Some magnetic properties of two recent lava flows from mount ngauruhoe, tongariro volcanic centre, New Zealand

Abstract

The andesite lava flow of 1954 June 30 and an earlier flow, believed to be between 500 and 1, 800 years old, each appear to contain three verticallydistributed, magnetic units: a basal unit of non-vesicular rocks, generally overlain by a unit of vesicular rocks, .and capped by clinker.The intensities of total magnetisation, intensities of remanent magnetisation, and Koenigsberger Ratio of specimens from the vesicular and nonvesicular units have log-normal distributions, similar to those reported in much older igneous rocks. Since these flows are very young, it is inferred that the log-normal distribution is a basic property gained by rocks when they acquire a thermoremanent magnetisation, and is not a result of weathering, or some other time-dependent process.Only in the non-vesicular unit of the Earlier Flow are the directions of magnetisation near-uniform; the directions in the other units are scattered, probably due to movements of parts of the flow during or subsequent to the rocks acquiring their magnetisation. © 1986 Journal of the Royal Society of New Zealand.