measured in the micro- rather than nanoscale). The most
common ingredient for the formation of allophane is volcanic glass (similar to window glass) which occurs in tephra
(often 50–90%), along with crystals of feldspar, quartz,
various mafic (or heavy) minerals, and rock fragments. The
volcanic glass in tephra weathers readily because the glass is
in the form of tiny shards (fragments) and is often vesicular
(with holes), so the particles have a large surface area that is
open to rapid attack by acids.
To form allophane the glass shards must first be weathered to release Si and Al species (along with interstitial ions
in the glass such as those of Ca, Mg, Na, K, and Fe) into
solution. Such weathering occurs primarily by the dissolution of the glass via hydrolysis, forming monosilicic acid (Si
(OH) 4 ) and gibbsitic compounds (Al 2 (OH) 3 ). Then, within
the soil solution, these two ingredients are precipitated as
allophane spheres. The formation of allophane is thus
dependent ultimately on the concentration of silicic acid and
the availability of aluminium species in soil solution. Such
concentrations are influenced by various environmental
conditions including annual rainfall, soil drainage, the nature
of the soil organic matter, soil acidity, tephra composition
(rhyolitic versus andesitic versus basaltic) and accumulation
rate, depth of burial, and vegetation type. These factors in
turn dictate the rate of silicon removal from a soil and form
the basis of the ‘silica-leaching model’ for the formation of
allophane (Fig. 2.5).
In the silicon-leaching model, if silicon concentrations in
soil solution are <*10 ppm, allophane will form, but if
concentrations are >*10 ppm then halloysite forms. Where
the silicon concentration is close to 10 ppm either or both
minerals may form. Studies involving Allophanic Soils have
shown that if about 250 mm of drainage water passes
through the profile annually, typically under an annual
rainfall of >*1600 mm, then the decreased silicon concentration (due to more leaching) will favour the formation
of allophane. If total drainage is <*250 mm, typically
where annual rainfall is <*1200 mm, then halloysite will
normally form. The formation of halloysite, rather than allophane, can be the result therefore of either low rainfall or
poor drainage, or both. Poor drainage may be caused by a
high water table or layers of fine material, a clayey paleosol,
or a pan, within the soil profile that supports a perched water
table. Thus Allophanic Soils do not form at sites where soils
are saturated for extended periods. Although rhyolitic and/or
andesitic tephras are the most common parent material for
formation of allophane, other materials may also weather
and the weathering products can form allophane or related
clays (such as imogolite, ferrihydrite, metal-humus complexes, and hydroxyl-interlayered vermiculite). Materials
that may weather to allophane, or related minerals, include
greywacke, schist, basaltic, tephras (scoria and ash) and
basalt, and the widely varying quartz- and feldspar-rich
parent materials of many Podzol Soils.
2.3 Soil-Landscape Relationships
2.3.1 Overview
Because of the relationship between tephra and the formation of allophane, the most common soil-landscape patterns
of Allophanic Soils relate to the accumulation of tephras to
sufficient depth to dominate the soil parent material. The soil
also needs to be free draining and under a moderately high
rainfall to provide conditions that promote the desilication
that favours the formation of common Al-rich allophane. On
flat to gently rolling slopes, within regions of regular tephra
additions, the consistent blanketing of the landscape results
in relatively uniform Allophanic Soils over wide areas.
However, in areas more distant from the volcanoes, and in
areas with steeper slopes or redistributed tephra materials,
then a variety of soil-landscape patterns occur. The particular
situations where Allophanic Soils occur in the vicinity of
andesitic volcanoes, and at greater distances from them, are
discussed along with an example of the pattern of Allophanic Soils that occurs on partly tephra-mantled alluvial
deposits in the central Waikato basin.
2.3.2 Allophanic Soils in Proximity to Andesitic
Source Volcanoes: The Taranaki Ring Plain
and Areas South of Tongariro
On the lower slopes of Mt Taranaki, and on the southern side
of Ruapehu (where the rhyolitic pumice, erupted from Taupo
in c. 232 AD, did not reach), Allophanic Soils are formed in
tephras derived predominantly from the frequent explosive
eruptions of the adjacent volcano.
Around Mounts Taranaki and Ruapehu, ring plains are
formed from repeated lahar (mud and debris) flows from
partial collapses of the volcano edifice or (in the case of
Ruapehu) from emptying the crater lake from time to time.
The lahar material is, in many places, overlain by tephra—
the older the lahar, and the closer to the mountain, the
thicker are the overlying tephra layers. The generally deep
mantle of tephras that covers the landscape provides a relatively uniform, stable, parent material that supports Allophanic Soil on many surfaces. In some places streams have
cut down to underlying lava, or lahar deposits, and so
Allophanic Soils form on the stable wide ridges but are
intersected by Recent and Raw Soils in the steep gullies.
The soils on the hummocky, undulating, lower parts of
the Taranaki ring plain are often mapped as a soil association
26
2 Allophanic Soils
common ingredient for the formation of allophane is volcanic glass (similar to window glass) which occurs in tephra
(often 50–90%), along with crystals of feldspar, quartz,
various mafic (or heavy) minerals, and rock fragments. The
volcanic glass in tephra weathers readily because the glass is
in the form of tiny shards (fragments) and is often vesicular
(with holes), so the particles have a large surface area that is
open to rapid attack by acids.
To form allophane the glass shards must first be weathered to release Si and Al species (along with interstitial ions
in the glass such as those of Ca, Mg, Na, K, and Fe) into
solution. Such weathering occurs primarily by the dissolution of the glass via hydrolysis, forming monosilicic acid (Si
(OH) 4 ) and gibbsitic compounds (Al 2 (OH) 3 ). Then, within
the soil solution, these two ingredients are precipitated as
allophane spheres. The formation of allophane is thus
dependent ultimately on the concentration of silicic acid and
the availability of aluminium species in soil solution. Such
concentrations are influenced by various environmental
conditions including annual rainfall, soil drainage, the nature
of the soil organic matter, soil acidity, tephra composition
(rhyolitic versus andesitic versus basaltic) and accumulation
rate, depth of burial, and vegetation type. These factors in
turn dictate the rate of silicon removal from a soil and form
the basis of the ‘silica-leaching model’ for the formation of
allophane (Fig. 2.5).
In the silicon-leaching model, if silicon concentrations in
soil solution are <*10 ppm, allophane will form, but if
concentrations are >*10 ppm then halloysite forms. Where
the silicon concentration is close to 10 ppm either or both
minerals may form. Studies involving Allophanic Soils have
shown that if about 250 mm of drainage water passes
through the profile annually, typically under an annual
rainfall of >*1600 mm, then the decreased silicon concentration (due to more leaching) will favour the formation
of allophane. If total drainage is <*250 mm, typically
where annual rainfall is <*1200 mm, then halloysite will
normally form. The formation of halloysite, rather than allophane, can be the result therefore of either low rainfall or
poor drainage, or both. Poor drainage may be caused by a
high water table or layers of fine material, a clayey paleosol,
or a pan, within the soil profile that supports a perched water
table. Thus Allophanic Soils do not form at sites where soils
are saturated for extended periods. Although rhyolitic and/or
andesitic tephras are the most common parent material for
formation of allophane, other materials may also weather
and the weathering products can form allophane or related
clays (such as imogolite, ferrihydrite, metal-humus complexes, and hydroxyl-interlayered vermiculite). Materials
that may weather to allophane, or related minerals, include
greywacke, schist, basaltic, tephras (scoria and ash) and
basalt, and the widely varying quartz- and feldspar-rich
parent materials of many Podzol Soils.
2.3 Soil-Landscape Relationships
2.3.1 Overview
Because of the relationship between tephra and the formation of allophane, the most common soil-landscape patterns
of Allophanic Soils relate to the accumulation of tephras to
sufficient depth to dominate the soil parent material. The soil
also needs to be free draining and under a moderately high
rainfall to provide conditions that promote the desilication
that favours the formation of common Al-rich allophane. On
flat to gently rolling slopes, within regions of regular tephra
additions, the consistent blanketing of the landscape results
in relatively uniform Allophanic Soils over wide areas.
However, in areas more distant from the volcanoes, and in
areas with steeper slopes or redistributed tephra materials,
then a variety of soil-landscape patterns occur. The particular
situations where Allophanic Soils occur in the vicinity of
andesitic volcanoes, and at greater distances from them, are
discussed along with an example of the pattern of Allophanic Soils that occurs on partly tephra-mantled alluvial
deposits in the central Waikato basin.
2.3.2 Allophanic Soils in Proximity to Andesitic
Source Volcanoes: The Taranaki Ring Plain
and Areas South of Tongariro
On the lower slopes of Mt Taranaki, and on the southern side
of Ruapehu (where the rhyolitic pumice, erupted from Taupo
in c. 232 AD, did not reach), Allophanic Soils are formed in
tephras derived predominantly from the frequent explosive
eruptions of the adjacent volcano.
Around Mounts Taranaki and Ruapehu, ring plains are
formed from repeated lahar (mud and debris) flows from
partial collapses of the volcano edifice or (in the case of
Ruapehu) from emptying the crater lake from time to time.
The lahar material is, in many places, overlain by tephra—
the older the lahar, and the closer to the mountain, the
thicker are the overlying tephra layers. The generally deep
mantle of tephras that covers the landscape provides a relatively uniform, stable, parent material that supports Allophanic Soil on many surfaces. In some places streams have
cut down to underlying lava, or lahar deposits, and so
Allophanic Soils form on the stable wide ridges but are
intersected by Recent and Raw Soils in the steep gullies.
The soils on the hummocky, undulating, lower parts of
the Taranaki ring plain are often mapped as a soil association
26
2 Allophanic Soils