Application of tephra volume models to ejecta volumes from subsurface explosion experiments
Deposit volume is a critical factor for reconstructing an explosive eruption. Volume estimate models typically used for large Plinian deposits have been adapted and improved repeatedly over the last few decades. Less work has been done to refine a method for estimating the volume from smaller deposits produced by discrete phreatic and phreatomagmatic explosions. The characterization of the volume and distribution of deposits is required to quantify the physical hazards presented by different explosion types and develop appropriate models of future eruptions. Six classic tephra volume models were assessed using a dataset from subsurface explosion experiments. The models typically did a poor job modelling the volume of proximal deposits as a component of total deposit volume of discrete explosion deposits. Models reproduced medial and distal deposit volumes with greater success, particularly the Exponential model and a more recent Linear Regression model. It is therefore recommended, when possible, to use digital elevation models produced from GPS or laser-based methods to characterize proximal deposits separately and to use tephra volume estimates for medial and distal deposits. Additionally, this dataset enabled the comparison of ejecta volumes with crater diameters and highlighted that this relationship only holds for simple crater scenarios without any lateral vent migration, collapse or erosion of the crater under study. The assessment and improvement of these methods are required to ensure accurate deposit volumes as they serve as one of the most important inputs to hazard assessments and numerical models.
Another look at the calculation of fallout tephra volumes
The calculation of volumes of fallout tephra layers is difficult because of the nonlinear dependence of thickness on area and because of the extrapolations required at the vent and in distal regions. Calculation using the trapezoidal rule, straight lines on log-log plots of area versus thickness, straight lines on plots of log thickness versus area1/2, and the crystal-concentration method are reviewed and the problems with each method discussed. The method using straight lines on plots of log thickness versus area1/2 is the most geologically reasonable because most deposits thin exponentially from source and therefore plot as straight lines using these coordinates. Errors and uncertainties in previous derivations for using this method are discussed and more general formulas presented. The method is also used to gain perspective on the “missing” distal volumes calculated by the crystal-concentration method compared to those calculated based only on isopach data.
Laschamp and Mono Lake geomagnetic excursions recorded in New Zealand
Eight basaltic lavas from the Auckland volcanic field, New Zealand, record three distinct sets of excursional geomagnetic field directions and low paleointensities, however the timing and therefore paleomagnetic significance of these records have been poorly understood. Radiocarbon, K–Ar, and thermoluminescence dating constrain these lavas to have erupted during the last 75 ka, a period during which as many as three excursions have been recorded differentially at several northern and fewer southern hemisphere sites. Forty 40Ar/39Ar incremental heating experiments conducted on groundmass from seven of these excursional lavas indicate that they erupted during at least two periods, at 39.1 ± 4.1 and 31.6 ± 1.8 ka, coincident with 40Ar/39Ar, astrochronologic, and 14C ages determined for the Laschamp and Mono Lake excursions, respectively. Samples from a lava flow associated with a third cluster of virtual geomagnetic poles (VGPs) are complicated by low concentrations of radiogenic argon in the presence of excess argon, and thus yield discordant age spectra and an imprecise age of 26.6 ± 8.1 ka. Our findings indicate that the Laschamp and Mono Lake excursions, until recently identified unequivocally and isotopically dated only in the Northern Hemisphere, were globally synchronized at 40 ± 1 and 32 ± 1 ka. However, the VGPs of lavas that record the Laschamp excursion in New Zealand and France are inconsistent with a simple clockwise looping geometry inferred from VGP paths obtained in eight marine sediment cores spanning the Laschamp excursion. We suggest that the differences in VGPs recorded at the various sites are significant and may point to non-axial dipole components and lower mantle control on transitional fields during short-lived excursions.