abandoning its previous channel, some Raw Soils may be
able to progress to become a Recent Soil because the profile
is on a stable site. Thus, once soil weathering and profile
formation become more advanced the Raw Soils develop
and transform into soils of the other soil orders.
A number of key stages of soil initiation from a newly
exposed substrate, or emplaced deposit, have been recognised, supported by a range of physical and chemical
weathering processes.
1. Surface disintegration of rock caused by a range of
processes including stress release due to removal of
overlying materials and thermal expansion and contraction through successive cycles of heat or cold. The
effectiveness of disintegration may be strongly controlled
by the nature of the rock such as pre-weathering,
pre-existing discontinuities, jointing, or rock fabric.
Unconsolidated material, such as alluvial or pyroclastic
deposits, have a ‘head start’ over hard, continuous rock
because they are fragmental and so have greater surface
areas at the outset. The mineralogical composition of the
rock or deposit also impacts the rate of disintegration as
some minerals are more susceptible to chemical weathering than others.
2. Penetration of air and water into the earth materials
provides for chemical weathering processes, including
oxidation and reduction, hydration, hydrolysis, and dissolution of minerals and the formation of clay materials,
to commence. Water and air also provide sustenance for
colonising roots and wide-ranging life forms.
3. Establishment of life is facilitated by accumulation of
seed, spores, dust, and microbes which may be dispersed
by wind, birds, animals, water, and lateral root spread.
Invasion of roots, and addition of leaf litter, from trees
growing nearby, introduce not only plant roots but also
passengers including bacterial and fungal/rhizobial flora,
and higher trophic level organisms. Some plants will be
nitrogen fixers and will build vital nitrogen fertility.
Plants also introduce organic materials, for example,
acids and chelating organic biochemicals, which further
facilitate weathering of the geologic substrate.
4. Further weathering follows the introduction of life,
along with water and air. Weathering drives the
increasing pace of the transformation of soil parent
material to clay, which, along with decomposing organic
matter, builds the essential soil fertility by providing
increasing cation and anion exchange for nutrient storage, pH buffering capacity, and water-holding capacity.
The rate and timing of ongoing soil development will
depend on the climate as well as ongoing landform stability,
parent materials, and other factors.
13.3 Soil-Landscape Relationships
The geomorphic processes that drive the formation of Raw
Soils are often obvious—for example, Sandy Raw Soils in
aeolian dune lands and Rocky Raw Soils on high and steep
mountain tops. The dynamism of the New Zealand landscape is ultimately controlled by the geological setting of
New Zealand on the boundary of the Pacific and Australian
tectonic plates and in the ‘roaring forties’ latitude (around
40°S) which means the New Zealand archipelago lies in a
belt of strong prevailing westerlies and is prone to high
winds and intense rainfall. Thus the combination of rapid
uplift, active erosion and deposition, volcanic activity, and
(most commonly) periodic storm events cause landscape
changes that lead to Raw Soil establishment.
Rocky Raw Soils are mainly formed in mountainous
areas, particularly the Southern Alps, where rapid and ongoing uplift, to high altitudes, leads to the land being
exposed to cold temperatures, high precipitation, and
gale-force winds that promote erosion and make plant
establishment difficult. A complex pattern of Raw and
Recent Soils often occurs on a local scale as plants become
established and pockets of Recent Soil form in areas with
more sheltered microclimates, while Raw Soils remain on
the most exposed faces. Some of the Raw Soils in high
altitude alpine areas, particularly adjacent to glaciers and on
shaded southerly faces, may contain permafrost, and many
exhibit some periglacial features such as down-slope
movement due to freeze-thaw processes. Small areas of
Rocky Raw Soil may also be observed where river
down-cutting or coastal erosion has exposed rocky cliff
faces.
Younger lava flows, such as those on Rangitoto Island, or
in Mangatepopo valley from eruptions of Ngauruhoe, where
there has been insufficient time for soil development to
progress, also support Rocky Raw Soils (Fig. 13.4). There is
a progression of weathering with age of lava flows from Raw
to Recent, then to Granular or Podzol, then Oxidic Soils.
However, many of the older lava flows in New Zealand are
subject to later additions of tephra that, if thick, reset the soil
development process back to Tephric Raw Soils that may
then progress through Recent to Pumice or Allophanic Soils.
Hydrothermal Raw Soils are scattered in small areas
through the central North Island where geothermal activity
results in areas of relatively inhospitable soils that often have
very low pHs (*3.4) and temperatures as high as 100 °C
(Fig. 13.5). As well as Ketetahi Springs in the northern
flanks of Tongariro, many of the more extensive sites, including Whakarewarewa, Waiotapu, Craters of the Moon,
hydrothermal areas on the margins of Lake Rotorua, and
Orakei Korako, are important tourist attractions. Vegetation
if present is typically stunted and shallow rooted.
13.2 Soil Profile Genesis
203
able to progress to become a Recent Soil because the profile
is on a stable site. Thus, once soil weathering and profile
formation become more advanced the Raw Soils develop
and transform into soils of the other soil orders.
A number of key stages of soil initiation from a newly
exposed substrate, or emplaced deposit, have been recognised, supported by a range of physical and chemical
weathering processes.
1. Surface disintegration of rock caused by a range of
processes including stress release due to removal of
overlying materials and thermal expansion and contraction through successive cycles of heat or cold. The
effectiveness of disintegration may be strongly controlled
by the nature of the rock such as pre-weathering,
pre-existing discontinuities, jointing, or rock fabric.
Unconsolidated material, such as alluvial or pyroclastic
deposits, have a ‘head start’ over hard, continuous rock
because they are fragmental and so have greater surface
areas at the outset. The mineralogical composition of the
rock or deposit also impacts the rate of disintegration as
some minerals are more susceptible to chemical weathering than others.
2. Penetration of air and water into the earth materials
provides for chemical weathering processes, including
oxidation and reduction, hydration, hydrolysis, and dissolution of minerals and the formation of clay materials,
to commence. Water and air also provide sustenance for
colonising roots and wide-ranging life forms.
3. Establishment of life is facilitated by accumulation of
seed, spores, dust, and microbes which may be dispersed
by wind, birds, animals, water, and lateral root spread.
Invasion of roots, and addition of leaf litter, from trees
growing nearby, introduce not only plant roots but also
passengers including bacterial and fungal/rhizobial flora,
and higher trophic level organisms. Some plants will be
nitrogen fixers and will build vital nitrogen fertility.
Plants also introduce organic materials, for example,
acids and chelating organic biochemicals, which further
facilitate weathering of the geologic substrate.
4. Further weathering follows the introduction of life,
along with water and air. Weathering drives the
increasing pace of the transformation of soil parent
material to clay, which, along with decomposing organic
matter, builds the essential soil fertility by providing
increasing cation and anion exchange for nutrient storage, pH buffering capacity, and water-holding capacity.
The rate and timing of ongoing soil development will
depend on the climate as well as ongoing landform stability,
parent materials, and other factors.
13.3 Soil-Landscape Relationships
The geomorphic processes that drive the formation of Raw
Soils are often obvious—for example, Sandy Raw Soils in
aeolian dune lands and Rocky Raw Soils on high and steep
mountain tops. The dynamism of the New Zealand landscape is ultimately controlled by the geological setting of
New Zealand on the boundary of the Pacific and Australian
tectonic plates and in the ‘roaring forties’ latitude (around
40°S) which means the New Zealand archipelago lies in a
belt of strong prevailing westerlies and is prone to high
winds and intense rainfall. Thus the combination of rapid
uplift, active erosion and deposition, volcanic activity, and
(most commonly) periodic storm events cause landscape
changes that lead to Raw Soil establishment.
Rocky Raw Soils are mainly formed in mountainous
areas, particularly the Southern Alps, where rapid and ongoing uplift, to high altitudes, leads to the land being
exposed to cold temperatures, high precipitation, and
gale-force winds that promote erosion and make plant
establishment difficult. A complex pattern of Raw and
Recent Soils often occurs on a local scale as plants become
established and pockets of Recent Soil form in areas with
more sheltered microclimates, while Raw Soils remain on
the most exposed faces. Some of the Raw Soils in high
altitude alpine areas, particularly adjacent to glaciers and on
shaded southerly faces, may contain permafrost, and many
exhibit some periglacial features such as down-slope
movement due to freeze-thaw processes. Small areas of
Rocky Raw Soil may also be observed where river
down-cutting or coastal erosion has exposed rocky cliff
faces.
Younger lava flows, such as those on Rangitoto Island, or
in Mangatepopo valley from eruptions of Ngauruhoe, where
there has been insufficient time for soil development to
progress, also support Rocky Raw Soils (Fig. 13.4). There is
a progression of weathering with age of lava flows from Raw
to Recent, then to Granular or Podzol, then Oxidic Soils.
However, many of the older lava flows in New Zealand are
subject to later additions of tephra that, if thick, reset the soil
development process back to Tephric Raw Soils that may
then progress through Recent to Pumice or Allophanic Soils.
Hydrothermal Raw Soils are scattered in small areas
through the central North Island where geothermal activity
results in areas of relatively inhospitable soils that often have
very low pHs (*3.4) and temperatures as high as 100 °C
(Fig. 13.5). As well as Ketetahi Springs in the northern
flanks of Tongariro, many of the more extensive sites, including Whakarewarewa, Waiotapu, Craters of the Moon,
hydrothermal areas on the margins of Lake Rotorua, and
Orakei Korako, are important tourist attractions. Vegetation
if present is typically stunted and shallow rooted.
13.2 Soil Profile Genesis
203