For convenience of geological description the large South Auckland Land District may be considered as two regions: one composed of mainly sedimentary rocks on the western side; the other dominantly volcanic and comprising the Coromandel Peninsula, Kaimai Range, and plateau of the central volcanic area.

The Sedimentary Area

In the sedimentary area basement rocks of the New Zealand Geosyncline are exposed in many places or lie at fairly shallow depths. Towards the west, fossiliferous Triassic and Jurassic strata are warped into a major downfold — the Kawhia Syncline — which was developed during the Rangitata Orogeny (seediagram 11). Although partly hidden by younger rocks, this syncline can be traced from Port Waikato to the Awakino River and probably continues under the Tertiary rocks of Taranaki. The Triassic and Jurassic rocks of the syncline are largely fossiliferous and of the kind that were deposited on the continental shelf. They include the New Zealand type Jurassic section. The Triassic rocks (Balfour Series) comprise some 10,000 ft thickness of conglomerates, siltstones, and sandstones; the Lower Jurassic strata (Herangi Series) consist of marine siltstone and sandstone overlain by conglomerates which contain tree trunks and other plant fossils. The Herangi Series ranges from about 2,000–6,000 ft in thickness. The Middle and Upper Jurassic strata (Kawhia Series), which succeed it, total some 8,000 ft in thickness. The lowest beds are partly non-marine, with plant fossils; the upper are marine, with abundant fossils, including ammonites and belemnites. The southern shores of Kawhia Harbour afford a rich collecting ground for these and other Jurassic fossils. The uppermost Jurassic rocks (Oteke Series), totalling some 10,000 ft in thickness, comprise siltstones, sandstones, and conglomerates, with abundant plant remains in the upper part. These young Jurassic strata are best exposed at Port Waikato, where they contain excellent plant fossils and are in places crowded with belemnites.

Far to the east of the Kawhia Syncline poorly fossiliferous rocks, of a kind that were deposited in the deep water of the geosyncline, project in numerous outcrops between Tongariro National Park and the Firth of Thames, some being encircled by ignimbrite erupted from the volcanic region. Among the largest areas of these rocks are the Hauhungaroa and Rangitoto Ranges (diagram 11). The Waikato basin (Hamilton lowlands) is almost enclosed by greywacke hills, which the Waikato River cuts through at Karapiro hydro-electric station and in the Taupiri Gorge. The high Hakarimata and Taupiri greywacke ranges are continued to the north by lower Triassic and Jurassic greywacke ranges that separate the lowlands of Huntly and Mercer from the Hauraki Plains. They are succeeded by the greywackes of the Hunua Hills (some 2,000 ft high) which lie west of the Firth of Thames.

Thick Tertiary sediments may lie below Pleistocene alluvium in major structural depressions, such as the Hamilton lowlands and Hauraki Plains, that were formed by the movements of the Kaikoura orogeny. The alluvium of the Hamilton lowlands (seediagram 10) includes pumice terraces built in the late Pleistocene and Holocene when the Waikato River brought pumice from eruptions in the central volcanic region. The youngest terrace was formed after the Taupo eruptions of approximately 1,800 years ago. Fine pumice sand is quarried in many pits at Horotiu near Hamilton.

Erosion has removed the cover of Tertiary strata from much of the region; the deposits that survive, mainly in faulted depressions, in few areas average more than a thousand feet in thickness. In the northern part of the region upper Eocene coal measures, resting on deeply leached greywacke, form the base of the Tertiary succession. A few mines work a small area south-west of Ngaruawahia, but the largest mines and opencast pits are in patches of coal measures within an area of some 40 sq. miles near Huntly, and at Kimihia and Kopuku north-east of Huntly (seediagram 10). Where coal measures are absent in the northern part of the region Oligocene marine strata are the oldest Tertiary rocks; they are mainly sandstone and prominent limestone. In the south-west part of the land district, Eocene coal measures are absent and thick Oligocene limestone (in which occur many sinkholes and caves, including those at Waitomo) is the dominant formation. Succeeding this to the south are thick layers of alternating Miocene sandstone and mudstone.

Mention has already been made of the prominent extinct basaltic volcanoes of south-west Auckland. Two large forest-covered eroded volcanoes, Mount Karioi (2,840 ft) at Raglan Harbour, and Mount Pirongia (3,156 ft) are the largest in a chain of cones that extends in a straight line for approximately 35 miles. South of Auckland city there are a number of Pleistocene basalt volcanoes, of which one of the largest and best preserved is Pukekohe Hill, a gently sloping, basalt lava cone.

A major fault marked by a high and greatly eroded scarp separates the Firth of Thames and the Hauraki Plains from the Coromandel Peninsula and its continuation as the Kaimai Range: Miocene andesites and Pliocene rhyolites and ignimbrites make up most of this region. The Jurassic greywackes that form the foundation of the range are exposed only in the north, for example in the Moehau Peninsula.

Central Volcanic Area

The Volcanic Plateau which forms the eastern part of this land district is an irregular, elevated area of some 10,000 sq. miles of dominantly acid volcanic rocks. Here the Taupo Volcanic Zone, a narrow, north-east-trending axial belt within the plateau, contains active volcanoes, geysers, and boiling springs, making the region one of New Zealand's principal tourist and holiday centres. Except at the southern extremity of the Kaimai Range, no sharp structural boundary separates the western part of the volcanic plateau from the remainder of the land district; the margin is highly irregular where erosion has nibbled at the edge of ignimbrite sheets that flooded out from the interior of the plateau in Pliocene and Pleistocene times. To the east and south-east, however, the plateau ends abruptly at the foot of mountain ranges which are in a large part fault bounded.

Although an elevated region, it is by no means a simple plateau, but rather the warped, broken, and partly dissected remnants of a plateau coated with thick deposits of pumice ash. These remnants (for example, the Kaingaroa, Mamaku, and West Taupo plateaus) owe their flat surfaces to the presence of hard ignimbrite as horizontal or gently tilted sheets (seediagram 11). Ignimbrite is a broad term covering a range of acid volcanic rocks, some hard, some soft, that were poured out in huge eruptions in the Pliocene and Pleistocene to form extensive thick layers, some of which may contain tens of cubic miles of this rock. It is not clearly understood in what form ignimbrites were erupted — they may have come out as greatly foamed-up lava, or as white-hot particles and lumps carried along in “glowing avalanche” eruptions.

Gravity measurements in recent years have revealed much about the general geological structure of the Volcanic Plateau. Greywacke basement rocks are thought to lie below the volcanic rocks which are only a few hundreds of feet thick in the marginal parts of the plateau, but they are enormously thick (as much as 2 miles) in a belt of structural depressions that extends from Tongariro National Park to the Bay of Plenty. These depressions, filled with ignimbrite, pumice, lava, and old lake sediments, have been formed after the expulsion of huge quantities of molten rock. Large blocks of the outer part of the crust have foundered along faults, and volcanic debris has accumulated.

The Taupo Volcanic Zone, to which is confined the most active faulting and volcanic activity, extends for some 120 miles from Ruapehu to White Island. North-east-trending faults bound the Taupo zone in many places, and many others can be followed within the zone: many of the hot springs, rhyolite domes, and craters lie on or close to the faults. Ruapehu (9,175 ft), Ngauruhoe (7,515 ft), and Tongariro (6,517 ft) are large, active, andesite volcanoes at the southern end of the zone. Pihanga (4,352 ft) and Kakaramea (4,266 ft) are the largest of a group of extinct forest-covered volcanoes of basaltic and basaltic-andesitic composition that lie between Tongariro and Lake Taupo.

Lake Taupo occupies a huge depression formed partly by faulting, partly by eruptions. Thick pumice deposits surround the lake, and radiocarbon dating of charcoal fragments shows that there has been a succession of pumice eruptions from the eastern side of the lake during the past 10,000 years. Some of these eruptions were small bursts, others gigantic explosions that spread pumice over thousands of square miles. The latest and perhaps grandest occurred approximately 1,800 years ago and showered pumice as far as Gisborne. A prominent landmark near Taupo township is Mount Tauhara (3,566 ft), an extinct dacite volcano that stands boldly above the ignimbrite plateau.

The Taupo Volcanic Zone is broadest in its middle section, between the northern end of Lake Taupo and the Rotorua lakes area. Here is the greatest concentration of hot springs and the most diversified picturesque scenery of the volcanic plateau. Domes and flows of glassy rhyolite lava are clustered at each end of this section in two “volcanic centres” – the Maroa centre to the south, the Okataina centre to the north. Between these centres the zone has been arched and then broken by parallel faults into a series of tilted blocks and a central graben. A striking fault scarp, the Paeroa scarp, marks one of these faults.

The Maroa centre, north of Lake Taupo, is a group some 10 miles across of rhyolite domes and flows, erupted along faults. Perlitic rock is quarried from some of these lavas and processed locally to make “expanded perlite”. The Okataina centre is a cluster about 15 miles across of domes and flows, among which are picturesque lakes such as Rotoiti, Rotoehu, Okataina, and Tarawera. The largest of these rhyolite domes are Haroharo and Tarawera. The latter is an elongated group of three steep-sided, rhyolite domes that have grown above the remnants of an older dome. The great Tarawera eruption of 10 June 1886 threw out basalt from a chain of new craters along the mountain's summit, and a further 6 miles of craters were blown open at the foot of the mountain and along the Waimangu Valley. Lake Rotomahana occupies the largest of these craters. Lake Rotorua occupies a sunken caldera further to the west; lake beaches cut in the surrounding hills show that the surface was once some 300 ft higher. Ash beds erupted from the Okataina centre mantled the landscape to a depth of many feet, the youngest of these being the Kaharoa shower, which may have been blown out as little as 800 years ago, probably from the site of Mount Tarawera.

Beyond the Rotorua lakes the Taupo zone becomes narrower and enters the sediment-filled Whakatane graben (fault trench) which is continued as the submarine White Island trench. Whale Island is an extinct, eroded andesite cone about 5 miles from Whakatane; White Island, some 30 miles to sea, is an active andesite volcano.