Geology of the Auckland Region

As with most of New Zealand, the basement rocks of the Auckland Region are composed of greywacke (indurated sandstone, siltstone and mudstone).

Murihiku Terrane Greywacke lies beneath the Auckland Region on the western side, and outcrops south of the Waikato River. The Murihiku Terrane is considered to be an accretionary wedge of mainly volcanogenic forearc sediments. It was formed in Late Triassic to Late Jurassic times (220-145 Ma).

A line of Dun Mountain-Maitai Terrane rocks are assumed to pass north–south through the centre of the Auckland Region, separating the Murihiku Terrane and Waipapa Composite Terrane. The existence of this terrane at depth is inferred from its distinctive magnetic anomaly (Junction Magnetic Anomaly or JMA) and xenoliths in the Auckland Volcanic Field.[1] Some Murihiku rocks occur to the east of the JMA, forming the Taupiri and Hakarimata Ranges, but it is assumed that these rocks have been shifted to their present position.

Hunua Terrane Greywacke lies beneath the Auckland Region on the eastern side, as far south as the Hunua Ranges, and outcrops in the Hunua Ranges, many islands in the Hauraki Gulf (Waiheke, Motutapu, Motuihe, Tiritiri Matangi, Kawau, etc.), and Tawharanui Peninsula. It sometimes includes Chert and Argillite. The rocks are generally fine grained and highly deformed. It was formed in Late Triassic to Late Jurassic times (220-145 Ma).

The Morrinsville Terrane Greywacke lies beneath the Auckland Region on the eastern side, south of the Hunua Ranges, and further east to the Coromandel Peninsula. The rocks are generally coarser grained, with massive sandstones and conglomerate. It was formed in Late Jurassic to Early Cretaceous times (160-120 Ma).

The Te Kuiti Group Rocks overlie the basement rocks, and are present in Northland, Auckland, the Waikato, and King Country, although they have often been eroded or covered. Rocks containing coal were formed from swampland in Late Eocene times (37-34 Ma). The land sank and the sea transgressed, and calcareous sandstone, mudstone, and limestone was deposited in Oligocene times (34-24 Ma).

The coal deposits outcrop near Huntly and Maramarua, where they are mined.

Flaggy limestone bluffs are located in the area south and west of the Waikato River.

In Early Miocene times (24-21 Ma), a series of thrust sheets were emplaced over Northland, extending as far south as the Kaipara Harbour and Albany areas. The rocks came from the northeast, and were emplaced in reverse order, but the right way up. The original rocks are of Cretaceous to Oligocene age (90-25 Ma), and include mudstones, limestone and basalt lava. These rocks outcrop around Silverdale, Warkworth, and Wellsford, reaching as far south as Albany.[2]

A volcanic arc became active to the west of the current land in Northland in Miocene times (23 Ma), and gradually moved south down to Taranaki. It produced mainly andesitic strato-volcanoes. Volcanoes were produced in what is now sea to the west of the Auckland Region, near the Kaipara Harbour and Waitākere Ranges (22-16 Ma), and Waikato Heads (14-8 Ma).

The Waitakere volcano has mainly been eroded, but conglomerate from the volcano formed the Waitākere Ranges. The Waitākere Ranges also contain many dikes, volcanic plugs, etc., that formed part of this volcano.

While the volcanic activity occurred to the west, a rapidly deepening basin developed further to the east, in the Auckland area.

These Early Miocene volcanoes, along with the Northland Allochthon, eroded and deposited most of the material that makes up the Waitemata sandstones and mudstones in the Waitemata Basin. The Waitemata sandstones and mudstones form the cliffs around the Waitematā Harbour and East Coast Bays, and land further north up to Cape Rodney, with outcrops further south down to Mercer and Miranda. Lahars produced the coarser Parnell Grit, found in many headlands around the East Coast Bays. Greywacke pebble beaches can be seen forming a layer in Waitemata sandstone cliffs on Motutapu Island.

Shortly after (18 Ma), a volcanic arc developed further east to create the Coromandel Ranges and undersea Colville Ridge. The initial activity was andesitic, but later became rhyolytic (12 Ma). In the Kauaeranga Valley, impressive volcanic plugs remain, as does a lava lake that now forms the top of Table Mountain. Active geothermal systems, similar to those that now exist near Rotorua, were present around 6 Ma, and produced the gold and silver deposits that were later mined in the Coromandel gold rush. Later (5-2 Ma), volcanic activity moved further south to form the Kaimai Ranges.

The Hauraki Plains and Hauraki Gulf are believed to represent a rift valley. The Coromandel Ranges used to be much closer to Auckland, and the rift valley is assumed to have developed about 2 Ma ago, due to the clockwise rotation of the eastern North Island, that stretched the land between Auckland and East Cape. The Waikato River used to flow from Karapiro, through the Hinuera Gap, and Hauraki Plains, to come out on the east coast in the Hauraki Gulf. The sediment from Oruanui eruption of Lake Taupo caused the river to change direction, and come out to the west. Rivers around Auckland, that now flow to the east, such as the rivers around Clevedon, used to flow to the west, and deposited rocks from the Coromandel Ranges in the Manukau Harbour.

The Auckland Volcanic Province comprises the Auckland, South Auckland, Ngatutura and Okete volcanic fields. Intra-plate basaltic volcanism in the Auckland region started in the south, at Okete, near Raglan in late Pliocene times (2.7-1.8 Ma), and has moved north through the Ngatutura, South Auckland and Auckland fields since then.

Over the last few million years, rhyolitic pumice deposits from the centre of the North Island have been washed down into the Auckland area, and form substantial deposits around the Hauraki Plains and Manukau Harbour, Western Waitematā Harbour, and even up into the Kaipara Harbour.

The Awhitu Peninsula (the west coast from Waikato Heads up to Manukau Heads), and the coastal barriers north and south of the entry to the Kaipara Harbour, are essentially consolidated sand dunes, built up over the last few million years.

The black sand on the west coast beaches is known for its high iron content, and is “mined” north of Waikato Heads, being extracted by electromagnets. The iron comes from the undersea volcanoes, to the west of the coast, and from the Tongariro Volcanic Centre, and Taranaki.

Quartz sand to the north of Auckland, around Tawharanui and Pakiri, appears white, due to its purity.

Rangitoto Island is a very recent volcano. Apart from the summit, it is composed of Pahoehoe (smooth) and ’A’a (broken) lava flows. A walk up to the scoria summit passes lava caves. The vegetation has pohutukawa forests, astelia growing on the lava, and a grove of kidney ferns.

The coast on Motutapu Island from Islington Bay to Administration Bay starts with Waitemata sandstone cliffs, and finishes with greywacke and chert near Administration Bay. Ancient greywacke pebble beaches can be seen embedded in the Waitemata sandstone cliffs. It is possible to walk from Rangitoto Island to Motutapu Island via a causeway.

Motuihe Island has a mixture of Waitemata sandstone, with Parnell Grit in the headlands, greywacke at the south, and even a coastal section with flaggy limestone on the west coast. It is also a bird sanctuary, with kiwi, saddleback, dotterel, oyster catcher, heron, and more common birds such as the fantail, tui, and pukeko.

Tiritiri Matangi Island is a rugged island composed of greywacke. It also a bird sanctuary, with rare birds, including saddleback, takahē, parakeet, North Island robin, kōkako, whitehead, little spotted kiwi, stitchbird and brown teal.

Waiheke Island is mostly composed of greywacke. At Stoney Batter, at the eastern end of the island are the remains of a volcano (7-8 Ma), with the remains of a lava flow in the form of large boulders.

Lake Pupuke on the North Shore is the crater lake of one of Auckland's earliest volcanoes. Water flows underground through cracks in the lava from the lake to Thorne Bay, where it is discharged by natural springs. At the northern end of Takapuna Beach are the remains of a lava flow, that flowed over a forest, preserving moulds of the tree stumps before incinerating them. Further north along the coast is a mould of a large Kauri tree.

Onepoto and Tuff Crater beside the North Shore motorway are examples of explosion craters, that have been breached and are now open to the sea. Onepoto basin has largely been reclaimed and now forms a park, while Tuff Crater hosts a tidal mangrove swamp.

Volcanic cones include Mount Eden, One Tree Hill, Mount Albert, Mount Roskill, Mount Hobson, Mount St John, Auckland Domain, Mount Mangere, North Head and Mount Wellington. Most have a large scoria cone and crater(s). They also often have the remains of a Māori pā (fortified village). Te Tokaroa Reef is a lava flow that flows 10 km from MT St John to the Waitematā Harbour, and almost managed to cross the harbour.

Many of the East Coast Bays beaches, such as Long Bay and Campbells Bay are good places to see the Waitemata sandstone and mudstone cliffs, with embedded Parnell Grit. Shakespear Park, at the end of Whangaparaoa Peninsula is also a good place to visit. Large blocks of basalt rock have been moved from the Waitākere Ranges to the coast just beyond Army Bay. Further north is Waiwera, with more Waitemata sandstone cliffs and Parnell Grit, and also commercial hot pools.

The Waitākere Ranges have streams which often flow down narrow gorges surrounded by conglomerate rocks formed from debris from the Waitakere volcano. The Piha Gorge, Pararaha Gorge and Karamatura Valley are examples.

Maori Bay, just south of Muriwai, has good examples of pillow lava. It is also the home of a gannet colony.

Te Henga Beach has large sand dunes behind it, and two lakes behind the same dunes. This area has been used to make several TV series, including “Xena: Warrior Princess”. On the way to the beach, it is worthwhile visiting the Auckland City Walk, to see the native bush, including large kauri trees.

Piha Beach is one of Auckland's surf beaches. The area to the south has blowholes, that the sea surges through. Whites Beach, to the north of Piha is also interesting. Further south is Mercer Bay, which has a headland, with an enormous hole that has eroded up a jointed lava dike. Still further south are Karekare Beach and Whatipu Beach, backed by steep cliffs. Sand dunes are accumulating along the shore at an amazing rate, and the beach has grown almost two kilometres wider in the last 100 years. Lakes are developing behind the dunes. All the beaches have black iron sand, that can be extracted by using a magnet.

Awhitu Peninsula, south of the Manukau Harbour, is essentially composed of consolidated sand, more than 100 m high. The cliffs are rapidly eroding, and preserved tree stumps and blocks of peat in the cliffs fall down to the beach.

The Hunua Ranges are an example of greywacke rock.

The route out to Waikato Heads gives examples of the volcanoes of the South Auckland volcanic field. Closer to the river mouth, the geology changes to Murihiku Terrane greywacke. A walk south from the beach gives good access to the greywacke. Travelling south from Waikato Heads on unsealed roads takes you through country roads, among cliffs composed of Te Kuiti sandstone and limestone, and finally out to the Waikato Plains at Te Kauwhata and State Highway 1.

• Geology of New Zealand

• Graham, Ian J. et al. A continent on the move : New Zealand geoscience into the 21st century - The Geological Society of New Zealand in association with GNS Science, 2008. ISBN 978-1-877480-00-3
• Lava and Strata: A guide to the volcanoes and rock formations of Auckland - Homer, Lloyd; Moore, Phil & Kermode, Les; Landscape Publications and the Institute of Geological and Nuclear Sciences, 2000. ISBN 0-908800-02-9.
• Vanishing volcanoes : a guide to the landforms and rock formations of Coromandel Peninsula - Homer, Lloyd; Moore, Phil; Landscape Publications and the Institute of Geological and Nuclear Sciences, 1992. ISBN 0-908800-01-0.
• City of volcanoes : a geology of Auckland, E.J. Searle, R.D. Mayhill, Longman Paul, 1981. ISBN 0-582-71784-1.
• Walks through Auckland's geological past : a guide to the geological formations of Rangitoto, Motutapu and Motuihe Islands, Peter F. Ballance and Ian E.M. Smith, Geological Society of New Zealand guidebook, no. 5.
• GNS Information on New Zealand Volcanoes, including the Auckland volcanic field
• Auckland Museum Information on New Zealand Volcanoes, including the Auckland volcanic field
• Auckland Regional Council Information on the Auckland volcanic field
• The age of volcanoes of the Auckland volcanic field, by Graham Leonard, Andy Calvert, Phil Shane, Jan Lindsay, Paul Augustinus, Ian Smith, Michael Rosenberg