refilled over about 15–20 years, reaching a height of 400 m
asl, about 30–40 m above its present level (357 m asl) to
form a terrace on which modern-day Taupo town is located.
Catastrophic failure of a pumiceous dam led to the
re-establishment of the Waikato River and the release of
about 20 km
3 of water, the peak discharge being 20,000–
40,000 m
3 sec
−1 , equivalent to the Mississippi River in flood.
Vern Manville and others traced the break-out flood deposits
(called Taupo Pumice Alluvium) about 220 km downstream
of Lake Taupo. Parts of Hamilton city are built on Pumice
Soils formed on Taupo Pumice Alluvium on low terraces
alongside the Waikato River. Other North Island rivers,
including the Whanganui, Rangitaiki, Mohaka, and Ngaruroro, were choked with fall deposits and Taupo ignimbrite
materials. The immediate aftermath of the eruption would
have seen erosion and reworking on a massive scale in the
river catchments (especially where the vegetation had been
entirely buried) involving rilling and debris flows, fan generation, and the development of overbank flood plains and
terraces underlain by Taupo Pumice Alluvium. The process
is still ongoing and it is still possible to find pumice, carried
down by the rivers and washed up on beaches on both the
east and west coasts of North Island.
12.4 Key Soil Properties
12.4.1 Soil Composition
Unconsolidated rhyolitic pumice is the dominant parent
material of Pumice Soils. The soils may also contain minor
amounts of ‘country rock’ (lithics) caught up in the eruption
at the vent, or picked up during the ignimbrite emplacement
as a pyroclastic flow, together with clasts of obsidian and
small crystals of felsic (feldspar and quartz) and mafic (iron,
magnesium, and titanium) minerals, and glass shards. Volcanic glass is an amorphous solid with poorly ordered
internal structure (hence is a mineraloid) comprising loosely
linked SiO 4 tetrahedra around which cations such as sodium
occur in the intermolecular spaces.
Charcoal fragments are ubiquitous in the uppermost
pumiceous ignimbrite layer throughout its distribution
(Fig. 12.6). In outcrops near Taupo and in road cuttings
along state highways between Turangi and Waiouru (e.g.
along the ‘Desert Road’) carbonised trees that were flattened
and burnt within the pyroclastic flow are abundant.
Clay minerals are dominantly allophane, with some ferrihydrite and imogolite, and occur as coatings around glass
or pumice particles. However, halloysite has also been
identified (by Peter McIntosh) 2 m below the land surface,
attributed to synthesis from a soil solution enriched in silicic
acid because of strong silicon migration through the highly
siliceous pumice materials above. Determining that halloysite, as well as allophane, can form directly in such young
materials was a notable advance in the understanding of the
formation of clays from the products of the dissolution and
hydrolysis of glass.
12.4.2 Physical Properties
Pumice Soils are sandy or gravelly soils dominated by
pumice or pumice-derived sand and silt (ash) with a high
content of volcanic glass. Many of the large pumice clasts
are notably rounded because of transport in a pyroclastic
flow. The uppermost layer in many profiles on the Taupo
eruptives is typically fine-grained and pumiceous (Taupo
ignimbrite), and it overlies coarser (pre-ignimbrite) fallout
beds.
In the fall deposits, the size of pumice clasts generally
decreases with distance from the eruption source while the
proportion of sand and silt size particles typically increases
with distance from source. However, pumice clasts in the
pyroclastic flow deposits show a complex relationship of
size with distance from source that depends on the specific
origin of the deposit. Clasts in the ignimbrite veneer deposits
(on ridge tops) decrease in size away from source but those
in the valley-ponded deposits remain uniform or increase
slightly in size away from source to about 40 km from vent
until decreasing beyond 60 km from vent.
Clay contents are low, generally less than about 5% or so
(Fig. 12.11, Table 12.1). Like the pumice from which it is
derived, soil dry bulk densities are generally less than 1 g
cm
−3 but may be higher in massive or firm, tightly packed
and compact horizons.
Pumice Soils are usually apedal earthy in the topsoil and
single-grain beneath, although some soils may have extremely fine spheroidal structures. The Pumice Soils have weak
to very weak soil strength, except in tightly packed layers
which are massive and may have higher strength. Such
compact materials, sometimes described as brittle, feature in
some of the Impeded Pumice Soils or Perch-gley Pumice
Soils. Pumice Soils are non-plastic and sensitive meaning
that they have low soil strength when disturbed.
Very high macroporosity enables rapid profile drainage at
low soil–water tensions and many Pumice Soils, despite
lacking much clay, are capable of storing large amounts of
water (high available water capacity) for plants. Partly, the
very fine vesicularity of the glass and pumice fragments
provides a higher (than expected for sand-sized grains) water
storage capacity. However, some of the shallower pumice
soils with pumice lapilli in the near-surface horizons drain
very rapidly and readily become droughty during dry spells.
Subsurface paleosols, with finer allophanic soil materials,
12.3 Soil-Landscape Relationships
191
asl, about 30–40 m above its present level (357 m asl) to
form a terrace on which modern-day Taupo town is located.
Catastrophic failure of a pumiceous dam led to the
re-establishment of the Waikato River and the release of
about 20 km
3 of water, the peak discharge being 20,000–
40,000 m
3 sec
−1 , equivalent to the Mississippi River in flood.
Vern Manville and others traced the break-out flood deposits
(called Taupo Pumice Alluvium) about 220 km downstream
of Lake Taupo. Parts of Hamilton city are built on Pumice
Soils formed on Taupo Pumice Alluvium on low terraces
alongside the Waikato River. Other North Island rivers,
including the Whanganui, Rangitaiki, Mohaka, and Ngaruroro, were choked with fall deposits and Taupo ignimbrite
materials. The immediate aftermath of the eruption would
have seen erosion and reworking on a massive scale in the
river catchments (especially where the vegetation had been
entirely buried) involving rilling and debris flows, fan generation, and the development of overbank flood plains and
terraces underlain by Taupo Pumice Alluvium. The process
is still ongoing and it is still possible to find pumice, carried
down by the rivers and washed up on beaches on both the
east and west coasts of North Island.
12.4 Key Soil Properties
12.4.1 Soil Composition
Unconsolidated rhyolitic pumice is the dominant parent
material of Pumice Soils. The soils may also contain minor
amounts of ‘country rock’ (lithics) caught up in the eruption
at the vent, or picked up during the ignimbrite emplacement
as a pyroclastic flow, together with clasts of obsidian and
small crystals of felsic (feldspar and quartz) and mafic (iron,
magnesium, and titanium) minerals, and glass shards. Volcanic glass is an amorphous solid with poorly ordered
internal structure (hence is a mineraloid) comprising loosely
linked SiO 4 tetrahedra around which cations such as sodium
occur in the intermolecular spaces.
Charcoal fragments are ubiquitous in the uppermost
pumiceous ignimbrite layer throughout its distribution
(Fig. 12.6). In outcrops near Taupo and in road cuttings
along state highways between Turangi and Waiouru (e.g.
along the ‘Desert Road’) carbonised trees that were flattened
and burnt within the pyroclastic flow are abundant.
Clay minerals are dominantly allophane, with some ferrihydrite and imogolite, and occur as coatings around glass
or pumice particles. However, halloysite has also been
identified (by Peter McIntosh) 2 m below the land surface,
attributed to synthesis from a soil solution enriched in silicic
acid because of strong silicon migration through the highly
siliceous pumice materials above. Determining that halloysite, as well as allophane, can form directly in such young
materials was a notable advance in the understanding of the
formation of clays from the products of the dissolution and
hydrolysis of glass.
12.4.2 Physical Properties
Pumice Soils are sandy or gravelly soils dominated by
pumice or pumice-derived sand and silt (ash) with a high
content of volcanic glass. Many of the large pumice clasts
are notably rounded because of transport in a pyroclastic
flow. The uppermost layer in many profiles on the Taupo
eruptives is typically fine-grained and pumiceous (Taupo
ignimbrite), and it overlies coarser (pre-ignimbrite) fallout
beds.
In the fall deposits, the size of pumice clasts generally
decreases with distance from the eruption source while the
proportion of sand and silt size particles typically increases
with distance from source. However, pumice clasts in the
pyroclastic flow deposits show a complex relationship of
size with distance from source that depends on the specific
origin of the deposit. Clasts in the ignimbrite veneer deposits
(on ridge tops) decrease in size away from source but those
in the valley-ponded deposits remain uniform or increase
slightly in size away from source to about 40 km from vent
until decreasing beyond 60 km from vent.
Clay contents are low, generally less than about 5% or so
(Fig. 12.11, Table 12.1). Like the pumice from which it is
derived, soil dry bulk densities are generally less than 1 g
cm
−3 but may be higher in massive or firm, tightly packed
and compact horizons.
Pumice Soils are usually apedal earthy in the topsoil and
single-grain beneath, although some soils may have extremely fine spheroidal structures. The Pumice Soils have weak
to very weak soil strength, except in tightly packed layers
which are massive and may have higher strength. Such
compact materials, sometimes described as brittle, feature in
some of the Impeded Pumice Soils or Perch-gley Pumice
Soils. Pumice Soils are non-plastic and sensitive meaning
that they have low soil strength when disturbed.
Very high macroporosity enables rapid profile drainage at
low soil–water tensions and many Pumice Soils, despite
lacking much clay, are capable of storing large amounts of
water (high available water capacity) for plants. Partly, the
very fine vesicularity of the glass and pumice fragments
provides a higher (than expected for sand-sized grains) water
storage capacity. However, some of the shallower pumice
soils with pumice lapilli in the near-surface horizons drain
very rapidly and readily become droughty during dry spells.
Subsurface paleosols, with finer allophanic soil materials,
12.3 Soil-Landscape Relationships
191