some input from Taranaki Maunga and Mayor Island. The
tephra cover generally thins with increasing distance away
from the central North Island source volcanoes. David Lowe
and David Palmer published a map in 2005 that shows the
dominance of andesitic versus rhyolitic tephra components,
making up soils, in relation to the main volcanic centres. In the
Waikato there is a gradient in tephra components from rhyolite
dominated in the east to andesite dominated towards the
southwest and King Country. All the Waikato soils contain
both andesitic and rhyolitc tephras, just in different proportions. David Lowe’s study of lake cores, where numerous fine
tephra layers have been preserved within the lake sediment,
has provided insight into the compositions of the tephras that
have merged to form the soils in the Waikato.
Near to Taupo a thick surface layer of rhyolitic pumice
dominates soil properties and Pumice Soils form (Chap. 12).
As the pumice deposit thins with increasing distance from
Taupo, gradually the finer underlying tephra and allophanic
soil properties, become nearer to the modern land surface,
and eventually dominant within the soil profile.
Buried-allophanic Orthic Pumice Soils marry the two orders
together nicely in many places. On the margins of the
Pumice Soils, Vitric Orthic Allophanic Soils occur where
there is 50% or more sand (including from coarse
glass-dominated pumice fragments) within the soil. Further
from Taupo, for example in the Tirau area, Typic Orthic
Allophanic Soils are formed mainly in incrementally
deposited thin tephra layers that form a fine, weakly developed, deep soil profile (similar to that shown in Fig. 2.1). On
alluvial plains (in the central Waikato Basin, Hauraki Plains,
and the Rangitaiki Plains in the Bay of Plenty) many of the
soils are formed in tephric alluvium where tephra and small
amounts of other materials have been reworked by rivers,
carried farther from the source volcanoes, and deposited on
the pre-modern river floodplains. Further north the tephra
layers thin and are preserved only intermittently, mainly on
flat or gently sloping surfaces, with Granular and Ultic Soils
on steeper slopes and in areas where younger tephras are not
present. Some of the younger tephra layers have been
transformed into halloysitic, not allophanic, clays (see
Chaps. 6 and 16).
2.3.4 Allophanic Soil Pattern on the Alluvial
Fans and Plains of the Waikato Basin
and Hauraki Area
The alluvial plains in the central Waikato (Hamilton) Basin,
such as the near-flat areas between Cambridge and Taupiri,
and in the Hinuera Valley and Matamata-Hauraki Plains,
were built up by material carried and deposited by the
ancestral Waikato River. During the last cold period in the
last glaciation, between about 32,000 and 18,000 years ago,
the ancestral Waikato River carried a high sediment load.
Such a load was partially due to increased erosion in the
headwaters as a result of less vegetation cover in the colder
and windier climate. Also contributing to the high sediment
load was a huge volume of loose volcanic material, in the
catchment, derived from repeated rhyolitic eruptions, particularly the caldera-forming Oruanui eruption of Taupo
volcano about 25,400 years ago. The Oruanui eruption
(generating the Kawakawa/Oruanui tephra) was the largest
eruption on Earth in the last 70,000 years and is classified as
a ‘super-eruption’ because of the huge volume of pyroclastic
material generated, around 530 km
3 (as dense rock equivalent) or 1170 km
3 as bulk loose material.
Because of the high bedload of loose material from the
Oruanui eruption, and the cold climate, the ancestral Waikato River was a high energy braided river system, somewhat like those of Canterbury today. As the river deposited
material and filled its bed it repeatedly overflowed and
moved to new channels, gradually filling the
Matamata-Hauraki Basin with volcanogenic alluvium ranging from coarse gravelly sands to silts. The river then began
infilling the Hamilton Basin with alluvium after shifting its
course (from the Hauraki to the Hamilton Basin), at Piarere,
about 23,500 years ago.
As climate warmed, commencing about 17,500 years
ago, forest vegetation became quickly re-established, erosion
rates declined markedly as the landscape became stabilised,
and the bedload in the river dropped. As a result the river
started to cut down through the earlier sediment deposits.
The downcutting process resulted in ‘degradational’ terraces
being formed, such as those evident in the Karapiro area.
Multiple abandoned channels are evident in the modern
landscape surface of the Hamlton Basin as shallow, wide
paleochannels. Eventually around, or soon after, about
17,000 years ago, the river became entrenched in the path
we see today, about 20 m lower than the now-abandoned
wider alluvial plain. The plain surface, such as that between
Hamilton and Cambridge (called the Hinuera Surface), was
left with the pattern of a braided river bed with many slightly
raised deposits of gravelly sands, and sands, on the former
levees (low ridges) and finer material, typically silts, in the
low-lying former back-swamp areas (swales).
Ongoing volcanic eruptions, since c. 17,000 years ago,
led to the gradual accumulation of thin tephra deposits on the
land surface, forming a composite, homogenous, tephra
mantle about 0.5–0.8 m thick, in which the upper soil has
formed through developmental upbuilding. The equivalent
thin tephras are preserved as separate layers in organic
sediments in lakes formed about 20,000 years ago in the
Hamilton Basin, and in peat bogs formed about 15,000 years
ago. Thus the land surface comprises a fine network of
28
2 Allophanic Soils
tephra cover generally thins with increasing distance away
from the central North Island source volcanoes. David Lowe
and David Palmer published a map in 2005 that shows the
dominance of andesitic versus rhyolitic tephra components,
making up soils, in relation to the main volcanic centres. In the
Waikato there is a gradient in tephra components from rhyolite
dominated in the east to andesite dominated towards the
southwest and King Country. All the Waikato soils contain
both andesitic and rhyolitc tephras, just in different proportions. David Lowe’s study of lake cores, where numerous fine
tephra layers have been preserved within the lake sediment,
has provided insight into the compositions of the tephras that
have merged to form the soils in the Waikato.
Near to Taupo a thick surface layer of rhyolitic pumice
dominates soil properties and Pumice Soils form (Chap. 12).
As the pumice deposit thins with increasing distance from
Taupo, gradually the finer underlying tephra and allophanic
soil properties, become nearer to the modern land surface,
and eventually dominant within the soil profile.
Buried-allophanic Orthic Pumice Soils marry the two orders
together nicely in many places. On the margins of the
Pumice Soils, Vitric Orthic Allophanic Soils occur where
there is 50% or more sand (including from coarse
glass-dominated pumice fragments) within the soil. Further
from Taupo, for example in the Tirau area, Typic Orthic
Allophanic Soils are formed mainly in incrementally
deposited thin tephra layers that form a fine, weakly developed, deep soil profile (similar to that shown in Fig. 2.1). On
alluvial plains (in the central Waikato Basin, Hauraki Plains,
and the Rangitaiki Plains in the Bay of Plenty) many of the
soils are formed in tephric alluvium where tephra and small
amounts of other materials have been reworked by rivers,
carried farther from the source volcanoes, and deposited on
the pre-modern river floodplains. Further north the tephra
layers thin and are preserved only intermittently, mainly on
flat or gently sloping surfaces, with Granular and Ultic Soils
on steeper slopes and in areas where younger tephras are not
present. Some of the younger tephra layers have been
transformed into halloysitic, not allophanic, clays (see
Chaps. 6 and 16).
2.3.4 Allophanic Soil Pattern on the Alluvial
Fans and Plains of the Waikato Basin
and Hauraki Area
The alluvial plains in the central Waikato (Hamilton) Basin,
such as the near-flat areas between Cambridge and Taupiri,
and in the Hinuera Valley and Matamata-Hauraki Plains,
were built up by material carried and deposited by the
ancestral Waikato River. During the last cold period in the
last glaciation, between about 32,000 and 18,000 years ago,
the ancestral Waikato River carried a high sediment load.
Such a load was partially due to increased erosion in the
headwaters as a result of less vegetation cover in the colder
and windier climate. Also contributing to the high sediment
load was a huge volume of loose volcanic material, in the
catchment, derived from repeated rhyolitic eruptions, particularly the caldera-forming Oruanui eruption of Taupo
volcano about 25,400 years ago. The Oruanui eruption
(generating the Kawakawa/Oruanui tephra) was the largest
eruption on Earth in the last 70,000 years and is classified as
a ‘super-eruption’ because of the huge volume of pyroclastic
material generated, around 530 km
3 (as dense rock equivalent) or 1170 km
3 as bulk loose material.
Because of the high bedload of loose material from the
Oruanui eruption, and the cold climate, the ancestral Waikato River was a high energy braided river system, somewhat like those of Canterbury today. As the river deposited
material and filled its bed it repeatedly overflowed and
moved to new channels, gradually filling the
Matamata-Hauraki Basin with volcanogenic alluvium ranging from coarse gravelly sands to silts. The river then began
infilling the Hamilton Basin with alluvium after shifting its
course (from the Hauraki to the Hamilton Basin), at Piarere,
about 23,500 years ago.
As climate warmed, commencing about 17,500 years
ago, forest vegetation became quickly re-established, erosion
rates declined markedly as the landscape became stabilised,
and the bedload in the river dropped. As a result the river
started to cut down through the earlier sediment deposits.
The downcutting process resulted in ‘degradational’ terraces
being formed, such as those evident in the Karapiro area.
Multiple abandoned channels are evident in the modern
landscape surface of the Hamlton Basin as shallow, wide
paleochannels. Eventually around, or soon after, about
17,000 years ago, the river became entrenched in the path
we see today, about 20 m lower than the now-abandoned
wider alluvial plain. The plain surface, such as that between
Hamilton and Cambridge (called the Hinuera Surface), was
left with the pattern of a braided river bed with many slightly
raised deposits of gravelly sands, and sands, on the former
levees (low ridges) and finer material, typically silts, in the
low-lying former back-swamp areas (swales).
Ongoing volcanic eruptions, since c. 17,000 years ago,
led to the gradual accumulation of thin tephra deposits on the
land surface, forming a composite, homogenous, tephra
mantle about 0.5–0.8 m thick, in which the upper soil has
formed through developmental upbuilding. The equivalent
thin tephras are preserved as separate layers in organic
sediments in lakes formed about 20,000 years ago in the
Hamilton Basin, and in peat bogs formed about 15,000 years
ago. Thus the land surface comprises a fine network of
28
2 Allophanic Soils