The steep slopes of the Waitākere Ranges rise in the west, and the Hūnua Ranges in the south-east. Between them lie the low undulating slopes of the isthmus on which the central city lies, dotted with small volcanic cones. The lowlands are mostly old marine deposits of silt and sand.
Mountains and dunes
The Waitākere Ranges, which extend 30 kilometres north of Manukau Harbour, were formed by lava hurled from volcanic eruptions beneath the Tasman Sea 22 million years ago. The battering of the waves along the coast has carved out dramatic high cliffs, caves and blowholes.
North and south of the cliffs are dunes. They consist of pumice and ash washed down the Waikato River from the Volcanic Plateau, mixed with black Taranaki sand that has been driven north by coastal currents. The huge sandspits that nearly enclose the Manukau and Kaipara harbours reach their highest point (285 metres) on the Āwhitu peninsula.
The rugged Hūnua Ranges, lifted above the sea 145 million years ago and now 688 metres high, are composed of blocks of greywacke and argillite. Their steep valleys have been dammed to supply much of Auckland’s water.
Auckland stands on an ancient basement of greywacke that is visible at a few sites along the east coast and on several islands in the Hauraki Gulf. Exposed along the eastern sea cliffs are the ‘Waitematā strata’ that cover most of the Auckland region: layers of sandstone and mudstone, and ash from the central volcanic zone, compacted into sedimentary rock over the last 5 million years.
The next big thing
The first sign of a volcanic eruption in Auckland is likely to be several small earthquakes and land uplifting above the magma rising in the earth. A column of rocks and ash would erupt in a volcanic explosion as the magma mixed with underground water. Lava would then be thrown upward as a fountain of glowing fragments. Ash and gas would probably make the area uninhabitable.
Although this terrain is moderately fertile, it is hard to work, puggy in winter and almost rock-hard in summer. In wide areas north and west of Auckland, the soil has been podzolised – impoverished by the acid litter of generations of kauri and other trees.
The most distinctive landmarks on the Auckland skyline are its volcanic cones. There are 49 discrete volcanoes in the Auckland volcanic field. Radiometric dating has placed the earliest eruption at about 150,000 years ago and the most recent, at Rangitoto, at just 600 years ago. The force of the volcanic field also created explosion craters such as the Ōrākei and Panmure basins, which have filled with sea water, and Lake Pupuke (fresh water). Although many of the volcanoes have been decapitated or scarred by quarrying, about 30 have been well preserved.
In the south the older Franklin volcanic field (500,000 to 1.5 million years old) runs from Bombay to Pukekohe and across to Waiuku.
Scoria rocks thrown from Auckland volcanoes have been put to many uses. Early settlers skilfully stacked them into walls to surround their farms and homes – many still stand. Denser blocks of basalt were used for buildings. The grimmest of these was the Mt Eden Prison, built to look like a fortress and completed in 1917. Still used as a prison, it is a daunting sight.
From the time of Māori occupation, market gardening has flourished in the region. Rich, loamy soil formed from the volcanic ash and lava that was strewn over two-fifths of the Auckland isthmus, and the whole Bombay–Pukekohe area.