3.4 Properties of Ash
Finer material (ash < 2 mm and lapilli 2-64 mm) is convected upwards in an eruption column (Self & Walker 1994) before settling out downwind to form pyroclastic fall deposits. These deposits are composed of various proportions of vitric, crystal or lithic particles. Vitric particles are glass shards or pumice derived from magma, while crystals are minerals derived from phenocrysts or mircolites developed in the magma. Different minerals reflect the composition of different magmas. The most common minerals are shown in Table 3.7.
TABLE 3.7 Composition of major phenocryst phases in magma (from Thorpe & Brown 1985)
Basalt |
Basaltic andesite |
Andesite |
Dacite |
Rhyolite |
|
plagioclase |
** |
*** |
*** |
*** |
** |
*** often present, ** frequently present, * rarely present, - absent or rare
Thickness and Particle Size: It is well known that thickness and median grain-size of ash deposits generally decrease exponentially with distance from a volcano (Walker 1971). The distribution of ash (Fig. 3.17) will depend on the initial grain-size distribution of the ejecta (reflecting fragmentation during the eruption), dynamics of the eruption column and the column's interaction with wind (Carey & Sparks 1986, Sparks et al. 1992).
FIGURE 3.17 Schematic illustration of the fall-out of particles from an umbrella eruption cloud showing decreasing thickness and mean grain-size with distance from source.
Density: The density of individual particles may vary from 700-1200 kg m-3 for pumice, 2350-2450 kg m-3 for glass shards, 2700-3300 kg m-3 for crystals and 2600-3200 kg m-3 for lithic particles (Shipley & Sarna-Wojcicki 1982). Pumice fragments may form mats of floating material if deposited on water. Since coarser and more dense particles are deposited close to source, fine glass and pumice shards are relatively enriched at distal locations (Fisher & Schmincke 1984).
The bulk density of any pyroclastic fall deposit can be variable, with reported dry bulk densities of newly fallen and slightly compacted deposits ranging from between 500 and 1500 kg m-3 (Moen & McLucas 1980; Scott & McGimsey 1994). Both increasing and decreasing bulk densities with distance from source have been reported (Scasso et al. 1994), but distal ash falls most commonly show slight increases in bulk density with distance from a volcano. Grain-size, composition (proportions of crystal, lithics, glass shards and pumice fragments) and particle shape appear to be the main features controlling bulk density. Less spherical particles (more irregular) will pack relatively poorly resulting in higher porosity and lower bulk densities. Particle aggregation prior to deposition will result in higher particle packing and therefore higher densities.
Abrasiveness: The abrasiveness of volcanic ash is a function of the hardness of the material forming the particles and their shape. Hardness values (on Moh's scale for hardness) for the most common particles are shown in Table 3.8. Ash particles commonly have sharp broken edges which makes them a very abrasive material.
TABLE 3.8 Moh's scale of hardness (mineral hardness from Deer et al. 1980).
Scale Number |
Mineral |
Metal |
Minerals in |
|
1 |
---- |
Talc |
||
2 |
---- |
Gypsum |
||
Aluminium |
||||
3 |
---- |
Calcite |
biotite (2.5-3) |
|
Brass |
||||
4 |
---- |
Fluorite |
||
Iron |
||||
5 |
---- |
Apatite |
||
Steel |
volcanic glass, |
|||
6 |
---- |
Orthoclase |
plagioclase, alkali-feldspar (H 6-6.5) |
|
7 |
---- |
Quartz |
olivine (H 6.5-7) magnetite (H 7.5-8) |
|
8 |
---- |
Topaz |
||
9 |
---- |
Corundum |
Chromium |
3.4.1 Properties of Ash - Soluble Components
Reservoir fluids from volcanic hydrothermal systems can be an important source of chemical compounds in volcanic fallout which, if ingested, could be potentially hazardous to livestock. These fluids, once ejected from a volcano during eruption, are typically transported downwind from the vent as aerosols, or are adsorbed onto ash particles. For eruption columns that are restricted to the troposphere, these aerosols and ash particles ultimately fall out to the earth's surface, usually within 10's to 100's of km proximity to the volcanic edifice.
Excellent examples of these fluids are found in crater lakes sitting atop
actively degassing volcanic vents such as that on Mt Ruapehu. As shown in Table 3.9, these
fluids are highly acidic, consisting essentially of mixtures of sulphuric, hydrochloric
and hydrofluoric acids, magmatic and meteoric waters, and dissolved rock components.
As described previously, soluble components on ash particles can also originate from
sorption of magmatic gases (eg. SO2, HCl, HF) onto aerosols and ash particle
surfaces during transport in the plume, and a third possible source of leachable material
derives from hypersaline magmatic fluid which is resident in the magma chamber. The latter
consists predominantly of chloro-salts of the typical alkali and alkaline earth rock
constituents (eg. NaCl, KCl, CaCl2, MgCl2, etc.).
Examples of the fairly typical leachate compositions from Mt Ruapehu are
listed in Table 3.9. The data are presented as mg of solute per litre of H2O;
and can be readily converted to mass of soluble salt per unit mass of ash. Not only is the
acidic nature of the leachate readily apparent from these data, but they also clearly
demonstrate the ash leachate as the source of the F which was responsible for fluorosis
fatalities associated with the 1995-96 eruptions of Ruapehu.
Time series trends in the leachate data point to a decrease in the total amount of
leachable material during each eruption episode, and this trend is also apparent over the
entire 1995/96 period of activity (Fig 3.18). This is interpreted as a result of
decreasing content of the hydrothermal end-member aerosols in the plume through time,
which is consistent with the observed early expulsion of these fluids from the lake and
vent conduits during each event.
One of the interesting features of the ash deposits, particularly those erupted early in each episode, is the presence of particulate elemental sulphur (Fig. 3.19). Although this phase, which originates in the hydrothermal system, only slowly oxidizes to sulphuric acid under atmospheric conditions, the reaction can be catalyzed by bacteria and/or certain transition metals, and therefore has the potential to cause long term soil acidity and/or corrosion problems.
It is common for aerosols and ash particles to dehydrate as they are transported away from the volcano, or after they have settled onto the ground surface. This may result in the formation of very concentrated acid solutions within the aerosols or on the ash particle surfaces. It is possible to form nearly pure sulphuric acid in this way, whereas HCl tends to revolatilise to the gas phase during dehydration. Loss of HCl gas is indicated in several of the leachate samples from 1995-96 eruptions of Mt Ruapehu.
In terms of agricultural impact, the greatest chemical hazard posed by volcanic eruptions is that caused by ingestion of soluble (acidic) magmatic constituents on ash particles, followed by vog and acid rain. Volcanoes that have the highest potential to cause such problems will be those that 1) have large, long-lived hydrothermal systems associated with them, or 2) magmas that have large natural abundance of S and/or halogen constituents (F, Cl, Br). With regard to the former, the highest risk volcanoes in NZ would be those such as Ruapehu and White Island, whereas the high-F bearing lavas of Mayor Island make this a good example of the latter.
Table 3.9 Ash leachate analyses. All concentrations are as mg/l.
SAMPLE No. |
Locality |
Eruption |
Ash Wt |
Wt H2O |
pH |
Li |
Na |
K |
Ca |
Mg |
Fe |
Al |
F |
Cl |
Br |
NO3 |
SO4 |
RuA 17069631 |
Kuratau Hydro. Rd. |
17.6.96 |
9.965 |
108.36 |
4.93 |
<.05 |
39.0 |
3.50 |
258 |
27.0 |
0.74 |
4.6 |
2.71 |
42.3 |
1.25 |
2.56 |
784 |
RuA 17069634 |
Kinloch marina |
17.6.96 |
9.317 |
100.00 |
4.47 |
<.05 |
30.0 |
0.14 |
343 |
30.0 |
1.90 |
5.8 |
1.43 |
29.4 |
0.24 |
0.91 |
1029 |
RuA 17069653 |
Opataka Hist Place |
17.6.96 |
10.047 |
99.42 |
4.69 |
0.12 |
16.1 |
2.40 |
116 |
10.6 |
0.58 |
4.1 |
1.53 |
9.2 |
0.15 |
355 |
|
RuA 17069654 |
Rotopounamu car pk |
17.6.96 |
10.770 |
98.68 |
4.66 |
<.05 |
18.4 |
2.80 |
121 |
11.4 |
0.38 |
4.1 |
2.24 |
9.8 |
377 |
||
RuA 17069655 |
Otara Road |
17.6.96 |
10.076 |
99.84 |
5.01 |
<.05 |
16.6 |
6.40 |
112 |
9.6 |
0.21 |
4.0 |
4.32 |
10.0 |
0.16 |
355 |
|
RuA 17069656 |
Whangamata Road |
17.6.96 |
10.318 |
99.32 |
4.70 |
<.05 |
30.3 |
2.60 |
193 |
24.4 |
0.81 |
4.1 |
2.38 |
27.1 |
613 |
||
RuA 18069613 |
Rotopounamu track |
18.6.96 |
10.027 |
100.02 |
5.05 |
<.05 |
21.0 |
6.40 |
162 |
12.6 |
<.1 |
5.2 |
5.86 |
16.6 |
0.01 |
2.24 |
479 |
RuA 18069618 |
Otara Road |
18.6.96 |
9.643 |
100.02 |
5.07 |
<.05 |
21.0 |
7.00 |
147 |
11.4 |
<.1 |
3.5 |
2.86 |
19.6 |
0.01 |
2.81 |
451 |
RuA 18069621 |
Rotoaira Farm |
18.6.96 |
4.028 |
100.01 |
5.03 |
<.05 |
20.0 |
10.20 |
147 |
13.4 |
0.19 |
3.7 |
3.36 |
19.1 |
0.01 |
3.58 |
472 |
RuA 18069619 |
Top of the Bruce Road |
18.6.96 |
9.982 |
100.01 |
4.72 |
<.05 |
23.0 |
3.90 |
146 |
19.7 |
0.33 |
4.3 |
2.58 |
19.3 |
0.38 |
2.15 |
481 |
RuA 18069625 |
Rotoaira fishing camp |
18.6.96 |
10.060 |
100.22 |
4.15 |
<.05 |
37.0 |
0.44 |
269 |
45.0 |
2.80 |
4.3 |
0.76 |
84.3 |
0.18 |
2.23 |
833 |
RuA 18069630 |
Wairakei Nth. |
18.6.96 |
9.942 |
100.01 |
4.11 |
0.08 |
68.0 |
2.70 |
388 |
66.0 |
3.90 |
3.5 |
1.18 |
79.9 |
0.07 |
3.37 |
1618 |
RuA 18069655 |
Rotorua |
18.6.96 |
10.046 |
100.04 |
4.46 |
0.10 |
74.0 |
1.30 |
353 |
54.0 |
1.10 |
5.8 |
1.48 |
54.5 |
1.18 |
118 |
|
RuA 19069610 |
Mangatepopo Track |
19.6.96 |
10.010 |
100.03 |
4.88 |
<.05 |
17.7 |
6.10 |
195 |
19.0 |
0.43 |
3.7 |
1.32 |
19.2 |
1.54 |
586 |
|
RuA 19069612 |
Top of the Bruce |
19.6.96 |
9.986 |
100.04 |
4.45 |
<.05 |
37.6 |
3.70 |
189 |
36.0 |
0.96 |
3.3 |
2.67 |
116.5 |
1.64 |
518 |
|
RuA 19069652 |
Top of the Bruce |
19.6.96 |
10.616 |
99.32 |
4.27 |
<.05 |
35.8 |
7.40 |
303 |
32.4 |
0.12 |
8.7 |
4.27 |
26.5 |
0.15 |
0.28 |
983 |
TOB-A |
Top of the Bruce |
8.7.96 |
10.283 |
99.25 |
4.91 |
<.05 |
14.6 |
7.00 |
101 |
9.4 |
0.10 |
2.2 |
2.42 |
11.0 |
319 |
||
TOB-B |
Top of the Bruce |
8.7.96 |
11.644 |
99.19 |
4.65 |
<.05 |
20.0 |
6.50 |
153 |
12.2 |
0.18 |
4.2 |
4.58 |
17.5 |
451 |
||
TOB-C |
Top of the Bruce |
8.7.96 |
10.469 |
100.00 |
4.62 |
<.05 |
14.9 |
6.00 |
118 |
9.5 |
0.12 |
2.9 |
3.84 |
12.0 |
338 |
||
Lochinvar |
Lochinvar Station |
12.10.95 |
10.223 |
99.20 |
3.70 |
<.05 |
85.0 |
0.39 |
276 |
124.0 |
8.40 |
120.0 |
8.13 |
61.8 |
0.72 |
0.57 |
2528 |
RuA 171095-01 |
Waipakahi Road |
17.10.95 |
3.050 |
100.02 |
4.72 |
<.05 |
35.0 |
1.10 |
206 |
25.0 |
1.50 |
4.1 |
1.16 |
61.6 |
1.84 |
612 |
|
RuA 171095-05 |
Rangipo Intake |
17.10.95 |
6.967 |
101.05 |
4.01 |
0.10 |
49.0 |
0.31 |
360 |
53.0 |
6.40 |
20.4 |
6.59 |
125.2 |
3.74 |
1154 |
|
RuA 171095-06 |
Whangaehu River ford |
17.10.95 |
10.096 |
100.28 |
3.93 |
<.05 |
3.0 |
0.35 |
20 |
2.8 |
6.60 |
5.4 |
tr |
12.3 |
3.56 |
89 |
Fig. 3.18 Volcanic eruptions inject water vapour (H2O), carbon dioxide (CO2),
sulphur dioxide (SO2), hydrochloric acid (HCl), hydrofluoric acid (HF) and ash
into the atmosphere. HCl and HF will dissolve in water and fall as acid rain whereas most
SO2 is slowly converted to sulphuric acid (H2SO4)
aerosols. Ash particles may absorb these aerosol droplets onto their surfaces providing an
acid leachate after deposition.
Fig: 3.19 Elemental sulphur grain in ash collected from Lochinver Station subsequent to the October 1995 eruptions of Ruapehu.
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