53

Minerals in Volcanic Ash 2:
Non-magmatic Minerals
Tsukasa OHBA1 and Mitsuhiro NAKAGAWA2
1

Department of Mineralogy, Petrology, Economic Geology
Graduate School of Science, Tohoku University
Aramaki Aza Aoba, Aoba-ku, Sendai, 980-8578 Japane-mail: ohbatu@mail.cc.tohoku.ac.jp
2
Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University
Kita-10 Nishi-8, Kita-ku, Sapporo, 060-0810 Japane-mail: nakagawa@ep.sci.hokudai.ac.jp

Abstract
Non-magmatic minerals are abundant in volcanic ash, especially in volcanic ash of phreatic andphreatomagmatic eruptions. Accidentally incorporated minerals from country rocks (rock into whichmagma or water intrude) are common in all types of eruptions. Any kind of rock-forming minerals in
the crust and mantle may be incorporated into volcanic ash by mechanical stripping from conduits ormagma chambers. Volcanic ash often contains hydrothermal minerals, such as smectite, kaolinite,
cristobalite and gypsum, that are formed by rock-fluid or magma-fluid interactions. Hydrothermal
minerals are common in the volcanic ash of phreatomagmatic and phreatic eruptions. Some kinds ofminerals in volcanic ash must have precipitated from volcanic fluids that involve hydrothermal water insubvolcanic aquifers, crater lakes, fumarolic gases, and volcanic gases. Sulfate, halide, and native
sulfur may be the major minerals precipitating from volcanic fluid, but they have not been well
documented.
Volcanic ash commonly contains hydrothermal clay minerals which are similar to soil clay formed
by weathering of magmatic fragments. Sulfide and halide minerals in volcanic ash have an influenceon human environments such as through soil properties and climate.
Key words: hydrothermal minerals, leachate, magma-water interaction, non-magmatic minerals,
rock-water interaction, xenocrysts

1. Introduction
Volcanic ash contains many kinds ofnon-magmatic minerals. We use here the term
“magmatic minerals” for the minerals crystallized
from magma, or for common rock-forming minerals
in igneous rocks, and “non-magmatic minerals” for all
the other minerals in volcanic ash. So far little
attention has been given to non-magmatic minerals in
volcanic ash because volcanologists have mainly
focused on magmatic minerals. Studying magmatic
minerals seems essential to understanding volcanic
activities since their features sensitively reflect
physical and chemical conditions of magmas. On theother hand, non-magmatic minerals have been considered of less importance because they are only
accidental or secondarily-produced materials.
In other research fields on volcanoes aside frommineralogy and petrology, external conditions outside
the magma have been long studied. Interactions

among volcanic gases, hydrothermal water, countryrock, and magma have been studied by researchers of
volcanic gas, water, geothermics, and economic
geology (e.g., Giggenbach, 1997). During the lasttwo decades, some geologists have discussed
magma-water interactions that cause explosive
eruptions (Wholetz and Heiken, 1992; Wholetz, 2002,
1986; Zimanowski et al., 1997; White, 1996; Barberi,
1985; etc.). Non-magmatic minerals are produced
through those interactions, and incorporated into
volcanic ash of explosive eruptions. Those minerals
appear to be clues to understanding the interactions
between magmas and the external circumstances(Heiken and Wohletz, 1992).
Even thosenon-magmatic minerals derived from country rocks(rock into which magma or water intrude) without any
interaction give us valuable information such as
depths of explosions. In this report, we will describe
the types and origins of non-magmatic minerals in
volcanic ash from the viewpoint of interaction among

54

T. OHBA and M. NAKAGAWA

magma, water, gas, and country rocks.
In spite of little attention to non-magmatic minerals, they are of great importance to environmental
science.
Non-magmatic minerals contain someelements, such as sulfur or halogens, that are essential
for the environment. Such minerals are more watersoluble than magmatic minerals, hence, their components easily enter environmental waters. There arealso clay minerals, such as kaolinite, which influencesoil characteristics. Thus the influence of non-magmatic minerals on the environment should be studiedin greater depth.

2. Non-magmatic-mineral-producing
Eruptions
Most volcanic eruptions discharge some amount of
non-magmatic minerals.
The abundance of
non-magmatic minerals in volcanic ash correlates to
eruptionstylesintermsofmagmatic,
phreatomagmatic, and phreatic eruptions. Volcanic
ash of magmatic eruptions is composed of abundantmagmatic materials (volcanic glass and magmatic
minerals) and a small amount non-magmatic materials(non-magmatic minerals and rock fragments), whereasthat of phreatic eruptions consists only of
non-magmatic
materials.
Constituentsofphreatomagmatic eruption products show a wide
range of ratios between magmatic and non-magmatic
materials.
Explosive magmatic eruptions are caused by degassing and bubbling under the condition ofvolatile-saturation by decompression.
Since
explosive fragmentation occurs within the magma
itself and since the explosivity is lower than for othertypes of eruptions, non-magmatic materials fromexternal circumstances are not necessarily containedin volcanic ash.
Nevertheless, non-magmatic
minerals are often contained in magmatic volcanic ash
such as “xenocrysts,” oxidized iron-oxide minerals,sulfates, and sulfides. “Xenocryst” is the term used
for accidentally incorporated minerals from countryrock into magma. Xenocrysts are derived fromvarious depths from the mantle to the surface.
Reddish-coloured scoria contains hematite and/or
goethite that could be regarded as non-magmatic
minerals. Those minerals are secondarily producedby oxidation or hydration during eruptions. Sulfate
and sulfide minerals are not usually regarded asmagmatic minerals, but both minerals were containedin the volcanic ash from eruptions of Mt. Pinatubo in
1991 (Hattori, 1997) and El Chihcon in 1982 (Luhr etal., 1984). Since the origin of sulfate and sulfide
minerals in magma is arguable, we will notdiscriminate
here
betweenmagmatic
andnon-magmatic minerals.
Phreatic eruptions, or steam explosions, are causedby the abrupt expansion of steam derived from a subvolcanic aquifer. Sudden steam expansion is caused

by decompression and/or heating of hydrothermal
fluid. Since magma is not discharged by the eruptions, the volcanic ash consists only of non-magmatic
materials. Non-magmatic materials in phreatic volcanic ash are derived mainly from subsurface country
rocks. Explosions caused by sudden steam expansion result in fragmentation of country rocks with highexplosivity. Volcanic ash from phreatic eruptions
also contains considerable amounts of minerals produced by hydrothermal alteration. Before a phreatic
eruption, country rocks under the crater may havereacted with the hydrothermal fluid that is possibly
essential for steam expansion during the phreatic
eruption. Some minerals may precipitate directlyfrom hydrothermal water. The mineral properties
reflect hydrothermal water chemistry, temperature,and pressure, hence, properties of the fluid may be
estimated from the mineralogy of the volcanic ash.
Phreatomagmatic eruptions are caused bymagma-water interaction such as magma intrusion
into subvolcanic aquifers or crater lakes.
The
explosive mechanism of magma-water interaction,e.g., the MFCI mechanism (Zimanowski, 1997), hasbeen a subject of controversy over the last few years.
High explosivity and dynamics of magma-waterinteraction are of great interest to modern volcanology.
Volcanic ash from the eruptions shows a wide range
of non-magmatic vs. magmatic mineral ratios. If
steam and magma are the only materials discharged
by a phreatomagmatic eruption, the volcanic ash
consists only of magmatic materials (glass andmagmatic minerals). However, volcanic ash ofphreatomagmatic
eruptions
usually
contains
considerable amounts of non-magmatic materials.
Some phreatomagmatic eruptions discharge only a
trace of magmatic minerals. For example, volcanic
ash from the phreatomagmatic eruption at Mt. Usu on
31 March 2000 contains trace amounts of magmatic
minerals and glass shards.
Some of thenon-magmatic minerals in phreatomagmatic volcanic
ash originate from water-magma interaction, in
addition to country rock and hydrothermal water.

3. Minerals in Volcanic Ash
3.1 Xenocrysts in magmatic and phreatomagmatic volcanic ash
Xenocrysts occur both as free crystals and in magmatic fragments (glass shards, pumice, and scoria) inmagmatic- and phreatomagmatic volcanic ash. Although any mineral may be incorporated into magma,
minerals recognized as xenocryst are limited to only a
few tens of mineral species, since most solid parts ofthe Earth are composed only of small numbers ofminerals called “rock-forming minerals.” Quartz,
feldspars, olivine, pyroxenes, amphiboles, and micascommonly occur as xenocrysts, but these minerals areoften indistinguishable from phenocrysts because theyalso commonly occur as phenocrysts. These xeno-

Minerals in Volcanic Ash 2: Non-magmatic Minerals

crysts often exhibit peculiar textures (kink band,corroded form, reaction rim, breakdown aggregations)that help to discriminate them from phenocrysts.
Phenocryst mineral assemblages can be roughly
estimated from magma compositions that are provided
by chemical analysis of glass shards, pumice or scoria.
Therefore, we can recognize xenocrysts if someminerals in the magmatic fragments are unlikely to bein equilibrium with the magma.
Quartz often occurs as xenocrysts in magmatic
fragments in which quartz is not equilibrated, such as
basalt, andesite, and alkaline rock fragments. Quartz
xenocrysts exhibit corroded forms caused by resorption in magma, or reaction rims caused by mineralcrystallization after quartz-magma reactions (Sato,
1975). Plagioclase xenocrysts also exhibit resorption
textures that are called spongy texture, sieved texture,or dusty zones. Resorption textures of these minerals
are caused not only by incorporation from country
rocks but also by mixing of two or more magmas.
Careful petrological observation is needed to distinguish xenocrysts from mixing disequilibration.
Hornblende and biotite also occur as xenocrysts.
When hornblende or biotite is brought to the surface
by hot magma, the mineral changes into an aggregation of other minerals (pyroxene, feldspar, iron oxide),which is called “breakdown.” Since breakdown texture is also formed by magma mixing, careful observation is needed to discriminate xenocrysts frommagma mixing.
Xenocrysts of olivine and pyroxenes are often interpreted as having a mantle-peridotite origin. Thoseminerals are the major constituents of peridotite in theupper mantle where basaltic magmas are generated.
Peridotite fragments, called “mantle xenolith,” sometimes occur in volcanic ash, e.g., the Ichinomegata
volcano, Oga Peninsula, NE Japan. Minerals disaggregated from mantle xenoliths are xenocrysts, butthey are sometimes confused with phenocrysts if there
are no signs of xenocrysts such as a kink band that is a
sharp bending plane in a crystal. Xenocrysts fromcrustal rocks show a wider variety than those frommantle rocks, since crustal rocks comprise larger
numbers of rock-forming mineral species than mantle
rocks, which consist of only a few kinds of minerals.
Major crustal minerals are quartz, feldspars,
pyroxenes, olivine, hornblendes, micas, garnet, aluminosilicates, carbonates, and Fe-Ti oxide. The
mineralogy of xenocrysts from crustal rocks sometimes becomes a good tool for estimating crustal
features (geothermal gradient, composition, etc.).
For example, Hacker et al. (2000) estimated thegeothermal gradient and development history of the
Tibetan Plateau from the mineralogy of xenocrysts.
Xenocrysts also give some clues for evaluating
thermal interaction between magma and the crust.
Barberi and Leoni (1980) discussed thermal interaction between a shallow magma chamber and crustal
carbonate rocks based on the mineralogy of carbonate

55

xenocrysts in the volcanic ash from Vesuvius.
3.2 Hydrothermal-alteration-related minerals inphreatic and phreatomagmatic volcanic ash
Volcanic ash of phreatic and phreatomagmatic
eruptions commonly contains considerable amounts ofminerals produced by hydrothermal alteration prior to
the eruptions. Hydrothermal minerals, such as smectite, kaolinite and cristobalite, commonly occur inthese volcanic ashes. Occurrence of the the samekinds of minerals have been well documented in
active geothermal fields (Hayashi, 1973; Browne,
1978; Henley and Ellis, 1983) and around hydrothermal ore deposits (e.g., Reed, 1997; Izawa, 1991),
whereas they have been less described for volcanicash (Ossaka and Ozawa, 1966; Kudo et al., 2000).
Since all these geological environments relate tovolcanic-hydrothermal systems, occurrence of theseminerals must be similar in each of these environments. According to studies on geothermal fields
and hydrothermal ore deposits, hydrothermal mineral
assemblages strongly depend on both the acidity andtemperature of the hydrothermal water. Therefore,hydrothermal mineral assemblages in volcanic ash
must reflect physical and chemical properties of thehydrothermal water. Some workers previously presented schemes of spatial chemical variation of hydrothemal water beneath volcanic edifices (e.g., Browne,
1978; Hayashi, 1978; Henly and Ellis, 1983). Compared with the schemes, hydrothermal minerals in
volcanic ash will appear in their original position in
the volcanic edifice and have their original properties
as well. Descriptions of hydrothermal minerals in
volcanic ash have been limited, but we list some description by recent researchers in Table 1. We willreport an example of hydrothermal minerals from thevolcanic ash of the 1997 eruptions of the Akitayakeyama volcano. The volcano erupted twice that
year; first on 11 May at the foot of the stratocone andsecond on 16 August at the summit crater. Mineral
assemblages of the volcanic ash differ between thetwo eruptions, as shown in Table 2. Compared withthe mineralogical works in geothermal fields (Browne,
1978; Hayashi, 1973; Henley and Ellis, 1983),
hydrothermal minerals in the volcanic ash of 11 May
appear to be equilibrated with hydrothermal water of
low temperature (<100°C) and their acidity is neutral
to weakly alkaline. On the other hand, those of 16
August were equilibrated with high-temperature andacidic hydrothermal water. Detailed observation of
the minerals in these volcanic ashes from
Akitayakeyama gives us a concrete picture of
subvolcanic hydrothermal systems. Itoh et al. (1997)
estimated the source depth of the phreatic eruption on
11 May based on a comparison between the volcanic
ash and drill holes. The presence of heulandite(zeolite) in the volcanic ash indicates alkalinehydrothermal water at a shallow depth, which is

56

T. OHBA and M. NAKAGAWA

consistent with the scheme of subvolcanic waterchemistry that involves alkaline hydrothermal water

shards. Palagonite is not a true mineral name but amixture of fine glass, smectite, iron oxide, and other

Table 1 Hydrothermal minerals in phreatic eruption deposits.

Volcanoes
Jogokunuma-Hakkoda
Iwate
BandaiYakedake
Kuju
Hokkaidokomagatake
Akita-YakeyamaUsu

Ages
500-300y.B.P.
500-100y
.B.P.
1888
1962
1995

Mineralscristobalite, tridymite, quartz, smectite, kaolinite

References
Kudo et al. (2000)
Itoh (1999)

1996

kaolinite, cristobalite, alunite, montmorillonite,
pyrophillite, quartz
kaolinite, illite/smectite mixed layer
montmorillonite, mica, quartz, cristobalite
cristobalite, quartz, alunite, pyrite, gypsum,
kaolinite, anatase, native sulfur
montmorillonite

1997

See Table 2

Itoh et al. (1997)

2000

quartz, smectite, chlorite, calcite, pyrite, zeolite

Website of Geological
Survey of Hokkaido

Yamamoto et al. (1999)
Ossaka and Ozawa (1966)
Taguchi et al. (1996)
Ui et al. (1996)

Table 2 Hydrothermal mineral assemblages in volcanic ash of phreatic eruptions of the Akita-Yakeyama volcano in 1997.

Date
locationminerals

estimated depth
estimated temperaturehydrothermal water acidity

11 May 1997
northeastern flankmontmorillonite, kaolinite,
tridymite, quartz, pyrite,
heulandite, Yugawalite(Itoh, et al. 1997)< 100 m (Itoh et al., 1997)<100℃
acid – weak alkaline

near the foot of the volcano (Henley and Ellis, 1983).
On the other hand, minerals in volcanic ash from thesummit crater indicate the core of the volcanic edifice
as the source region. Quartz aggregation is quite
abundant in the ash, which means there is a “silica
baggy” (Giggenbach, 1997) that is formed by leachingof most elements by acidic water at the core of the
volcanic edifice. The presence of sulfides (pyrite)and sulfates (gypsum and alunite) indicates aParagenesis ofhigh-sulfidation environment.
pyrophillite + quartz and andalusite + quartz in alteredrock fragments indicates hydrothermal conditions ofhigh temperature (230-330°C) and high acidity.
Since water cannot exist in liquid form under suchhigh temperatures or at low pressure conditions, highpressure conditions at a depth of hundreds of metres
would be needed to crystallize the minerals.
3.3 Magma/water-interaction-related minerals inphreatomagmatic volcanic ash
It is well known that direct contact between
magma and water induces explosions, and it is also
known that some minerals are produced by thereaction between magma and water at the time ofcontact. “Palagonite” is the term used for the material that is considered to be produced by direct interaction between magma and water. Palagonite usuallyforms on the surface of lava, pumice, scoria, and glass

16 August 1997
summit cratermontmorillonite, pyrophillite,
kaolinite, gypsum, cristobalite,
quartz, alunite, pyrite, anatase,diaspore, andalusite
0 - 1000 m< 330℃
intense acid - weak acidminerals (Heiken and Wohletz, 1992).
Hematite (Fe2O3) and goethite (FeOOH) are notcommon magmatic minerals but are produced in magmatic fragments by oxidation and/or hydration throughwater-magmaorair-magma
interactions.
Reddish-coloured magmatic fragments (scoria,
pumice, glass shards) contain those iron-oxide minerals that are formed by oxidation or hydration of ironin volcanic glass or magnetite.
3.4 Other non-silicate minerals in magma
Except for a couple of Fe-Ti oxide minerals (magnetite and ilmenite), zircon, and apatite, non-silicate
minerals in magmatic fragments have not been welldescribed regardless of whether they are magmatic or
non-magmatic. Some kinds of oxide, sulfide, andsulfate minerals are considered to be key minerals in
understanding the behavior of magma. We will
discuss here those non-silicate minerals even thoughsome of them might be magmatic minerals.
Spinel and chromite in volcanic ash might bemagmatic minerals, in particular, in basaltic magmas.
But they also might be xenocrysts especially frommantle peridotite, the same as pyroxene and olivine
xenocrysts. As stated above, hematite and goethite
are produced by oxidation and/or hydration of iron inmagma. Sulfide and sulfate minerals, such as isocubanite or gypsum, in volcanic ash have been focus-

Minerals in Volcanic Ash 2: Non-magmatic Minerals

ed on during the last decade, since those minerals in
volcanic ash from active volcanoes are considered togive an understanding of the behavior of volcanic gas
and magma (Hattori, 1997).
The presence ofanhydrite (CaSO4) in the volcanic ash of the eruption
at Mt. Pinatubo in 1991 is vital for considering theinconsistency between excess sulfur gas emission andthe sulfur budget in the magma. Similar excess
sulfur emission was observed at the eruption of El
Chihcon in 1982 (Luhr et al., 1984).
Those
examples are not exceptional with regard to thepresence of sulfide and sulfate minerals, even thoughthose minerals have not been well described inmagmatic fragments or volcanic rocks.
Since
economic geologists have recognized sulfur-bearing
minerals in magmatic ore deposits, (Naldrett, 1989),
those minerals seem to be common magmatic
fragments, but they have been ignored by petrologists.

4. Origin of Non-magmatic Minerals
Mechanical incorporation of country rocks
Since magmas rise through the crust to the surface,
any minerals along the path may be accidentally
incorporated into volcanic ejecta.
Minerals are
stripped from the wall of the conduit by the ascending
magma. Minerals incorporated by this process occur
as both free crystals and xenocrysts in magmatic
fragments. Non-magmatic eruptions, or phreatic
eruptions, also discharge country rock fragments thatcome from the shallow crust. In this case, theminerals are stripped from country rocks by ascending
steam or water.
Non-magmatic minerals in volcanic ash ofphreatomagmatic eruptions are brought up by steam,water, and magma. Because of the wide variety ofrock-forming minerals in crustal rocks, many kinds ofrock-forming minerals can be incorporated by thismechanical process.
Since many magmas passthrough the mantle, minerals from mantle peridotite,57

accidentally included in volcanic ash.
Alteration of country rock and magmas
Hydrothermal water commonly interacts withcountry rocks under volcanic edifices, and an area ofhydrothermal alteration develops around the volcanic
edifices. The interaction causes mineralization ofhydrothermal minerals, then the country rocks change
into altered rocks. Hydrothermal water beneath a
volcano has a peculiar character because of heating bymagma and injection of volcanic gas into the water,
hence, species in the altered rocks around the volcano
are unique. Most hydothermal minerals in volcanic
ash are derived from altered rocks that formed prior to
the eruption. The process of incorporation of theminerals into volcanic ash is the same as that ofcountry rock fragments, but the minerals are different
from common rock-forming minerals in crustal andmantle rocks (Table 3).
Direct interaction between water and magma also
produces such hydrothermal minerals. This mineralinteraction only lasts for a short period during theeruption. Even during such a short time this interaction induces mineralization of hydrothermal minerals,
but hydrothermal alteration does not penetrate all of
the magma. The interaction often results in thincrusts of hydrothermal minerals (palagonite) surrounding magmatic fragments. If the instantaneousinteraction results in only oxidation or hydration of
the iron in the magma, the magmatic fragmentschange their color into red by formation of hematite orgoethite.
Precipitation from water and gas
In volcanic environments, many kinds of minerals
continuously precipitate from volcanic fluid (hydrothermal water in subvolcanic aquifers, crater lakewater, volcanic gas, and fumarolic gas). When avolcano erupts explosively, the volcanic plume mightmechanically carry those minerals that have precipi-

Table 3 Potent non-magmatic minerals in volcanic ash and their origins.

origins of minerals
crustal and mantle rocks

related to external water(hydrothermal alteration,water-magma reactions,precipitation from volcanic
fluids)potent minerals
plagioclase, quartz, alkali-feldspar,mica (biotite, phlogopite, etc.),
amphiboles, pyroxenes, olivine,
garnet, Fe-Ti oxide, spinel, calcite,
dolomite, etc.
quartz, cristobalite, tridymite,smectite (montmorillonite, saponite,etc.), sericite, illite, mixed layer,
kaolinite, zeolite (mordenite,heulandite, etc.), alunite, gypsum,
anhydrite, pyrophillite, chlorite,
albite, aduralia, pyrite, anatase,diaspore, native sulfur, etc.

such as olivine, pyroxenes and spinel, are also

incorporation mechanisms
mechanical stripping from country
rocks by ascending magma, steam, or
water･mechanical stripping fromhydrothermally altered country rocks
by ascending magma, steam, or
water･water-magma interactions･mechanical incorporation fromaquifers or surface water into plume･direct precipitation from volcanic
fluidstated prior to the eruption. Species of minerals that

58

T. OHBA and M. NAKAGAWA

precipitate from volcanic fluids are similar to those inhydrothermally altered rocks, but direct precipitationfrom fluid results in a wider variety of mineral species.
Rare minerals are often found especially aroundfumaroles, such as orpiment (As2O3). Major minerals precipitated from volcanic fluids include quartz,
cristobalite, gypsum, alunite, native sulfur, and ironoxide. These minerals must commonly occur in volcanic ash, but so far they have not been well described.
Native sulfur was reported from the volcanic ash of
the 1995 eruption of the Ruapehu volcano(Christenson, 2000).
Some minerals might precipitate from volcanic
fluids even at the time of the eruption. During aneruption, physical conditions of volcanic fluids change
dramatically, involving phase transition of fluids,
decompression, and rapid cooling. Since the periodof sustainment in the volcanic column is short,mineral precipitation in the volcanic plume requires a
rapid crystallization rate, which is feasible for somekinds of sulfate and halide (chloride and floride)
minerals but is unlikely for silicate minerals. If theminerals precipitate in the volcanic plume, they mustaccount for a certain part of the volcanic ash. However, they are hardly recognized as constituents of
volcanic ash, because they occur as very fine grains
and because such minerals are so water soluble thatrainfall can flush them away. In fact, there is no
description of certain minerals that are precipitatedfrom volcanic fluids during an eruption. That is why
we did not describe such minerals in the above
sections. Nevertheless, such minerals evidently precipitate from volcanic fluids in the volcanic plume.
During the last two decades, the volcanologicalsignificance of “leachate,” or soluble components on
the surface of volcanic ash particles, has beenemphasized by reaserchers on volcanic fluids (e.g.,
Casadevall, et al., 1983). They consider that when
volcanic ash is suspended in a plume, a certain portionof the gas phase condenses onto the surface of ash
particles. They found that the condensates are soluble sulfates, chlorides, and florides. If the volcanic
gas phase is condensed in the free space in the plume,the condensate becomes an “aerosol” and is suspended
in the air for long periods.

5. Environmental SignificanceNo one has been able to evaluate the effects ofnon-magmatic minerals in volcanic ash on the humanenvironment since insufficient descriptions of theminerals are available. We will mention here two
possible effects on human environments based on thedescriptions in above sections. One is effects on soil
and the other is that on climate.
Properties of soils are dominated by clay minerals
that are thought to be formed by weathering ofmagmatic fragments after deposition. Clay minerals
formed by weathering of magmatic fragments include

smectite, kaolinite, mica, etc. However, as statedabove, volcanic ash commonly contains those clay
minerals that are formed by hydrothermal activities
around volcanic edifices. Pedological effects of the
primarily contained clay minerals in volcanic ash
should be evaluated for distribution, properties, andthe genesis of soils. Sulfate and halide minerals in
volcanic ash are of great significance with regard toproperties of soils. Even small amounts of soluble
minerals govern chemical properties of soils since
they bring some essential anions such as SO4 2–, Cl–,
F–. Penetration of these anions into fields sometimes
causes disastrous effects. Floride-rich volcanic ash
from the Hekla volcano in 1970 caused 7,000 sheep to
die from feeding on polluted grass.
Volcanic ash is one source of aerosols that dominate the global climate. Magmatic and non-magmatic rock fragments may become aerosols if they are
fine-grained enough.
Condensate from volcanic
fluids will be more significant for aerosol formation.
Condensation often occurs at high levels in the
volcanic plume, sometimes even in the stratosphere.
Grains of the condensate are usually small enough tobe suspended in the air for years and spread aroundthe world, whereas most magmatic and rock fragments fall onto the ground around the volcano.
Aerosol from the volcanic fluid discharged by theeruption effects climate cooling. This cooling cannegatively affect agriculture. Furthermore, solublecomponents in the aerosol can increase rainfall
acidity.

Aknowledgements
We would like to thank professor Nanjo andprofessor Taniguchi of Tohoku University for giving
us the opportunity to write this paper, and we thankMr. M. Passalacqua for reading the entire text in its
original form.
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(Received 7 October 2002, Accepted 20 December 2002)