with lake sediment, was erupted, depositing Rotomahana
Mud (Fig. 12.7).
The Kaharoa eruption was a complex, multi-episode
rhyolitic event comprising up to 13 separate and very
explosive (Plinian-style) eruption phases in two stages. The
early stage generated seven pumice lapilli fall units dispersed solely to the southeast of the volcano, and the later
stage produced six fall units that were dispersed almost
entirely to the north and northwest. The elongated NW–SE
shape of the Kaharoa isopachs (Fig. 12.4) thus reflects the
180°-change in wind direction during the eruption episode.
Within about 5 km of Tarawera, fine-textured ash beds occur
interbedded with the fallout deposits and represent deposition from pyroclastic flows. The explosive eruptive activity
during the Kaharoa eruption lasted about two weeks in total
(subsequent lava dome formation, and block-and-ash flows,
continued for another five years or so), with the total magma
volume being about 9 km
3 (equivalent to about 15 km
3 as
bulk pyroclastic material). Break-out flood deposits were
also formed during the Kaharoa eruption.
The Kaharoa tephra deposits are everywhere unconsolidated, and Pumice Soils, formed entirely on the fall beds,
occur in the central and eastern parts of the Bay of Plenty
where pumice layers are >25 cm thick. Many soils are
multi-layered because the Kaharoa tephra thins relatively
quickly away from the source and so the pumice layers are
only between about 25 and 75 cm thick in most locations.
12.2.5 The Development of Pumice Soils
The short durations of the explosive phases of the Taupo and
Kaharoa eruptions means that large volumes of pyroclastic
material were deposited rapidly over large swathes of land in
central and eastern North Island. Pre-existing soils were
buried in an instant, turning them into paleosols. The
emplacement of the Taupo ignimbrite within about 10 min
provided a pumice-rich parent material that is exactly the
same age wherever it occurs. It means that studies, such as
those looking at the effects of different land use and management, can be undertaken on Taupo soils with the surety
that the age of the parent material is not a compounding
factor. In addition, soil development under differing conditions can also be studied with the time factor constant. The
Pumice Soils on the Taupo and Kaharoa eruptives thus
provide an exceptional example of retardant upbuilding pedogenesis (Fig. 12.8). Soil formation in the newly buried
soil, instantly isolated from surface processes, was stopped
in its tracks, and pedogenesis was forced to start again from
time zero in the freshly arrived pumice at the land surface.
The pre-existing soils (now paleosols) were sterilised by a
combination of heating and anoxia. Where pumice deposits
are less than 1 m thick, the profiles contain identifiable older
tephra layers and buried paleosols that together form
multi-layered soils (Sect. 12.3, Fig. 12.10). Figure 12.8
shows developmental and retardant upbuilding pedogenesis,
Fig. 12.8 Developmental and retardant upbuilding pedogenesis, and a resultant multi-layered Buried-allophanic Orthic Pumice Soil on Kokako
Rd near Lichfield (cal BP = calendar years before 1950, ka = 1000 years ago) After McDaniel et al (2012)
12.2 Soil Profile Genesis
187
Mud (Fig. 12.7).
The Kaharoa eruption was a complex, multi-episode
rhyolitic event comprising up to 13 separate and very
explosive (Plinian-style) eruption phases in two stages. The
early stage generated seven pumice lapilli fall units dispersed solely to the southeast of the volcano, and the later
stage produced six fall units that were dispersed almost
entirely to the north and northwest. The elongated NW–SE
shape of the Kaharoa isopachs (Fig. 12.4) thus reflects the
180°-change in wind direction during the eruption episode.
Within about 5 km of Tarawera, fine-textured ash beds occur
interbedded with the fallout deposits and represent deposition from pyroclastic flows. The explosive eruptive activity
during the Kaharoa eruption lasted about two weeks in total
(subsequent lava dome formation, and block-and-ash flows,
continued for another five years or so), with the total magma
volume being about 9 km
3 (equivalent to about 15 km
3 as
bulk pyroclastic material). Break-out flood deposits were
also formed during the Kaharoa eruption.
The Kaharoa tephra deposits are everywhere unconsolidated, and Pumice Soils, formed entirely on the fall beds,
occur in the central and eastern parts of the Bay of Plenty
where pumice layers are >25 cm thick. Many soils are
multi-layered because the Kaharoa tephra thins relatively
quickly away from the source and so the pumice layers are
only between about 25 and 75 cm thick in most locations.
12.2.5 The Development of Pumice Soils
The short durations of the explosive phases of the Taupo and
Kaharoa eruptions means that large volumes of pyroclastic
material were deposited rapidly over large swathes of land in
central and eastern North Island. Pre-existing soils were
buried in an instant, turning them into paleosols. The
emplacement of the Taupo ignimbrite within about 10 min
provided a pumice-rich parent material that is exactly the
same age wherever it occurs. It means that studies, such as
those looking at the effects of different land use and management, can be undertaken on Taupo soils with the surety
that the age of the parent material is not a compounding
factor. In addition, soil development under differing conditions can also be studied with the time factor constant. The
Pumice Soils on the Taupo and Kaharoa eruptives thus
provide an exceptional example of retardant upbuilding pedogenesis (Fig. 12.8). Soil formation in the newly buried
soil, instantly isolated from surface processes, was stopped
in its tracks, and pedogenesis was forced to start again from
time zero in the freshly arrived pumice at the land surface.
The pre-existing soils (now paleosols) were sterilised by a
combination of heating and anoxia. Where pumice deposits
are less than 1 m thick, the profiles contain identifiable older
tephra layers and buried paleosols that together form
multi-layered soils (Sect. 12.3, Fig. 12.10). Figure 12.8
shows developmental and retardant upbuilding pedogenesis,
Fig. 12.8 Developmental and retardant upbuilding pedogenesis, and a resultant multi-layered Buried-allophanic Orthic Pumice Soil on Kokako
Rd near Lichfield (cal BP = calendar years before 1950, ka = 1000 years ago) After McDaniel et al (2012)
12.2 Soil Profile Genesis
187