Harrow and Hillingdon Geological Society

Previous Meetings

New Zealand – Geology of the South Island

Ruth Weinberg
Brent Geological Society

Known as the “Land of the Long White Cloud”, New Zealand lies midway between the equator and Antarctica. It is 1500km east of Australia and is composed of > 700 islands. In area it is 270x10³ km² and the three major islands are North, South and Stewart. The total population is about 4x10⁶ most of which are in Christchurch and Auckland and the capital is Wellington. About 75% of the people are on the North Island. In addition there are 40x10⁶ sheep, 60% of the country is >300m in height and 70% is hilly or steep.

The North Island is sub-tropical with rainforests, mangrove swamps and tidal estuaries. The Bay of Islands is an area of limestone, Rotorua is a region of high volcanic activity and Wai-O-Tapei is famous for its boiling mud.

On the south Island, the Southern Alps run for an unbroken 650km on the western side but not on the coast, with Mount Cook the highest at 3754m. There are numerous glaciers with the Tasman and Franz Joseph being one of the largest outside the polar regions.

New Zealand is associated with the Pacific plate fault system dating to 250 Ma. There are Gondwana sediments from Australia composed mainly of quartz and feldspar dating back to the Permian ice age of 10⁸ years ago. These sediments were deposited to form turbidites and the area is highly deformed, but there are no fossils. The youngest rocks occur on the Canterbury plain and date from the Jurassic to Cretaceous. They contain plant fossils and their sedimentation lasted about 200 Ma. A subduction trench formed, leading to volcanoes and more ash sediments. Arc rocks are the product of volcanoes. During the Triassic, 200 Ma ago, compression increased and sediments were pushed up giving rise to the Rangitata orogeny. This occurred in two phases at 100 and 40 Ma ago and mineral composition of the Harst Schist shows that the rocks came from different parts of Gondwana. The land was raised above sea level and the Eskhead melange which is limestone, basalt and mudstone were all squeezed together and concreted.

The Rangitata orogeny stores a magnetic anomaly. It is so rich in Mg, Fe, olivines and pyroxines together with Cr and Ni that a strong magnetic anomaly exists. The Ragitata orogeny ended about 105 Ma ago. A new trench opened to form the Tasman Sea and this lasted about 30 Ma. By the mid Cretaceous, the land was crossed by rivers and the soil was fertile. The fossil record showed plants and particularly ferns. Coal and petroleum were deposited. Only two dinosaurs have been discovered but bivalves, belemnites and ammonites have been found in the sea.

Following erosion of the mountains during the Tertiary 65 Ma ago, schists and softer sandstones, limestones and mudstones were formed. Pancake rocks (flat) are to the north west of South Island and were formed as the soft sediments were washed away leaving layers. By the mid-Tertiary, 35 Ma ago, only a small chain of islands was left but by 5 Ma ago, large areas of land had risen from the sea

About 25 Ma ago a tectonic collision occurred between the Pacific plate and the Australian plate giving rise to a sub-duction zone running from east to west. The Pacific plate went under the Australian plate forming a line of volcanoes. Australia is now continental plate and not oceanic and further out the Pacific becomes continental, however, in the south, the sub-duction zone occurs west to east on south Island. This process is still going on. The Alpine fault divides the new part from the old part on South Island, the rocks at the top of the mountains were below sea level only 1 Ma ago. These plates are still moving at 37mm per annum. Most of the uplift in New Zealand is on the Hope Fault. The Hanmer springs are thermal and compression and uplift occurs again at Kaikura orogeny north of Christchurch. The last earthquake occurred in 1717 at Millford Sound.

Erosion

Four major cycles have occurred over 250, 000 years of retreat and advancement and every feature of glaciation can be seen.

Weathering

14-12-91, Mount Cook was reduced in height by 10m due to rock fall which produced 14 x 10⁶ cubic metres of debris. Rainfall can also be very high, up to Im in 24hr. The sediments which contain many different minerals accumulate on the Canterbury Plains and then pass back into the sea.

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