canism, such as in the circum-Pacific ‘Ring of Fire’. However, elsewhere allophane is relatively uncommon (soils
dominated by allophane—the Andisols of ‘Soil Taxonomy’—make up only about 1% of the world’s soils). Allophanic Soils, especially on mafic (scoriaceous) materials,
may also contain small to moderate quantities of the iron
oxide clay mineral called ferrihydrite, in effect an iron-rich
mineral equivalent to allophane. Both allophane and ferrihydrite are now recognised as being nanocrystalline clays (or
nanominerals)—that is, they comprise tiny structural units in
the nanoscale range (1–100 nm), as described in Sect. 2.4.
Allophanic Soils have an unusually low soil dry bulk density, high porosity, and high to very high soil-water holding
capacity, and may also be sensitive and/or thixotropic (i.e. they
lose strength when remoulded). They are formed mainly in
glassy pyroclastic material, also referred to collectively as
tephra (volcanic ash). Allophanic Soils form mainly in andesitic or rhyolitic materials, very commonly both together.
They can also form in coarse pumice, basaltic scoria, and
tephra-derived materials reworked and deposited by rivers,
wind, or gravity (described as volcaniclastic). Occasionally
soils containing allophane, but with insufficient amounts or
thicknesses to qualify for Allophanic Soils per se, are formed
on tuffaceous greywacke (fine sandstone) and in strongly
leached loess and other regolith materials in high country situations where strong leaching and desilication have occurred.
A key concept is that Allophanic Soil profiles, in contrast
to those of many other soil orders, typically form by
upbuilding pedogenesis as younger tephras are deposited on
top of older ones so that the resulting profile character is a
reflection of the interplay, or ‘competition’, between the rate
at which tephras have accumulated on the land surface (by
geological processes) and their modification by topdown
soil-forming or pedogenic processes that transform the
material into soil horizons. Thus soil stratigraphy is at the
heart of understanding the genesis of many Allophanic Soils.
2.1.2 Areas of Occurrence
The Allophanic Soils cover 5% of New Zealand (Fig. 2.2).
They occur only in the North Island, with concentrations in
Taranaki, Waikato, and western Bay of Plenty, and to the
south and southeast of the Tongariro Volcanic Centre in the
central North Island. Location is mainly controlled by the
distribution of andesitic and rhyolitic tephra-fall deposits,
tephric loess, or volcanogenic alluvium derived mainly from
reworked tephra deposits. Allophanic Soils also form on the
weathering products of andesite lahar mounds on the Taranaki and Ruapehu ring plains, and from some mafic pyroclastic deposits including scoria, such as in Auckland and
Northland on basaltic scoria cones.
Fig. 2.1 Allophanic Soil and associated landscape. Left: a Typic Orthic
Allophanic Soil in the Waikato formed in incrementally accumulating
thin tephras derived from central North Island and Taranaki volcanic
eruptions. The 1 m deep soil is one of the oldest Andisols in the world
(containing Rotoehu Ash, c. 45,000 years old at *1 m depth and
Kawakawa/Oruanui Tephra at *55 cm). Every part of the profile has
been an A-horizon at the surface at some time. Right: friable Allophanic
Soils are readily ploughed to form an exceptionally fine seed-bed
22
2 Allophanic Soils
dominated by allophane—the Andisols of ‘Soil Taxonomy’—make up only about 1% of the world’s soils). Allophanic Soils, especially on mafic (scoriaceous) materials,
may also contain small to moderate quantities of the iron
oxide clay mineral called ferrihydrite, in effect an iron-rich
mineral equivalent to allophane. Both allophane and ferrihydrite are now recognised as being nanocrystalline clays (or
nanominerals)—that is, they comprise tiny structural units in
the nanoscale range (1–100 nm), as described in Sect. 2.4.
Allophanic Soils have an unusually low soil dry bulk density, high porosity, and high to very high soil-water holding
capacity, and may also be sensitive and/or thixotropic (i.e. they
lose strength when remoulded). They are formed mainly in
glassy pyroclastic material, also referred to collectively as
tephra (volcanic ash). Allophanic Soils form mainly in andesitic or rhyolitic materials, very commonly both together.
They can also form in coarse pumice, basaltic scoria, and
tephra-derived materials reworked and deposited by rivers,
wind, or gravity (described as volcaniclastic). Occasionally
soils containing allophane, but with insufficient amounts or
thicknesses to qualify for Allophanic Soils per se, are formed
on tuffaceous greywacke (fine sandstone) and in strongly
leached loess and other regolith materials in high country situations where strong leaching and desilication have occurred.
A key concept is that Allophanic Soil profiles, in contrast
to those of many other soil orders, typically form by
upbuilding pedogenesis as younger tephras are deposited on
top of older ones so that the resulting profile character is a
reflection of the interplay, or ‘competition’, between the rate
at which tephras have accumulated on the land surface (by
geological processes) and their modification by topdown
soil-forming or pedogenic processes that transform the
material into soil horizons. Thus soil stratigraphy is at the
heart of understanding the genesis of many Allophanic Soils.
2.1.2 Areas of Occurrence
The Allophanic Soils cover 5% of New Zealand (Fig. 2.2).
They occur only in the North Island, with concentrations in
Taranaki, Waikato, and western Bay of Plenty, and to the
south and southeast of the Tongariro Volcanic Centre in the
central North Island. Location is mainly controlled by the
distribution of andesitic and rhyolitic tephra-fall deposits,
tephric loess, or volcanogenic alluvium derived mainly from
reworked tephra deposits. Allophanic Soils also form on the
weathering products of andesite lahar mounds on the Taranaki and Ruapehu ring plains, and from some mafic pyroclastic deposits including scoria, such as in Auckland and
Northland on basaltic scoria cones.
Fig. 2.1 Allophanic Soil and associated landscape. Left: a Typic Orthic
Allophanic Soil in the Waikato formed in incrementally accumulating
thin tephras derived from central North Island and Taranaki volcanic
eruptions. The 1 m deep soil is one of the oldest Andisols in the world
(containing Rotoehu Ash, c. 45,000 years old at *1 m depth and
Kawakawa/Oruanui Tephra at *55 cm). Every part of the profile has
been an A-horizon at the surface at some time. Right: friable Allophanic
Soils are readily ploughed to form an exceptionally fine seed-bed
22
2 Allophanic Soils