deposited in the sea with some of the sediment ultimately
forming the ‘papa’ mudstones that now make up much of the
land on the east coast of North Island and through the
Rangitikei region along with some parts of South Island. The
land continued to subside beneath the sea until by about 25
million years ago, during the Oligocene Period, much, but
not all, land was submerged and little further sediment
was produced. Shellfish and other calcareous organisms
accumulated in the shallow sea waters, and ultimately
become cemented into limestones such as those at Waitomo
in the King Country and in scattered locations throughout
New Zealand.
By about 20 million years ago the modern plate tectonic
setting was established and uplift and mountain building
began, driven by the impact of the Pacific Plate colliding
obliquely with the Australian Plate. A rapidly expanding
alpine zone began developing from about 5 million years
ago. The areas uplifted the most, such as the Southern Alps
in South Island and the Tararua, Ruahine, Kaweka, and
Kaimanawa ranges in North Island, typically had younger
overlying sediments eroded off, leaving the greywacke
exposed. The younger mudstones remain, and are particularly concentrated, on the east coast of the North Island,
pushed up as the oceanic lithosphere of the Pacific Plate
descends beneath the lighter continental crust of Zealandia at
the Hikurangi Trough to the east of North Island (which runs
northward into the Kermadec Tonga Trench, Fig. 1.1).
As the Pacific Plate descends into the Earth’s hot mantle,
the crustal rocks are heated and water and other volatiles are
boiled off. The effect of the water is to lower the melting
point of rocks in the solid mantle above the subducting plate,
allowing magma to form. The magma is mostly made up of
basalt which is relatively low in silica (SiO 2 ). The magma
evolves into andesite, with intermediate silica content, that
erupts to form cone or stratovolcanoes such as Mt Ruapehu
in central North Island. Heat from the basalt magma causes
the continental crust (mainly greywacke) to melt, leading to
huge rhyolite eruptions, with high silica content, that can
result in collapse of the land to form calderas such as those
occupied by lakes Taupo and Rotorua.
The recently active volcanoes, other volcanic landforms,
and associated geothermal activity, are concentrated in a
linear zone, stretching from Ruapehu in the south to White
Island (Whakaari) in the north, the Taupo Volcanic Zone
(TVZ). Magma erupted from Taranaki Maunga (Mt Taranaki) in the western North Island derives from a much
deeper source than that beneath the TVZ. Large volumes of
pyroclastic (fragmental) material or tephra (volcanic ash)
have been explosively erupted from volcanoes in the TVZ,
and from Taranaki, and deposited over the landscape of the
central North Island and beyond. Hence many soils in the
central North Island are formed from tephra deposits which,
because of their intermittent deposition, often comprise
multiple layers of tephra beds with buried soil horizons
(paleosols) within them. Both lavas and tephras have been
erupted from the most recently active intraplate basaltic
volcanism in the Auckland Volcanic Field, but they are more
localised in extent.
The Last Glaciation, which extended from about 115,000
to 11,700 years ago, with a glacial maximum between about
31,000 and 18,000 years ago, had marked impacts on New
Zealand. The Southern Alps were sufficiently high to allow
an extensive ice cap to develop on them and small glaciers
occurred on Taranaki, Tongariro and Ruapehu, and in the
Tararua Range. The average temperature in New Zealand
was about 6–6.5 °C colder, and it was about 25% drier,
frostier, and windier. Sea level was about 135 m lower than
that at present. The treeline was lowered by about 800 m,
and forest in most places (except Northland) was replaced by
grassland or shrublands, or both, except in small sheltered
areas (refugia) where patches of beech and/or conifer forest
remained.
The extended glaciers and the cold, drier, and windier,
climate meant that erosion increased in the high country and
mountains. The action of glacier ice, and freeze-thaw processes elsewhere, helped shatter rocks. The glacial retreat
commenced about 18,000 years ago with warming temperatures. Large, energetic, rivers were able to transport, and
break up, eroded rock materials, depositing the resulting
gravels, sands, and silts, on the lowlands to build up the
extensive plains such as those in Canterbury and Hawkes
Bay. Silt material (some formed by the grinding of rocks
carried in glaciers, but most derived from abrasion and
breakdown of rocks as they were transported by the vigorous
rivers) was blown from the wide riverbeds and deposited on
the surrounding landscape, at relatively slow rates, to form
deposits known as loess. The resulting aeolian (wind
deposited) loess mantles many of the terraces and rolling
hills of the lower North Island and the eastern South Island.
Soils were formed in the loess at the same time as it slowly
accumulated (i.e., loess deposition and soil formation
occurred concurrently).
From about 18,000 years ago, when the glaciation started
to transition towards the warmer Holocene period that began
11,700 years ago, sea level rose and many former river
valleys were drowned to form extensive harbours and fjords.
As climate warmed, forest vegetation was re-established,
initially in the north from 17,500 years ago and then towards
the south by the start of the Holocene, or soon after. The rate
of erosion, and reworking, declined in many regions as the
landscape was stabilised. However, ongoing volcanic
activity in the Taupo Volcanic Zone and at Taranaki, and
river floodplain deposition, has provided some new substrates for soil formation.
Thus our active geological history has led to the wide
range of geological materials that underpin the soils of New
4
1 Introduction
forming the ‘papa’ mudstones that now make up much of the
land on the east coast of North Island and through the
Rangitikei region along with some parts of South Island. The
land continued to subside beneath the sea until by about 25
million years ago, during the Oligocene Period, much, but
not all, land was submerged and little further sediment
was produced. Shellfish and other calcareous organisms
accumulated in the shallow sea waters, and ultimately
become cemented into limestones such as those at Waitomo
in the King Country and in scattered locations throughout
New Zealand.
By about 20 million years ago the modern plate tectonic
setting was established and uplift and mountain building
began, driven by the impact of the Pacific Plate colliding
obliquely with the Australian Plate. A rapidly expanding
alpine zone began developing from about 5 million years
ago. The areas uplifted the most, such as the Southern Alps
in South Island and the Tararua, Ruahine, Kaweka, and
Kaimanawa ranges in North Island, typically had younger
overlying sediments eroded off, leaving the greywacke
exposed. The younger mudstones remain, and are particularly concentrated, on the east coast of the North Island,
pushed up as the oceanic lithosphere of the Pacific Plate
descends beneath the lighter continental crust of Zealandia at
the Hikurangi Trough to the east of North Island (which runs
northward into the Kermadec Tonga Trench, Fig. 1.1).
As the Pacific Plate descends into the Earth’s hot mantle,
the crustal rocks are heated and water and other volatiles are
boiled off. The effect of the water is to lower the melting
point of rocks in the solid mantle above the subducting plate,
allowing magma to form. The magma is mostly made up of
basalt which is relatively low in silica (SiO 2 ). The magma
evolves into andesite, with intermediate silica content, that
erupts to form cone or stratovolcanoes such as Mt Ruapehu
in central North Island. Heat from the basalt magma causes
the continental crust (mainly greywacke) to melt, leading to
huge rhyolite eruptions, with high silica content, that can
result in collapse of the land to form calderas such as those
occupied by lakes Taupo and Rotorua.
The recently active volcanoes, other volcanic landforms,
and associated geothermal activity, are concentrated in a
linear zone, stretching from Ruapehu in the south to White
Island (Whakaari) in the north, the Taupo Volcanic Zone
(TVZ). Magma erupted from Taranaki Maunga (Mt Taranaki) in the western North Island derives from a much
deeper source than that beneath the TVZ. Large volumes of
pyroclastic (fragmental) material or tephra (volcanic ash)
have been explosively erupted from volcanoes in the TVZ,
and from Taranaki, and deposited over the landscape of the
central North Island and beyond. Hence many soils in the
central North Island are formed from tephra deposits which,
because of their intermittent deposition, often comprise
multiple layers of tephra beds with buried soil horizons
(paleosols) within them. Both lavas and tephras have been
erupted from the most recently active intraplate basaltic
volcanism in the Auckland Volcanic Field, but they are more
localised in extent.
The Last Glaciation, which extended from about 115,000
to 11,700 years ago, with a glacial maximum between about
31,000 and 18,000 years ago, had marked impacts on New
Zealand. The Southern Alps were sufficiently high to allow
an extensive ice cap to develop on them and small glaciers
occurred on Taranaki, Tongariro and Ruapehu, and in the
Tararua Range. The average temperature in New Zealand
was about 6–6.5 °C colder, and it was about 25% drier,
frostier, and windier. Sea level was about 135 m lower than
that at present. The treeline was lowered by about 800 m,
and forest in most places (except Northland) was replaced by
grassland or shrublands, or both, except in small sheltered
areas (refugia) where patches of beech and/or conifer forest
remained.
The extended glaciers and the cold, drier, and windier,
climate meant that erosion increased in the high country and
mountains. The action of glacier ice, and freeze-thaw processes elsewhere, helped shatter rocks. The glacial retreat
commenced about 18,000 years ago with warming temperatures. Large, energetic, rivers were able to transport, and
break up, eroded rock materials, depositing the resulting
gravels, sands, and silts, on the lowlands to build up the
extensive plains such as those in Canterbury and Hawkes
Bay. Silt material (some formed by the grinding of rocks
carried in glaciers, but most derived from abrasion and
breakdown of rocks as they were transported by the vigorous
rivers) was blown from the wide riverbeds and deposited on
the surrounding landscape, at relatively slow rates, to form
deposits known as loess. The resulting aeolian (wind
deposited) loess mantles many of the terraces and rolling
hills of the lower North Island and the eastern South Island.
Soils were formed in the loess at the same time as it slowly
accumulated (i.e., loess deposition and soil formation
occurred concurrently).
From about 18,000 years ago, when the glaciation started
to transition towards the warmer Holocene period that began
11,700 years ago, sea level rose and many former river
valleys were drowned to form extensive harbours and fjords.
As climate warmed, forest vegetation was re-established,
initially in the north from 17,500 years ago and then towards
the south by the start of the Holocene, or soon after. The rate
of erosion, and reworking, declined in many regions as the
landscape was stabilised. However, ongoing volcanic
activity in the Taupo Volcanic Zone and at Taranaki, and
river floodplain deposition, has provided some new substrates for soil formation.
Thus our active geological history has led to the wide
range of geological materials that underpin the soils of New
4
1 Introduction