pumice-rich and dense, lithic- and crystal-rich zones that
underlie layer 2, which is dominated by pumiceous materials
consisting of coarse clasts (lapilli, blocks) and abundant fine
material (ash) (Fig. 12.5). In places, the flow tore up or
overturned parts of the underlying soil and incorporated
them into the ignimbrite as rip-up clasts.
The violent flow flattened and engulfed an entire forest,
turning it to charcoal and ingesting about 1 km
3 of timber in
total. Charred logs and charcoal are ubiquitous in the ignimbrite (Fig. 12.6). Until the charcoal-bearing unit was
identified as a pyroclastic flow deposit, it was a puzzle as to
how the deposit had remained so hot throughout its
160-km-wide distribution. Leslie Grange estimated, in 1931,
that the charcoal-bearing unit must have been at least 250 °
C. More recent estimates (from charring experiments and
thermal remnant magnetic work) indicate the ignimbrite’s
temperature was 400–500 °C at sites beyond 40 km from
vent and, surprisingly, cooler (150–300 °C) at sites within
30–40 km of the vent.
The extreme violence and fluidity of the Taupo pyroclastic flow caused the ignimbrite to be spread as an
essentially continuous, but thin, sheet over the entire landscape (its average thickness is about 1.5 m). Unlike more
conventional ignimbrites, the Taupo ignimbrite’s powerful
emplacement was not controlled by the existing topography,
or only barely. The Taupo ignimbrite covered a near-circular
area and reached about the same distance after crossing
several substantial mountain ridges as it did after travelling
in other sectors over nearly level ground. The thickness
varies according to the underlying topography, however, and
the resultant Pumice Soils also vary somewhat in their
properties (Sect. 12.3).
George Walker and Colin Wilson, who have studied
Taupo volcano and its products intensely, commented that
the widespread nature of the ignimbrite reflected an extremely powerful and dangerous event despite its thinness.
During emplacement, the flows infilled valleys as well as
surmounting slopes as steep as 30°, spilling over ridges into
neighbouring catchments, and swinging around valley
bends, as a speedway-car would take a speedway curve, and
splaying out to cover plains. Taupo ignimbrite is thus thicker
in valleys, forming deposits about 5–70 m deep, typically >5 m (known as valley-ponded deposits). As the flow
crossed topographic highs, it emplaced thin deposits 0.25–
8 m deep, typically 0.5–2 m (known as ignimbrite-veneer
deposits).
Unlike a number of older ignimbrites, the Taupo ignimbrite is everywhere loose and unconsolidated, a state
referred to as non-welded. In volcanology, the term ‘welded’
means transformed by the sintering together of hot pumiceous fragments and glass shards under a compactional load
to form hard rock, typically where the deposits exceed 550 °
C and are thick. Welded ignimbrites tend to have prominent
cliffs, often with vertical joints, such as that occur in the
Hinuera Valley. Originally the term ‘ignimbrite’ (meaning
‘fiery storm-clouds’), introduced in 1932, was associated
with hard (welded) rocks and it took many decades for unconsolidated pyroclastic flow deposits to be recognised also
Fig. 12.6 Carbonised logs and
charcoal fragments scattered
through pyroclastic flow deposit
in cutting in Palmer Mill Road
north of Wairakei. Cutting tool
30 cm long
12.2 Soil Profile Genesis
185
underlie layer 2, which is dominated by pumiceous materials
consisting of coarse clasts (lapilli, blocks) and abundant fine
material (ash) (Fig. 12.5). In places, the flow tore up or
overturned parts of the underlying soil and incorporated
them into the ignimbrite as rip-up clasts.
The violent flow flattened and engulfed an entire forest,
turning it to charcoal and ingesting about 1 km
3 of timber in
total. Charred logs and charcoal are ubiquitous in the ignimbrite (Fig. 12.6). Until the charcoal-bearing unit was
identified as a pyroclastic flow deposit, it was a puzzle as to
how the deposit had remained so hot throughout its
160-km-wide distribution. Leslie Grange estimated, in 1931,
that the charcoal-bearing unit must have been at least 250 °
C. More recent estimates (from charring experiments and
thermal remnant magnetic work) indicate the ignimbrite’s
temperature was 400–500 °C at sites beyond 40 km from
vent and, surprisingly, cooler (150–300 °C) at sites within
30–40 km of the vent.
The extreme violence and fluidity of the Taupo pyroclastic flow caused the ignimbrite to be spread as an
essentially continuous, but thin, sheet over the entire landscape (its average thickness is about 1.5 m). Unlike more
conventional ignimbrites, the Taupo ignimbrite’s powerful
emplacement was not controlled by the existing topography,
or only barely. The Taupo ignimbrite covered a near-circular
area and reached about the same distance after crossing
several substantial mountain ridges as it did after travelling
in other sectors over nearly level ground. The thickness
varies according to the underlying topography, however, and
the resultant Pumice Soils also vary somewhat in their
properties (Sect. 12.3).
George Walker and Colin Wilson, who have studied
Taupo volcano and its products intensely, commented that
the widespread nature of the ignimbrite reflected an extremely powerful and dangerous event despite its thinness.
During emplacement, the flows infilled valleys as well as
surmounting slopes as steep as 30°, spilling over ridges into
neighbouring catchments, and swinging around valley
bends, as a speedway-car would take a speedway curve, and
splaying out to cover plains. Taupo ignimbrite is thus thicker
in valleys, forming deposits about 5–70 m deep, typically >5 m (known as valley-ponded deposits). As the flow
crossed topographic highs, it emplaced thin deposits 0.25–
8 m deep, typically 0.5–2 m (known as ignimbrite-veneer
deposits).
Unlike a number of older ignimbrites, the Taupo ignimbrite is everywhere loose and unconsolidated, a state
referred to as non-welded. In volcanology, the term ‘welded’
means transformed by the sintering together of hot pumiceous fragments and glass shards under a compactional load
to form hard rock, typically where the deposits exceed 550 °
C and are thick. Welded ignimbrites tend to have prominent
cliffs, often with vertical joints, such as that occur in the
Hinuera Valley. Originally the term ‘ignimbrite’ (meaning
‘fiery storm-clouds’), introduced in 1932, was associated
with hard (welded) rocks and it took many decades for unconsolidated pyroclastic flow deposits to be recognised also
Fig. 12.6 Carbonised logs and
charcoal fragments scattered
through pyroclastic flow deposit
in cutting in Palmer Mill Road
north of Wairakei. Cutting tool
30 cm long
12.2 Soil Profile Genesis
185