the use of this parameter in the correlation of glass shards. The use of refractive index for this purpose had previously been abandoned because of its lack of sensitivity. (b) Specific gravity: The determination of density was investi…
abstract: Field evidence is no longer adequate to resolve the problems of tephrostratigraphy, particularly in areas far from the likely volcanic source. In such areas the deposits are thin, and consequently more strongly weathered, as they are more or less continually involved in soil development processes. Under such conditions lithologic resemblance of a given ash to the same material closer to source may be lacking. The identification and correlation of tephra in the Waikato Basin of the North Island of New Zealand is one such example. These tephra form the parent materials of the agricultural soils of the region, and are at considerable distances from the likely sources, these being the Egmont volcanic centre in Taranaki, the Okataina volcanic centre in the Bay of Plenty, and the Tongariro and Taupo volcanic centres in the Central North Island. In particular, this thesis sought to resolve the problem of the identity of the Tirau and Mairoa Ash beds; whether they merely represent the products of pedogenesis of the same parent materials under differing weathering conditions. In the resolution of this problem, many laboratory techniques were exhaustively and systematically investigated, the methods being reviewed and extended in their applicability and their sensitivity being improved where possible. Physical properties of the rhyolitic glass associated with the samples that were found to provide useful data were refractive index, density, and magnetic properties. (a) Refractive index: This classical method of determination of a parameter indicative of bulk composition was improved an order of magnitude over that typical for petrologic investigation by the development of a thermal variation method. The increased sensitivity so obtained enabled the use of this parameter in the correlation of glass shards. The use of refractive index for this purpose had previously been abandoned because of its lack of sensitivity. (b) Specific gravity: The determination of density was investi…
author: Hodder, A.P.W.
license: https://researchcommons.waikato.ac.nz/bitstreams/0c3699af-e7ee-4751-a40e-d9911f886a31/download
date published: 1974
date modified: 2026-07-24T05:57:24Z
provider: University Waikato
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: The University of Waikato