Blake, S.; Wilson, C.J.N.; Smith, I.E.M.; Leonard, G.S. 2006 Lead times and precursors of eruptions in the Auckland Volcanic Field, New Zealand : indications from historical analogues and theoretical modelling. Lower Hutt, N.Z.: GNS Science. GNS Science report 2006/34 22 p.
Abstract: The monogenetic basaltic eruptions in the Auckland Volcanic Field (AVF) are fed from the upper mantle by dikes, with little or no evidence from geochemical data for magma stalling at intermediate depths. This report attempts to quantify the likely duration of pre-eruptive unrest prior to a future AVF eruption that such a transport system will present. In the absence of historical information from the AVF itself, the report presents a review of precursor phenomena from historical events that can be considered to represent plausible analogues to AVF eruptions, plus a theoretical modelling study of the speeds of dike propagation and mafic magma ascent to the surface. Eleven historical mafic eruptions that share the AVF’s traits of being deeply sourced alkali basalts, or show little evidence for having stalled at and then ascended from shallow depths, are reviewed. Reports from these events indicate that instrumental and/or felt seismicity occurred for periods of between about a day and months to years before eruption, with a majority having less than 2 weeks of felt seismicity. In addition, data show that the epicentres of the earthquakes were not always directly above the eventual vent sites. Other precursors (e.g. thermal/chemical changes or ground deformation) were absent, or occurred closer in time to the eruption than the seismicity. Theoretical modelling of dike propagation also suggests transit times through the crust of the order of up to 2 weeks, but mechanical models predict dike widths and lengths that are seemingly too narrow and too great, respectively, to match surface observations from AVF volcanoes. It is argued that these models neglect important thermal effects that, under suitable conditions favoured by low magma flow rates, lead to focussing of the upward flow of magma. This may cause the magma to rise in a complex branching pattern that can focus magma to a single point or very short dike segment at the surface. The study identifies the need for (i) increased definition of the state of the upper mantle beneath Auckland and (ii) petrologic studies and modelling of crystal zoning patterns in AVF basalts in order to calculate actual transit times for specific eruptions. (auth)