Eychenne, J., Rust, A., Cashman, K., & Wobrock, W. (2017). DistalEnhanced Sedimentation From Volcanic Plumes: Insights From theSecondary Mass Maxima in the 1992 Mount Spurr Fallout Deposits. Journalof Geophysical Research: Solid Earth, 122(10), 7679-7697.https://doi.org/10.1002/2017JB014412

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Journal of Geophysical Research – Solid Earth

Supporting Information for

Distal enhanced sedimentation from volcanic plumes: insights from the secondary

mass maxima in the 1992 Mount Spurr fallout deposits

Julia Eychenne ab*, Alison Rust b, Katharine Cashman b, Wolfram Wobrockc

a Laboratoire Magmas et Volcans, Université Clermont Auvergne - CNRS - IRD, OPGC, Campus Universitaire des Cézeaux, 6 Avenue Blaise Pascal, 63178 Aubière Cedex

b School of Earth Sciences, University of Bristol, United Kingdom

c Laboratoire de Météorologie Physique, Université Clermont Auvergne - CNRS, OPGC, Campus Universitaire des Cézeaux, 4 Avenue Blaise Pascal, 63178 Aubière Cedex

* Corresponding author: julia.eychenne@uca.fr

Contents of this file

Figures S1 to S9

Introduction

The supporting information present additional figures cited in the text.

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Figure S1. Deconvolution of the bimodal grain size distributions. (A) List of the bimodal samples

in the August 18, 1992 Spurr fallout deposit and legend. (B) Illustration of the deconvolution

method using the automated code DECOLOG 5.0 (Bellotti et al. 2010; Caballero et al. 2014). The

bimodal probability density function of the bulk grainsize distribution is deconvolved into a

coarse (red) and a fine (blue) subpopulation. (C) Bulk cumulative grain size distributions of the

bimodal samples from the August 18, 1992 Spurr fallout deposit. (D) Deconvolved

subpopulations of the bimodal grain size distributions in (C). Thick curves = coarse

subpopulations. Thin curves = fine subpopulations. Samples in (C) and (D) are colour coded by

their distance from vent (see legend in (A)).

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Figure S2. Altitude of different sizes of tan (A) and grey pumices (B) of sphericity 0.8 in the wind

field of August 18, 1992, as a function of the horizontal distance travelled. Terminal velocity of

different sizes of tan (C) and grey pumices (D) of sphericity 0.8 in the wind field of August 18,

1992, as a function of the horizontal distance travelled.

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Figure S3. Wind profiles (speed in red and direction in blue) measured at Anchorage International

Airport around the times of the August and September 1992 Spurr eruptions. The thick lines in

both plots represent the profiles used to run the settling simulations.

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Figure S4. Example of shapes with 3D sphericity values varying between 1 and 0.2.

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Figure S5. Density variations with grainsize for tan and grey pumices. (A) Density profile used by

default as input in the settling model. (B) Density profile used for comparison in discussion.

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Figure S6. Mean Circle Equivalent (CE) Diameter of dense (filled diamonds) and vesicular

particles (open diamonds) measured in the Vt groups identified within tephra sample 44 (Fig. 2)

of the August 18, 1992 Mount Spurr fallout deposit (data after Riley et al (2003)). This figure

complements Fig. 8 of the main text. Here, for a sphericity of 0.8, Vt at sea level was also

recalculated for particles expected to reach the location of sample 44 assuming that the density

of the particles reached the solid density at 15.6 µm rather than 125 µm (stars with red contour,

T = tan pumices, G = grey pumices). See Fig. S.4 for the new density profile. We note that the

variations in density have a small impact on the size and Vt of the particles expected to reach the

location of sample 44.

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Figure S7. Comparison of the distance travelled by particles of different grainsizes in a strong

wind field (wind profile from 18 August 1992 16:00 ADT), and a weaker wind field (wind profile

from 19 August 1992 04:00 ADT). Closed symbols represent samples located on the deposit axis.

We note that the decrease in horizontal wind speed in the high velocity atmospheric layer

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(altitude of 10 to 12 km) results in significantly shorter travel distances for particles coarser than

~100 µm.

Figure S8. Mean roughness and sphericity of dense (filled symbols) and vesicular particles (open

symbols) in each Vt group. Data from Riley et al (2003). Roughness and sphericity are calculated

from the 2D projected shape of the grains as: Roughness = Convex Perimeter / Perimeter of

particle, and Sphericity = (4π x Area of particle) / (Perimeter of particle)².

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Figure S9. Horizontal (arrows) and vertical (coloured areas) wind patterns in the Alaska region

downwind of Mount Spurr on August 18 and September 17, 1992, about 4 hours after the

eruptions started. The wind field is simulated at the mesoscale with the NCAR Clark-Hall cloud

scale model (Clark et al. 1996) using meteorological data from the ECMWF-ERA-interim

database. (A & B) Maps of the wind pattern at 6 km a.s.l. (C & D) Maps of the wind pattern at 8

km a.s.l. The arrow length and direction represent the speed and xy direction of the horizontal

component of the wind, respectively. The vertical component of the wind is colour-coded (see

legend), with positive and negative velocities corresponding to upward and downward winds,

respectively. The scales for vertical and horizontal wind velocities are the same in (A & B) as (C

& D). The black and grey lines represent the East-West traverse represented in the profiles of

Fig. 7A & B, and the plume trajectories, respectively.