The Hauraki Fault, which runs down the west side of the Coromandel Peninsula, is the axis of the Hauraki–Coromandel region. Movement on the faultline created both the mountains to its east and the basin to its west.
The oldest mountain rocks
The base rock of the Coromandel Peninsula is greywacke, a sedimentary rock deposited on the sea floor about 150 million years ago. Its later uplift and folding formed the Coromandel Range and the peninsula’s general shape. Greywacke is exposed only in parts between Tapu and Cape Colville; it is overlain elsewhere by more recent volcanic rocks.
Early volcanic rocks
Between 20 and 10 million years ago a string of large andesitic volcanoes dominated the peninsula. They erupted intermittently, spewing lava and scattering ash and debris. All that remains of them are the ‘plugs’ or molten magma at their centres, the surrounding rock long eroded away. Castle Rock (Motutere), south of Coromandel town, and Camel’s Back (Maumaupaki), inland from Tapu, are well-known examples.
Later volcanic rocks
A later volcanic period beginning about nine million years ago produced rhyolites in place of the older andesites. Instead of forming volcanic cones, rhyolites exploded from calderas – large, deep craters – and formed sheets of ignimbrite in places. Remnant spines and domes of calderas are found south of Whitianga, at Kapowai west of Tairua, and close to Waihī.
Low and flat
The gradient of the Hauraki Plains is slight, rising only about 3 metres over the 25 kilometres from the Firth of Thames to Paeroa. Flood protection of the surrounding countryside is a huge, ongoing task.
The Hauraki depression
The sea (the Firth of Thames) moved into the northern part of the depression on the western side of the peninsula, while the Waikato River flowed through the southern part (the Hauraki Plains) to the Firth of Thames. Vast quantities of river-transported muds, sands and gravels were deposited on the Hauraki Plains. About 20,000 years ago debris from a volcanic eruption at Rotorua blocked the Waikato River and re-directed it to its present course through Hamilton to the Tasman Sea. The Waihou and Piako rivers are responsible for the recent alluvial deposits on the Hauraki Plains.
Islands and estuaries
Coromandel’s coastal features were sculpted by changes in sea level over the last 70,000 years. When ice sheets expanded the sea fell, and when they contracted the sea rose. During ice ages streams cut deeply into the rocks to form steep-sided valleys. The rise in sea level since the last ice age, about 20,000 years ago, drowned many river systems, especially along the east coast: Whangamatā, Tairua and Whitianga. It also converted many coastal high-points into inshore islands, a particular scenic feature of the peninsula.