12
Pumice Soils
Born of pyroclastic eruption
Pumice Soils host wood production
Pine roots reach to paleosols
nutrients and water are their goals.
Abstract
Pumice Soils cover about 16% of North Island, formed
mainly in unconsolidated rhyolitic (silica-rich) pumice
deposited by the Taupo eruption (AD 232 ± 10) from the
huge caldera now occupied by Lake Taupo, and the
Kaharoa eruption (AD 1314 ± 12) from Mt Tarawera.
Other areas of Pumice Soils are formed in 3500-year-old
Waimihia pumice deposits and on fluvially reworked
pumice alongside rivers. Pumice Soils comprise sandy to
gravelly pumice materials. They are weakly weathered
with a low clay content and have A, and weakly
developed Bw, horizons within pumice deposits that are
at least 25 cm thick. Pumice Soils have low reserves of
major nutrients and are likely to be deficient in trace
elements including cobalt, copper, molybdenum, boron,
iodine, and selenium. For agricultural development, it is
necessary to add fertiliser and trace elements, particularly
cobalt as its deficiency causes ‘bush sickness’ (vitamin
B 12 deficiency) which leads to death of sheep and cattle.
Pinus radiata trees have been successfully grown,
supporting a major plantation forestry industry. However,
in recent decades, many plantations have been converted
to dairying. Pumice Soils are readily eroded by water, and
if not protected by vegetation, severe gully erosion can
form in a single rainstorm.
12.1 Important Features of Pumice Soils
12.1.1 Concept and Key Features of the Soil
Order
Pumice Soils are formed in unconsolidated, weakly or
non-weathered coarse-textured rhyolitic pumice fragments
(clasts) deposited largely within the last 1800 years from the
Taupo and Kaharoa eruptions (Sect. 12.2). Consequently,
the soil properties relate strongly to those of the (geologically young) parent materials (Sect. 12.4). Pumice is a
low-density, glass-rich, vesicular (i.e. containing vesicles or
holes) material derived from pyroclastic eruptions of viscous
(‘sticky’) magma. The term ‘pyroclastic’ describes fragmental material (as opposed to lava) of all grain sizes
erupted explosively in a volcanic eruption. Rhyolitic pumice
is pale coloured and enriched in silica (70–78 wt% SiO 2 )
along with potassium, sodium, and other elements. The
vesicularity of pumice clasts means that soil horizons have a
low bulk density (usually <1 t m
−3 ), and pumiceous sand
grains, unlike equivalent grains of quartz or feldspar, contain
vesicles filled with air and/or water. Pumice clasts of the
Kaharoa deposits are harder, whiter, and less vesicular than
those of the Taupo deposits, which are typically pale cream
coloured. Taupo lapilli (clasts 2–64 mm in diameter) are
often crushable between thumb-nails, whereas Kaharoa
lapilli are not.
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
A. E. Hewitt et al., The Soils of Aotearoa New Zealand, World Soils Book Series,
https://doi.org/10.1007/978-3-030-64763-6_12
179
Pumice Soils
Born of pyroclastic eruption
Pumice Soils host wood production
Pine roots reach to paleosols
nutrients and water are their goals.
Abstract
Pumice Soils cover about 16% of North Island, formed
mainly in unconsolidated rhyolitic (silica-rich) pumice
deposited by the Taupo eruption (AD 232 ± 10) from the
huge caldera now occupied by Lake Taupo, and the
Kaharoa eruption (AD 1314 ± 12) from Mt Tarawera.
Other areas of Pumice Soils are formed in 3500-year-old
Waimihia pumice deposits and on fluvially reworked
pumice alongside rivers. Pumice Soils comprise sandy to
gravelly pumice materials. They are weakly weathered
with a low clay content and have A, and weakly
developed Bw, horizons within pumice deposits that are
at least 25 cm thick. Pumice Soils have low reserves of
major nutrients and are likely to be deficient in trace
elements including cobalt, copper, molybdenum, boron,
iodine, and selenium. For agricultural development, it is
necessary to add fertiliser and trace elements, particularly
cobalt as its deficiency causes ‘bush sickness’ (vitamin
B 12 deficiency) which leads to death of sheep and cattle.
Pinus radiata trees have been successfully grown,
supporting a major plantation forestry industry. However,
in recent decades, many plantations have been converted
to dairying. Pumice Soils are readily eroded by water, and
if not protected by vegetation, severe gully erosion can
form in a single rainstorm.
12.1 Important Features of Pumice Soils
12.1.1 Concept and Key Features of the Soil
Order
Pumice Soils are formed in unconsolidated, weakly or
non-weathered coarse-textured rhyolitic pumice fragments
(clasts) deposited largely within the last 1800 years from the
Taupo and Kaharoa eruptions (Sect. 12.2). Consequently,
the soil properties relate strongly to those of the (geologically young) parent materials (Sect. 12.4). Pumice is a
low-density, glass-rich, vesicular (i.e. containing vesicles or
holes) material derived from pyroclastic eruptions of viscous
(‘sticky’) magma. The term ‘pyroclastic’ describes fragmental material (as opposed to lava) of all grain sizes
erupted explosively in a volcanic eruption. Rhyolitic pumice
is pale coloured and enriched in silica (70–78 wt% SiO 2 )
along with potassium, sodium, and other elements. The
vesicularity of pumice clasts means that soil horizons have a
low bulk density (usually <1 t m
−3 ), and pumiceous sand
grains, unlike equivalent grains of quartz or feldspar, contain
vesicles filled with air and/or water. Pumice clasts of the
Kaharoa deposits are harder, whiter, and less vesicular than
those of the Taupo deposits, which are typically pale cream
coloured. Taupo lapilli (clasts 2–64 mm in diameter) are
often crushable between thumb-nails, whereas Kaharoa
lapilli are not.
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
A. E. Hewitt et al., The Soils of Aotearoa New Zealand, World Soils Book Series,
https://doi.org/10.1007/978-3-030-64763-6_12
179