The use of palaeomagnetism and rock magnetism to understand volcanic processes: introduction

This Special Publication provides a snapshot of our understanding of volcanic processes through the use of palaeomagnetic and rock magnetic techniques. Here, we provide a context for the book, placing individual chapters within the milieu of previous work, including some magnetic techniques that were not used in the particular studies described herein. Thermoremanent magnetization is a powerful tool to understand processes related to heating and cooling of rocks, including estimating the temperature of emplacement of pyroclastic deposits, which may allow us to better understand the rates of cooling during eruption and transport. Anisotropy of magnetic susceptibility and anisotropy of remanence are used primarily to investigate rock fabrics, and allow the interpretation of flow dynamics in dykes, lava flows and pyroclastic deposits, as well as the location of the eruptive vents. Rock magnetic characteristics can help in the correlation of volcanic deposits but also provide means to date volcanic deposits and to better understand the processes of cooling of the deposits, as the magnetic minerals can change with temperature. In addition, volcanic rocks may be key recorders of past magnetic fields, allowing a better understanding of changes in field intensity and, perhaps, providing clues of how the magnetic field is formed. Over the past several decades, palaeomagnetic and rock magnetic techniques have been applied to numerous problems in volcanology. Since Aramaki & Akimoto’s pioneering study in 1957, which used natural remanent magnetization (NRM) data to qualitatively estimate the temperature of emplacement of selected pyroclastic deposits in Japan (Aramaki & Akimoto 1957), many studies have utilized and refined the approach of using progressive thermal demagnetization data to estimate emplacement temperatures of pyroclastic deposits and have demonstrated the utility of this approach in many settings. Another extensively utilized technique is that of anisotropy of magnetic susceptibility (AMS), which was first applied to pyroclastic volcanic rocks by Ellwood (1982) on ignimbrites in the San Juan Mountains, SW Colorado, USA, to estimate flow fabrics and, in that study, to infer source locations through triangulation. It is staggering to think of the number and diversity of AMS studies on volcanic rocks that have been undertaken since that time, in a bit more than three decades! Remanent magnetization characteristics have long been used to correlate volcanic rocks and such data, together with rock magnetic properties, have been employed to study crystallization and cooling processes (e.g. Schlinger et al. 1988; Wells & Hillhouse 1989). As techniques have been refined, and the sensitivity, precision, reliability and/or speed of measurements for an array of modern instrumentation all have improved, workers have been able to study volcanic problems in more detail, pulling out details of the processes that have not previously been considered. In addition, the types of problems addressed have diversified considerably. We intend this Special Publication to provide a representative suite of contributions that help to summarize the current array of studies that apply palaeomagnetism and rock magnetism to volcanic processes, and to stimulate further research in this exciting science! This book is a result of two sessions on volcanic processes and palaeomagnetism held at the International Union of Geodesy and Geophysics Congress in Melbourne, Australia, in July 2011, and at the American Geophysical Union Annual Meeting in San Francisco, USA, in December 2011. From: Ort, M. H., Porreca, M. & Geissman, J. W. (eds) 2015. The Use of Palaeomagnetism and Rock Magnetism to Understand Volcanic Processes. Geological Society, London, Special Publications, 396, 1–11. First published online January 20, 2015, http://dx.doi.org/10.1144/SP396.17 # 2015 The Geological Society of London. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Angharad Hills at the Geological Society of London graciously and tactfully guided this book and the editors through the entire process from chapter solicitation to publication. Thermoremanent magnetization in volcanic rocks and emplacement temperature