(2) soil formation (pedogenesis) subsequently transforms
the (stable) parent material through topdown processes,
the horizons developing in a downward moving front
according to site conditions.
This model, well entrenched in almost all text books,
applies to stable landscapes and ‘set’ parent materials where
there are effectively no ongoing geological processes in play,
such as the deposition of loess, tephra, colluvium, or
alluvium.
The frequency and thickness of tephra accumulation
determines, in part, how much impact topdown pedogenic
processes have on the resulting soil profile, and if ‘developmental upbuilding’ or ‘retardant upbuilding’, or both, take
place. Developmental upbuilding occurs when the rate of
addition of tephra is sufficiently slow (in thin increments)
that topdown soil formation effectively keeps pace, as the
soil surface gradually rises, and the separate thin tephra
layers are modified to form soil horizons. Because the land
surface is slowly rising, there is not much time for horizon
features to develop and soils tend to be reasonably uniform
(Fig. 2.1). A key point is that every part of the profile has
been an A-horizon at one time and so will have some imprint
of pedogenesis.
The soil profile in Fig. 2.1 (an example of a soil formed
as a result of developmental upbuilding, to *1 m depth) has
a composite age of c. 45,000 years, i.e. it has an age
range rather than a specific ‘single’ age—because the profile
has taken 45,000 years to accumulate thin distal tephra
layers while being simultaneously modified by pedogenesis.
The profiles of many Allophanic Soils (formed by developmental upbuilding) in New Zealand have composite ages
of 20,000 years or more. By way of contrast, the soil formed
on the pumiceous Taupo ignimbrite, which was every
where emplaced within about 10 min (Chap. 12), has a
single, specific age of *1788 years (± 10 years) before
2020.
Retardant upbuilding (as exemplified in Fig. 2.4) occurs
when a relatively thick layer of tephra is ‘instantaneously’
added to the surface, or where the rate of accumulation of
thinner additions is fast, so that the original soil is rapidly
buried. Thus the buried topsoil (now a paleosol) is isolated
from the new land surface, and its development is effectively
retarded. At the ground surface, soil formation begins anew
with a Raw Soil. Over time, if conditions remain stable, the
Raw Soil will develop into a Recent Soil and then to a
Pumice or an Allophanic Soil. Retardant upbuilding is more
likely to occur close (proximal) to the source volcano,
whereas developmental upbuilding occurs further away (at
medial to distal sites) where the layers of tephra that fall are
thinner and/or less frequent. Both forms of upbuilding can be
recognised as taking place in some Allophanic Soils.
2.2.2 Formation of Allophane
Allophane, the presence of which imparts the key properties
of an Allophanic Soil, is a clay mineral that comprises tiny
hollow spherules which are 3.5–5 nm in diameter (which is
much finer than many other clay minerals which are
Fig. 2.4 Complex soil profile near Mt Tarawera chosen to illustrate
retardant upbuilding pedogenesis. The sequence shows a series of five
tephra layers dating back to 9500 years ago (as indicated on the labels)
with soil horizons on each of them developed when at the land surface.
The deposition of each new tephra layer has buried the pre-existing soil
horizons, forming a paleosol, and isolating them from the land surface,
for example, between the Taupo and Kaharoa tephras. The uppermost
soil horizons are formed in material erupted from Mt Tarawera on 10
June 1886 (mainly the Rotomahana Mud). The modern multi-layered
soil profile (to *1 m depth) includes the soil on Rotomahana Mud and
the paleosols on the Kaharoa and Taupo tephra layers. The soil is
classified as Buried-pumice Tephric Recent Soil (which does contain
some allophanic soil materials)
2.2 Soil Profile Genesis
25
the (stable) parent material through topdown processes,
the horizons developing in a downward moving front
according to site conditions.
This model, well entrenched in almost all text books,
applies to stable landscapes and ‘set’ parent materials where
there are effectively no ongoing geological processes in play,
such as the deposition of loess, tephra, colluvium, or
alluvium.
The frequency and thickness of tephra accumulation
determines, in part, how much impact topdown pedogenic
processes have on the resulting soil profile, and if ‘developmental upbuilding’ or ‘retardant upbuilding’, or both, take
place. Developmental upbuilding occurs when the rate of
addition of tephra is sufficiently slow (in thin increments)
that topdown soil formation effectively keeps pace, as the
soil surface gradually rises, and the separate thin tephra
layers are modified to form soil horizons. Because the land
surface is slowly rising, there is not much time for horizon
features to develop and soils tend to be reasonably uniform
(Fig. 2.1). A key point is that every part of the profile has
been an A-horizon at one time and so will have some imprint
of pedogenesis.
The soil profile in Fig. 2.1 (an example of a soil formed
as a result of developmental upbuilding, to *1 m depth) has
a composite age of c. 45,000 years, i.e. it has an age
range rather than a specific ‘single’ age—because the profile
has taken 45,000 years to accumulate thin distal tephra
layers while being simultaneously modified by pedogenesis.
The profiles of many Allophanic Soils (formed by developmental upbuilding) in New Zealand have composite ages
of 20,000 years or more. By way of contrast, the soil formed
on the pumiceous Taupo ignimbrite, which was every
where emplaced within about 10 min (Chap. 12), has a
single, specific age of *1788 years (± 10 years) before
2020.
Retardant upbuilding (as exemplified in Fig. 2.4) occurs
when a relatively thick layer of tephra is ‘instantaneously’
added to the surface, or where the rate of accumulation of
thinner additions is fast, so that the original soil is rapidly
buried. Thus the buried topsoil (now a paleosol) is isolated
from the new land surface, and its development is effectively
retarded. At the ground surface, soil formation begins anew
with a Raw Soil. Over time, if conditions remain stable, the
Raw Soil will develop into a Recent Soil and then to a
Pumice or an Allophanic Soil. Retardant upbuilding is more
likely to occur close (proximal) to the source volcano,
whereas developmental upbuilding occurs further away (at
medial to distal sites) where the layers of tephra that fall are
thinner and/or less frequent. Both forms of upbuilding can be
recognised as taking place in some Allophanic Soils.
2.2.2 Formation of Allophane
Allophane, the presence of which imparts the key properties
of an Allophanic Soil, is a clay mineral that comprises tiny
hollow spherules which are 3.5–5 nm in diameter (which is
much finer than many other clay minerals which are
Fig. 2.4 Complex soil profile near Mt Tarawera chosen to illustrate
retardant upbuilding pedogenesis. The sequence shows a series of five
tephra layers dating back to 9500 years ago (as indicated on the labels)
with soil horizons on each of them developed when at the land surface.
The deposition of each new tephra layer has buried the pre-existing soil
horizons, forming a paleosol, and isolating them from the land surface,
for example, between the Taupo and Kaharoa tephras. The uppermost
soil horizons are formed in material erupted from Mt Tarawera on 10
June 1886 (mainly the Rotomahana Mud). The modern multi-layered
soil profile (to *1 m depth) includes the soil on Rotomahana Mud and
the paleosols on the Kaharoa and Taupo tephra layers. The soil is
classified as Buried-pumice Tephric Recent Soil (which does contain
some allophanic soil materials)
2.2 Soil Profile Genesis
25