where Orthic Allophanic Soils form on the higher parts of
the landscape with Gley Soils in hollows and Impeded
Allophanic Soils in the intermediate positions between.
Where steeper slopes occur, such as on lahar mounds, much
of the tephra may have been eroded off and soils tend to be
bouldery, stony, and gravelly. Some are still sufficiently
Allophanic to qualify as Allophanic Soils (Opua soils).
Rainfall increases with altitude and hence more strongly
leached Allophanic Soils are recognised at higher altitudes
(250–1150 m). Above about 1150 m, on Mt Taranaki,
younger volcanic deposits (Burrell Formation, erupted about
1655 AD, and the Tahurangi Formation, erupted about 1755
AD) have Recent Soils developed on them that grade into
Raw Soils in alpine areas where rock is exposed by erosion
and minimal soil development has occurred. In the Taranaki
region, where rainfall is high, iron has often been moved
down through the profile and accumulated to form distinct,
hard, iron pans in the subsoil made up mainly of the iron
oxide minerals goethite and ferrihydrite. East of Mt Taranaki, soils become progressively more gravelly with inputs
of the young rhyolitic pumice from the Taupo volcano.
On the steep mudstone hill country between Mt Taranaki
and Mt Ruapehu, tephra mantles less-steep slopes on flattish
ridge tops, terraces that are above the modern flood plain,
and the rolling or gently sloping foot slopes. Where a tephra
mantle is preserved, Allophanic Soils have formed. Eruption
of the pumiceous Taupo Tephra from Tauop volcano
resulted in the infilling of many lower lying areas with
pumice, forming lower terraces on which Pumice Soils have
formed (Chap. 12). On steeper slopes, any tephras may have
been eroded off, leaving soils formed in the exhumed,
underlying, mudstone parent materials. Brown Soils, Recent
Soils, or Raw Soils form on the exposed mudstone parent
materials, depending on how recently material has been
eroded or deposited.
2.3.3 The Waikato-Bay of Plenty Region
Much of the central Waikato area, along with the Bay of
Plenty, is mantled by tephra-fall deposits, predominantly
derived from the central North Island volcanoes, but also with
Fig. 2.5 Relationship between environmental conditions and the formation of allophane and other minerals according to the silicon-leaching
model and the availability of aluminium
2.3 Soil-Landscape Relationships
27
the landscape with Gley Soils in hollows and Impeded
Allophanic Soils in the intermediate positions between.
Where steeper slopes occur, such as on lahar mounds, much
of the tephra may have been eroded off and soils tend to be
bouldery, stony, and gravelly. Some are still sufficiently
Allophanic to qualify as Allophanic Soils (Opua soils).
Rainfall increases with altitude and hence more strongly
leached Allophanic Soils are recognised at higher altitudes
(250–1150 m). Above about 1150 m, on Mt Taranaki,
younger volcanic deposits (Burrell Formation, erupted about
1655 AD, and the Tahurangi Formation, erupted about 1755
AD) have Recent Soils developed on them that grade into
Raw Soils in alpine areas where rock is exposed by erosion
and minimal soil development has occurred. In the Taranaki
region, where rainfall is high, iron has often been moved
down through the profile and accumulated to form distinct,
hard, iron pans in the subsoil made up mainly of the iron
oxide minerals goethite and ferrihydrite. East of Mt Taranaki, soils become progressively more gravelly with inputs
of the young rhyolitic pumice from the Taupo volcano.
On the steep mudstone hill country between Mt Taranaki
and Mt Ruapehu, tephra mantles less-steep slopes on flattish
ridge tops, terraces that are above the modern flood plain,
and the rolling or gently sloping foot slopes. Where a tephra
mantle is preserved, Allophanic Soils have formed. Eruption
of the pumiceous Taupo Tephra from Tauop volcano
resulted in the infilling of many lower lying areas with
pumice, forming lower terraces on which Pumice Soils have
formed (Chap. 12). On steeper slopes, any tephras may have
been eroded off, leaving soils formed in the exhumed,
underlying, mudstone parent materials. Brown Soils, Recent
Soils, or Raw Soils form on the exposed mudstone parent
materials, depending on how recently material has been
eroded or deposited.
2.3.3 The Waikato-Bay of Plenty Region
Much of the central Waikato area, along with the Bay of
Plenty, is mantled by tephra-fall deposits, predominantly
derived from the central North Island volcanoes, but also with
Fig. 2.5 Relationship between environmental conditions and the formation of allophane and other minerals according to the silicon-leaching
model and the availability of aluminium
2.3 Soil-Landscape Relationships
27