Story: Geology – overview
Page 4 – Building a continent
Until about 300 million years ago, much of the rock that makes up New Zealand did not exist. Around this time, sediment from the Australian and Antarctic sections of Gondwana and its offshore islands began to accumulate in the ocean.
Greywacke forms the mountain ranges in both the North and South islands. This rather drab-looking rock consists of beds of muddy grey sandstone alternating with thinner layers of darker mudstone.
Greywacke occurs in other parts of the world. How it formed remained a matter of debate until the 1960s, when exploration of the deep ocean floors began. Large fans of sediment were discovered on the sea floor at the foot of valleys and canyons cut in the continental slopes. Sandy sediment dumped by rivers onto the continental shelves intermittently cascaded down the canyons as turbidity currents – soupy mixtures of sediment and water – spreading blankets of sand on the fans. During periods between the turbidity currents, thin layers of mud settled slowly out of the ocean and covered the sands.
Over 200 million years, tens of thousands of metres of these sediments built up off the edge of Gondwana. They were eventually buried, deformed and hardened to become the rocks known as the Torlesse greywackes. Today, Torlesse rocks make up more than half of the New Zealand landmass. They cover a vast area, extending from Otago to East Cape, and below the ocean across to the Chatham and Auckland Islands.
Source of the greywackes
The Torlesse greywackes, which are named after the Torlesse Range of inland Canterbury, contain large amounts of quartz and feldspar, the main minerals in granite. Detailed studies of the mineral grains suggest that much of the Torlesse greywacke is derived from granitic rocks in north-east Australia.
Western Arc and Murihiku rocks
While greywacke sediments were accumulating far offshore from the Gondwana supercontinent, sediments of a quite different type were being deposited in shallower coastal waters. Stretching for more than 1,000 kilometres along Gondwana’s eastern coastline was a chain of volcanic islands. For nearly 200 million years, ash from eruptions and sediment from their erosion built up on the sea floor. The layers hardened to form the Western Arc and Murihiku rocks. These rocks once formed a continuous band, but have been separated by later movement along the Alpine Fault. They are now found in Southland and from east Nelson to South Auckland.