Geology of the Auckland region

The Auckland region of New Zealand is built on a basement of greywacke rocks that form many of the islands in the Hauraki Gulf, the Hunua Ranges, and land south of Port Waikato.

The Waitākere Ranges in the west are the remains of a large andesitic volcano, and Great Barrier Island was formed by the northern end of the Coromandel Volcanic Zone.

The Auckland isthmus and North Shore are composed of Waitemata sandstone and mudstone, and portions of the Northland Allochthon extend as far south as Albany.

Murihiku terrane greywacke lies beneath the Auckland region on the western side, and outcrops south of the Waikato River.

[1] 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.

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.

[4] 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.

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.

Shortly after (18 Ma), a volcanic arc developed further east to create the Coromandel Ranges and undersea Colville Ridge.

[9] At Maungatawhiri the initial eruption was of fine-grained volcanic tuffs, thought to be comminuted by water interacting with magma, followed by strombolian activity and lava flows.

North of Papanui Point violent phreatomagmatic eruptions have formed tuff rings of airfall and pyroclastic surge deposits.

Activity then moved to the Ngatutura volcanic field, south of Port Waikato in early Pleistocene times (1.8-1.5 Ma).

Activity then moved to the South Auckland volcanic field, in the area from Pukekawa north to Waiuku and Papakura, in mid Pleistocene times (1.5-0.5 Ma), producing over 100 eruptions.

The rich soils used for market gardening in this area are the product of the Hamilton Ash tephra formation, not the weathering of the South Auckland volcanic field.

It covers the area from Wiri in the south, through the Auckland isthmus, to Lake Pupuke and Rangitoto Island in the north, and contains around 50 vents.

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.