Size of volcanic ash
William I Rose
Michigan Technological University
Houghton, MI 49931 USA
Ashfall Graduate Class 2009
Lecture #4
Tephra is classified on the basis of
pyroclast size:
ASH -- Very fine-grained fragments (< 2 mm),
generally dominated by broken glass shards, but
with variable amounts of broken crystal and lithic
(rock) fragments. Courtesy of USGS.
LAPILLI -- Pea- to walnut-size pyroclasts (2 to 64
mm). They often look like cinders. In water-rich
eruptions, the accretion of wet ash may form
rounded spheres known as accretionary lapilli (left).
Courtesy of USGS.
BLOCKS AND BOMBS -- Fragments >64 mm. Bombs
are ejected as incandescent lava fragments which
were semi-molten when airborne, thus inheriting
streamlined, aerodynamic shapes. Blocks (not
shown) are ejected as solid fragments with angular
shapes. Courtesy of J.P. Lockwood, USGS.
www.geology.sdsu.edu/how_volcanoes_work
Like many natural and man made
materials, volcanic ash is made up of
materials of variable size. Some
times a sample of ash has a simple
size range that can be described in a
straightforward way…
But there are some methods that
have developed to make it easier to
compare samples.
Lognormal size
distributions are
“expected” and
we use a
“biased” system
to define them
Φ phi
Φ = - log2 d (mm)
BOMBS, BLOCKS
LAPILLI
ASH
NON-GENETIC CLASSIFICATION OF PYROCLASTIC ROCKS
Ash tuff - rock dominated by ash; sometimes simply
referred to as tuff.
Lapilli tuff - rock dominated by lapilli.
Tuff breccia - rock containing 25% to 75% blocks and/or
bombs.
Pyroclastic breccia - rock containing at least 75% blocks
and bombs.
Agglomerate - rock containing at least 75% bombs.
Agglutinate - rock composed of fused, largely
unrecognizable, basalt spatter fragments.
www.geology.sdsu.edu/how_volcanoes_work
Pyroclastic fall deposits
Ashfall -- finer than 2 mm
Scoria fall -strombolian/cinder cone
Pumice fall – highly
vesiculated silicic
pyroclasts from plinian
eruptions
www.geology.sdsu.edu/how_volcanoes_work
d < 1000 μm
Φ>0
d < 30 μm
Φ>5
There is a revolt about
using phi system for
description, (see
papers by S.
Dartevelle)
Metric scale
parameters:
First moment
(arithmetic metric
mean) (mm)
SD (metric sorting)
(mm)
SSA Specific Surface
Area (m2/m3 or m-1)
Specific surface area
(m2/g)
from S. K. Friedlander,
Smoke, Dust and Haze
2000
Fu74-49
12.0
10.0
8.0
Particle Diameter ()
6.0
4.0
2.0
Sieve range
35.0
30.0
Class Weight (%)
25.0
0.0
“Normal” ashfall, with
dominant coarse mode
20.0
15.0
10.0
5.0
Because this fine-grained mode is minor
and was grouped in the “pan” portion of
sieved samples, it is underappreciated and
is quite common.
0.0
1
10
100
1000
Particle Diam eter 
( m)
Ashfall sample from Fuego Volcano,
Guatemala 14 Oct 1974
Rose et al., 2007 BV
-2.0
How to measure the fine ash
Forward Fraunhofer Diffraction of laser light
Particle Size Analysis
Malvern Mastersizer 2000; Cambridge University
Dispersion unit
Particle Diameter ()
12.0
10.0
8.0
6.0
4.0
2.0
-2.0
VF 74-103
10.0
17 Oct 1974 Fuego.
8.0
Class Weight (%)
0.0
This distal fall has only
a very minor coarse
mode, with a dominant
mode at ~5 .
6.0
4.0
2.0
0.0
1
10
Particle Diam eter ( m )
100
Sieve range 1000
Laser diffraction expands the range of precise GSD work to submicron
diameters.
Rose et al., 2007 BV
New Laser diffraction data on distal/fine ashfall
samples from 10 - >1000 km distance:
Volcano
Magma
Style
VEI
Fuego
San Miguel
Spurr
Colima
Redoubt
Augustine
Pinatubo
El Chichón
St Helens
Santiaguito
Bruneau-J
Basalt
subplinian
2- 4
Basalt
strombolian? 1
Andesite subplinian
3-4
Andesite peleean/
1-3?
Andesite peleean
2-3
Andesite peleean
2-4
Andesite plinian
6
Trachyandesite plinian 5
Dacite
plinian
5
Dacite
peleean
1-2
Rhyolite plinian
8
Dates
1973-74
1970
1992
2000-2006
1989-90
1986
1991
1982
1980
1968-2006
11 ma
Rose et al., 2007 BV
Fu74-17
12.0
10.0
Particle Diameter ()
6.0
4.0
8.0
2.0
0.0
14.0
12.0
Class Weight (%)
10.0
8.0
6.0
4.0
A typical example of the ashes we have
studied, this sample has two obvious
modes, a coarse mode at 0-1 and a
fine mode at 4-5. These two modes
are unlikely partners. Why do they
occur together?
We suggest that, like
raindrops, big pyroclasts
capture smaller ones as they
fall.
2.0
0.0
1
10
Particle Diameter (m)
100
1000
-2.0
Fu74-40
12.0
10.0
8.0
Particle Diameter ()
6.0
4.0
2.0
0.0
16.0
14.0
Class Weight (%)
12.0
10.0
8.0
6.0
4.0
2.0
0.0
1
10
Particle Diam eter 
( m)
100
1000
-2.0
Fu74-200
12.0
10.0
8.0
Particle Diameter ()
6.0
4.0
2.0
0.0
16.0
14.0
Class Weight (%)
12.0
10.0
8.0
6.0
4.0
2.0
0.0
1
10
Particle Diam eter 
( m)
100
1000
-2.0
Fu74-1
12.0
10.0
8.0
Particle Diameter ()
6.0
4.0
2.0
0.0
18.0
16.0
Class Weight (%)
14.0
12.0
10.0
8.0
Fine grained
fine skewed
GSD with Md
of 4-5
6.0
4.0
2.0
0.0
1
10
Particle Diam eter 
( m)
100
1000
-2.0
Fu74-5
12.0
10.0
8.0
Particle Diameter ()
6.0
4.0
2.0
0.0
12.0
Class Weight (%)
10.0
8.0
6.0
4.0
2.0
0.0
1
10
Particle Diam eter 
( m)
100
1000
-2.0
Fu74-17
12.0
10.0
8.0
Particle Diameter ()
6.0
4.0
2.0
0.0
14.0
12.0
Class Weight (%)
10.0
8.0
Bimodal GSD with
two subequal
modes.
6.0
4.0
2.0
0.0
1
10
Particle Diam eter 
( m)
100
1000
-2.0
Fu74-22
12.0
10.0
8.0
Particle Diameter ()
6.0
4.0
2.0
0.0
20.0
18.0
16.0
Class Weight (%)
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
1
10
Particle Diameter ( m)
100
1000
-2.0
Fu74-30
12.0
10.0
8.0
Particle Diameter ()
6.0
4.0
2.0
0.0
30.0
Class Weight (%)
25.0
20.0
15.0
10.0
5.0
0.0
1
10
Particle Diam eter 
( m)
100
1000
-2.0
Fu74-32
12.0
10.0
8.0
Particle Diameter ()
6.0
4.0
2.0
0.0
35.0
30.0
Class Weight (%)
25.0
20.0
15.0
10.0
5.0
0.0
1
10
Particle Diam eter 
( m)
100
1000
-2.0
Fu74-49
12.0
10.0
8.0
Particle Diameter ()
6.0
4.0
2.0
0.0
35.0
30.0
Class Weight (%)
25.0
20.0
“Normal” ashfall, with
dominant coarse
mode
15.0
10.0
5.0
Because this fine-grained mode is
minor and was grouped in the “pan”
portion of sieved samples, it is
underappreciated and is quite
common.
0.0
1
10
Particle Diam eter 
( m)
100
1000
-2.0
General pattern of
decreasing Md
reflects changes
only in the coarse
mode
:
Fu-74-22
20
16
12
Wt %
8
4
0
-2
-1
0
1
2
3
4
5
6
7
8
9
10
11
-4
Phi Size
The bimodal GSDs can be unscrambled into lognormal
components using mathematical methods. We analyzed the
bimodal GSD by looking for lognormal subpopulations, using an
SFT software developed by K Wohletz et al., 1989, Jour
Geophys Res 94: 15703-15721
12
13
Related ashes such as
these from Fuego follow a
pattern with stable fine
modes at about 4-5  and
variable coarse modes
which shift to finer
medians with distance and
then merge with the fine
mode. The fine skewed
tails appear to contain one
or more finer modes
which represent no more
than a few percent of any
fall sample.
Rose et al., 2007 BV
Fu74-1
12.0
10.0
8.0
Particle Diameter ()
6.0
4.0
2.0
0.0
18.0
16.0
Class Weight (%)
14.0
12.0
10.0
8.0
Fine grained
fine skewed
GSD with Md
of 4-5
6.0
4.0
2.0
0.0
1
10
100
1000
Particle Diam eter 
( m)
Rose et al., 2007 BV
-2.0
:
20
15
Wt %
10
5
0
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10
11
12
6
7
8
9
10
11
12
13
Phi Size
:
40
30
Wt %
20
10
GSD patterns of distal fall deposits
are bimodal with a coarse population
that varies systematically with
distance and a persistent fine
population at about 4-5 . As the
overlap becomes more pronounced
the GSD looks skewed.
0
-2
-1
0
1
2
3
:
4
5
Phi Size
15
:
10
20
Wt %
5
16
0
-1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
12
14
Phi Size
Wt %
:
20
8
15
4
Wt %
10
0
5
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1
2
3
4
5
6
7
-4
Phi Size
Phi Size
8
9
10
11
12
1
:
Fu-74-22
20
15
Wt %
10
5
0
-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10
11
12
Phi Size
Simple idea: A bimodal grain size distribution suggests the larger ash particles
falling, colliding and capturing smaller, drifting ash particles that are too small
to fall as simple particles.
But, it is correct?
Fu-74-40
:
20
16
2.5
12
56%
Wt %
8
4
0
0
1
2
3
4
4.6
6.9
34%
10%
5
6
-4
Phi Size
7
8
Finer modes appear to
exist, which is why fine
skewing occurs.
9
10
11
12
13
14
15
Fine mode is located to
WNW, while the bulk of
the coarse ash is W to
WSW
HYSPLIT shows that 4
km winds are to WNW
while winds at 8-12 km
are W and WSW
Origin of particles in volcanic clouds
Explosive vesiculation-As pressure drops in ascending magma--overpressured bubbles burst
Hydrothermal explosions--rock fractured by
thermal shock
from contact between magma and water
Milling--
abrasion and grinding of particles can occur in pyroclastic
flows and in the vent
Chemical and meteorological processes-condensation, sublimation, surface chemical reactions forming acids,
salts, hydrometeors and aggregates of mixed origin
Others: Breakage of glass at crystal faces, Breakage of crystals from
melt inclusion overpressures, etc
Particle Diameter ()
12.0
10.0
8.0
6.0
4.0
2.0
0.0
-2.0
14.0
Ashfall of elutriated fine ash
above a block and ash flow
at Santiaguito Volcano on 1
Dec 1996. We have
analyzed many co-PF
ashes and they generally
show this type of fine (Md=
~5), fine-skewed GSD
pattern.
12.0
Class Weight (%)
10.0
8.0
6.0
4.0
2.0
0.0
1
10
Particle Diam eter ( m )
100
1000
Particle Diameter ()
12.0
10.0
8.0
6.0
4.0
2.0
0.0
-2.0
16.0
A distal ashfall sample fro
the August 1992 eruption of
Crater Peak/Spurr also
shows a fine (Md= 5) and
fine skewed GSD. Thus this
type of GSD can result from
eruptions that have no
pyroclastic flows.
14.0
Class Weight (%)
12.0
10.0
8.0
6.0
4.0
2.0
0.0
1
10
Particle Diam eter ( m )
100
1000
Voronoi method of weighting data
Bonadonna & Houghton, 2005
Overall the Fuego subplinian eruptions have only a few percent of the fine mode, which appears
to come from the pyroclastic flow milling. Larger plinian eruptions likely have a much higher
proportion of co-PF ash.
Rose et al., 2007 BV, in press
18 May 1980
Mount St Helens
Fall deposit
Total Grain-size distribution weighted by mass and by isopach volume, compared
to Carey and Sigurdsson [1982].
Crater Peak/Spurr, Sept 1992
Andesite, Sub-plinian
17 September 1992 Crater Peak TGSD
25
Wt.%
20
15
Total
GSD
10
5
0
-6 -5 -4 -3 -2 -1
0
1
2
3
4
5
Size Class Interval (phi)
6
7
8
9
10 11 12
hydrometeor—Any product of condensation or deposition of
atmospheric water vapor, whether formed in the free atmosphere or
at the earth's surface; also, any water particle blown by the wind
from the earth's surface.
1) liquid or solid water particles formed and remaining suspended in the air,
for example, damp (high relative humidity) haze, cloud, fog, ice fog, and
mist;
2)
liquid precipitation, for example, drizzle and rain;
3) freezing precipitation, for example, freezing drizzle and freezing rain;
4) solid (frozen) precipitation, for example, snow, hail, ice pellets, snow
pellets (soft hail, graupel), snow grains, and ice crystals;
5)
falling particles that evaporate before reaching the ground, for example,
virga;
6)
liquid or solid water particles
liftedglossary…
by the wind from the earth's surface,
From AMS
for example, drifting snow, blowing snow, and blowing spray.
These are probably more abundant than volcanic particles in many or
most volcanic clouds!
Conclusions:
All ash falls contain significant pyroclasts that are finer than 50µm in
diameter.
A very common feature of distal ashfalls is a mode at 4-5  (31-63 µm
diameter) which may be paired with a coarse mode, or alone with a
skewed shoulder toward finer particles.
Bimodal ashes seem to represent cases where larger pyroclasts sweep
up smaller particles as they fall.
Most ash falls from the finer modes much more quickly (< 24 hrs) than
simple fall would suggest, even when the particles are not being swept.
This suggests either aggregation or a role for hydrometeors.
The fine ash mode may surely be generated by pyroclastic flows and
elutriated upward where this ash may either join the plinian column or be
dispersed by lower level winds.
In some cases, however, milling by pyroclastic flows does not occur and
fine ash is still present. This may be evidence for milling within the vent.
The sizes of ash particles sensed
optimally by these methods are
about 1-25 µm (~5-9 ).
Based on the 1992 Crater Peak/Mt Spurr eruptions
Stage 1: First hour--rapid fallout of large( >500 µm diam) near the
volcano (<25 km), affecting a small area (<~300 km 2 ) and forming a primary
fallout blanket
Stage 2: During first 12-24 hours: Volcanic cloud expansion by
advection and diffusion by winds; aggregate fallout of 90% of fine ash (<25
µm diam) forming secondary fallout region of ~ 5 x 104 km2
Stage 3: During several
days: movement with winds and
decreasing area, and slowly
decreasing ash and SO2 masses
Rose et al, 2002, J Geology, 109: 677-694
Eruption style and the size of ash
produced.
The “total grain size distribution” of an explosive eruption can be estimated from
the deposit, if enough is known about all parts of the fall deposit.
Experiments suggest that energetic eruptions produce more fine ash by
vesiculation alone.
Several examples show clearly that pyroclastic flows contribute large fine
fractions to the overall GSD, presumably by milling (Soufrière Hills--Bonadonna et
al, 2002; Pinatubo--Dartevelle et al., 2002; Fuego, 1974; Rose et al., 2007;
Colima 2005-2006; Evans et al., Geology, in press)
Fallout of fine ash during stage 2
Aggregation observed in laboratory and the field
Accumulation by collision/coalescence-- analogy with
meteorological clouds
Areas of secondary maxima marked by bimodal SD
fallout of particles of expected size occurs WITH much
smaller particles
Highest masses of fine ash may occur on dispersal axis
or on the side
Evidence for fallout with ice noted
Observations of mammatus clouds
Remote sensing shows that a vast majority of fine ash