Submitted by admin on April 22, 2009 - 21:21
Since the beginning of the Miocene, volcanoes have been active only in the southern half of the South Island and in the northern half of the North Island. Dunedin and neighbouring areas of east Otago were the sites of eruptions of basaltic and alkaline volcanic rocks from the Miocene until the late Pliocene; Dunedin stands upon the deeply eroded remnants of these volcanoes. Banks Peninsula has been formed by the eruption of a huge, twin basaltic shield volcano that has had a long history of eruptions. These volcanoes attained most of their size in the Pleistocene and erosion has since breached two craters and carved them into enormous erosion calderas which have been flooded to form Lyttelton and Akaroa Harbours. One of the latest centres of volcanic activity in the South Island was the Solander Islands in Foveaux Strait. These small islands, composed of hornblende-andesite lavas and agglomerates, are the wave-battered remnants of a large volcano, possibly of upper Pleistocene age: the nearest andesitic rocks of similar age are in Taranaki, more than 600 miles away.
As the map shows, considerably larger areas of young volcanic rocks are preserved in the North Island: the largest expanse is the central volcanic region between Ruapehu and the coast of the Bay of Plenty, where lie all New Zealand's active volcanoes and all the geysers and boiling springs.
This enormous accumulation of volcanic rocks, several thousands of cubic miles in volume, is a product of a very late phase of the North Island's volcanic activity — it began to form late in the Pliocene period. Activity began earlier in the volcanic regions to the north — in the Coromandel Peninsula, in North Auckland Peninsula, and in the islands to the east of it. The Coromandel Peninsula, Kaimai Range, and Great Barrier Island were the scene of eruptions of thick accumulations of andesite lava and agglomerate during the Miocene. Cracks in these rocks were later filled with quartz containing the valuable gold deposits of the Hauraki goldfield. The andesite lavas and agglomerates that now form the craggy hills of Whangarei Heads were erupted then also. Dacite lavas were erupted in eastern North Auckland in the late Miocene and early Pliocene. The early Pliocene saw the eruption also of a chain of rhyolitic volcanoes off the eastern coast of North Auckland. The Poor Knights, Great Mercury, Mokohinau, and Aldermen Islands are the remnants of them. On the eastern side of Coromandel Peninsula rhyolite lavas, tuff, and ignimbrite were erupted at this time.
The latest eruptions of the northern North Island have been mainly basalt. During the Pliocene thick sheets of this lava spread out over hundreds of square miles of western North Auckland south of Hokianga Harbour: erosion has since lowered the land around them so that they stand high as the Tutamoe and Waipoua plateaux. A chain of basalt volcanoes grew west of the Waikato basin in the Pleistocene. The most westerly is Mount Karioi, south of Raglan Harbour; Mount Pirongia, and two small volcanoes near Te Awamutu succeed it to the south-east. Mount Maungatautari, near Lake Karapiro, is an eroded andesite volcano of similar age. Basalt lava was erupted from many vents in North Auckland, Auckland City, and south Auckland in Pleistocene and later times: sheets of this lava built the Kerikeri plateau. The huge andesite cone, Mount Egmont, is one of a chain of volcanoes that grew in Taranaki in the Pleistocene.
The earliest of the long series of eruptions that have slowly built the central volcanic region of the North Island to its present plateaulike form began in the late Tertiary. The greatest eruptions there were in the late Pliocene and Pleistocene, when more than 3,000 cubic miles of ignimbrite, rhyolite lava, and pumice were poured out. The andesite volcanoes of Tongariro National Park grew during the Pleistocene. The central volcanic region continues north-eastwards to White Island, Kermadec Islands, Tonga, and Samoa.