Story: Geology – overview
Page 8 – Quaternary mountains and glaciers
Forming the landscape
Immense changes which have occurred in the last 1.8 million years – the Quaternary period – have created the New Zealand landscape of today. The Southern Alps have risen thousands of metres, eruptions have created lofty volcanoes and buried large areas of the central North Island under rock, and huge glaciers have spread out from the mountains. During the Quaternary period, marine sediments continued to accumulate in coastal basins. Terrestrial rocks and sediments from this period cover the surface of much of New Zealand, and include coastal sand dunes, the sediment in river beds, and the scree on mountain slopes.
The Southern Alps
The uplifting of the Southern Alps has gradually accelerated, and today they are among the fastest-rising mountains in the world. Many of New Zealand’s mountain ranges have long straight fronts because blocks of bedrock are being pushed up along major faults. The highest rate of uplift is at the plate boundary, along the Alpine Fault. The land east of the fault is rising at average rates of 1–2 metres per century. The rock forming the summit of Aoraki (Mt Cook) was below sea level less than a million years ago. In other areas the rock is being bent, crumpled and squeezed up. Erosion has kept pace with uplift, however, so the mountains have rarely been much higher than they are now. Rivers, glaciers and gravity have, during the Quaternary period, carved out the entire landscape we see in the Southern Alps.
About 2.6 million years ago, a little before the start of the Quaternary period, earth plunged into cycles of repeated climate cooling known as ice ages. During glacial periods, average temperatures dropped by as much as 4.5°C, and lots of heavy snow fell on New Zealand’s high mountains. The steadily accumulating snow hardened into ice, forming huge glaciers that moved downhill into lower regions. At the height of glacial periods, glaciers blanketed the mountains from Fiordland to west Nelson, with smaller glaciers in the North Island’s Tararua and Ruahine ranges and on the central volcanoes.
Glaciers act as giant conveyor belts, moving rock debris from the mountains to lowland areas, and dumping it in great ridges, called moraines, along the flanks and front ends of the glaciers. When the glaciers later melted, these ridges were left, outlining the former extent of the ice. On the South Island’s West Coast, moraine ridges hundreds of metres high extend down to the coast and out under the sea. In the eastern South Island, remnants of moraines indicate that ice once reached the top of the Canterbury Plains. Rock debris carried by the glaciers was also flushed down rivers, filling river valleys with thick gravelly deposits.
During the ice ages, massive glaciers and ice caps formed and retreated many times worldwide. Few deposits of early glaciations survive in New Zealand – they were usually overrun and destroyed by glaciers during later advances. In addition, in the rapidly rising mountains, glacier debris tended to be quickly eroded away by rivers.
All of the ice-sculpted landforms of the Southern Alps are the product of advances and retreats of the ice in the South Island during the last 250,000 years. The most extensive moraines are from the most recent glaciation, the Ōtira Glaciation, which reached its maximum around 18,000 years ago. As ice has retreated, the depressions behind some of these moraines have filled with water, creating some of New Zealand’s most scenic lakes, such as Te Anau, Wakatipu, Tekapo and Pūkaki.