For almost all of the 85 million years after New Zealand separated from the Gondwana supercontinent, the land did not have mountains. The present ranges only emerged within the last five million years, so they are relatively young, and very dynamic.
New Zealand has outstanding examples of the various processes that form mountains.
Uplift: tectonic plates
New Zealand straddles the boundary between the Pacific and Australian tectonic plates. As they collide, the plates push up the land, forming mountains.
The Southern Alps are one of the most rapidly rising mountain ranges in the world. The total uplift in the Aoraki/Mt Cook region during the past two to three million years could be as much as 20,000 metres. Dislocation of river terraces across the Alpine Fault shows that over the past 10,000 years, this uplift has continued at an average of 10 millimetres or more a year. Surprisingly, despite the rapid rise, there have been no major earthquakes in the central Southern Alps over the past 150 years.
Uplift rates elsewhere are more modest – about 5 millimetres per year along the west side of the Southern Alps and in the Kaikōura Ranges.
As well as uplift, there has been major sideways movement along the plate boundary. Over 15–20 millions years, this action has moved rocks apart by 480 kilometres: separate mountains, in the north-west and south-west of the South Island, contain rocks that were once in the same place.
A hard rain
New Zealand’s heavy annual rainfall has helped shape the mountains. More than 10,000 millimetres fall on the western side of the Southern Alps, south of Hokitika. Much of the alps, Fiordland and the north-west ranges receive over 3,200 millimetres. In the North Island the highest annual rainfall occurs on Mt Taranaki (Mt Egmont), including a record 844 millimetres in 48 hours in 1967.
Countering uplift: rainfall and erosion
While plate movement is pushing the mountains up, high rainfall is wearing them down.
New Zealand’s mountains and the Chilean Andes are the only significant barrier to the moisture-laden westerly winds that circle the Southern Ocean. On reaching the mountains the air rises, dropping rain on the west coasts.
In the Southern Alps, rock is split by the stresses of rapid uplift, and temperatures often shift from freezing to thawing. Combined with high rainfall, this results in very rapid rates of weathering and erosion. The results of this vigorous erosion is apparent in the widespread cones of eroding scree and the expanses of boulder and gravel that make up the beds of the larger rivers.
Erosion has also produced the eastern expanse of the Canterbury Plains and the high coastal hills (up to 680 metres) of South Westland. Both areas have been formed by ice-age glaciers and rivers carrying eroded debris to the coast.
Rock and roll
In a dramatic scene of alpine erosion, a huge rock avalanche broke away from the High Peak of Aoraki/Mt Cook in December 1991. About 14 million cubic metres of rock and ice fell onto the surface of the Tasman Glacier, reducing the height of New Zealand’s highest mountain by 10 metres. It also damaged the mountain’s elegant profile.
Glaciers: carving valleys and lakes
For much of the past two million years, the rising Southern Alps and Fiordland mountains were buried under huge ice-age glaciers up to 1,000 metres thick. These glaciers filled valleys as far as 100 kilometres east of the main divide of the Southern Alps, and extended westward beyond today’s coastline.
The glaciers carved out the sheer-walled, U-shaped valleys and fiords of Fiordland, and the deep (up to 444 metres) southern lakes. In the Southern Alps the legacy of ice age glaciers remains in the deep, ice-straightened valleys, large lakes and vast deposits of rocks, debris and gravels.
The most common ‘parent rocks’ from which the mountains originate are a sandstone known as greywacke, though there are mountain regions of schist, granite and other hard rocks.
The Southern Alps are formed from 100–300 million-year-old sedimentary rocks. These are greywacke that contains narrow bands of siltstone in the east, grading into schist west of the main divide. Greywacke sandstones also provide the ‘parent rocks’ in the Kaikōura ranges and North Island axial ranges.
Nothing but grey
The mountain ranges that form the backbone of New Zealand are made of ancient greywacke, which is being rapidly uplifted and eroded. Boulders in many of the rivers are made almost entirely of this rock. If you identify a grey rock as greywacke, there is a good chance you will be correct. Much of the sand on the beaches is made of tiny fragments of greywacke – which is why most New Zealand beaches are grey.
Harder rocks: Fiordland and north-west Nelson
Fiordland’s hard, crystalline rocks (granite, gneiss and diorite) are far more resistant to erosion than the greywacke ranges. This has resulted in a sharper topography of sheer-walled valleys, fiords and chiselled peaks.
Granite and gneiss also feature in the ranges in north-west Nelson. Here you will find New Zealand’s most diverse topography of ridges and crests, and the only sizeable areas of alpine karst (limestone and marble) terrain.
In the North Island the distinctive shape of the volcanic mountains is a result of thick lava flowing out of vents and forming high cones.