Story: Natural environment
Page 1 – Geography and geology
Remote in the Southern Seas
New Zealand lies about 1,600 km from both Australia and Polynesia. Its three main islands are the North and South islands and Stewart Island, or Rakiura, lying due south of the South Island. The South Island (150,437 sq km), often referred to as ‘the mainland’ by its inhabitants, is larger than the North Island (113,729 sq km). Stewart Island is 1,680 sq km.
The terms ‘deep south’ for Southland and ‘far north’ for Northland hint at the length of the main islands. The Māori names also signify the country’s long, narrow shape – the North Island is Te Ika a Māui (the fish of Māui, a legendary character), and the South is Te Waka o Māui (the canoe of Māui). The three main islands stretch 1,500 km across latitudes 34° to 47° south.
New Zealand is an archipelago with over 700 offshore islands. Most are small and lie within 50 km of the coast. The islands are the visible surface of an extensive submarine plateau, and enable the country to enjoy a huge exclusive economic zone (fishing grounds).
The Chatham Islands are 800 km east of the South Island. There are also several uninhabited island groups: the Kermadec Islands lie about 1,000 km north-east of Auckland; the Bounty, Snares, Antipodes, Auckland and Campbell islands lie south of the mainland.
The North Island is mainly rolling hill country, much of which is farmed. A series of narrow ranges (Tararua, Ruahine and Kaimanawa) form a roughly north-east belt of higher country that rises up to 1,700 m. Much of the surviving forest cover is found here and in other mountainous areas. In the central North Island, volcanoes that have been active over the past million years jut up thousands of metres near Lake Taupō. This is the country’s largest lake, formed by water filling a volcanic crater. Nearby, Rotorua’s mud pools boil and geysers erupt.
While the North and South islands are separated by only 20 km of water, they have quite different landscapes. Mountaineer T. H. Scott found this when he moved south. In 1950 he wrote:
‘I felt immediately and overwhelmingly that I had come to a quite different country …. Here then was a vast kind of land, whatever its size on the map, giving a sense of great distance where herds of grazing animals might roam, where grain plants might grow and a people wander. This had always been for me the meaning of continents.’ 1
The South Island is divided by the Southern Alps, which traverse most of its length and rise over 3,000 m. To the west of the alps lie rainforests. To the east are the farmlands of the Canterbury Plains, formed by rivers flowing from the mountains. In the south, a series of large lakes formed in depressions that were scoured out by huge glaciers.
Stewart Island is mainly low rolling hills. Unlike the two main islands, it remains almost entirely covered in native vegetation.
New Zealand is a sliver of the supercontinent Gondwana. The islands are only the visible part of a much larger submerged subcontinent that separated from Australia, on the eastern margin of Gondwana, around 85 million years ago.
Since the Cambrian geological period (over 500 million years ago), sea levels and the land have risen and fallen many times. Periods of mountain building have been followed by epochs when mountains eroded away. Huge volcanoes erupted and massive earthquakes and landslides have ravaged the earth. Great glaciers have lain over much of the land, melted away and then returned as the climate has repeatedly cooled and warmed over time spans hard to comprehend.
Earthquakes and volcanoes
When the climate warmed about 13,000 years ago, the glaciers retreated. In the Monowai Valley in Southland, a glacier that had been propping up the side of a valley melted. A nine-km stretch of mountain range collapsed, generating a massive landslide. Over time large landslide ponds, including Fiordland’s Green Lake, formed in hollows in the rumpled terrain of this landslide. When the next big earthquake occurs on the Alpine Fault in the Southern Alps there will be many catastrophic landslides, cutting road links. It is unlikely they will be as big as the Green Lake landslide, although geologists tell us that you never know.
After drifting to its current position over millions of years, New Zealand now sits on the boundary of the Indo-Australian and Pacific tectonic plates. Much of the history of mountain building, earthquakes and volcanic activity is due to its location on the Pacific ‘Ring of Fire’. The Southern Alps are the result of two massive plates meeting and sliding past each other along the Alpine Fault.
Māori oral traditions refer to earthquakes and tsunamis. In the mid-1400s a tsunami of over 10 m in height is thought to have swept over many Māori coastal settlements. Sometimes known as ‘the shaky isles’, New Zealand undergoes damaging earthquakes every few hundred years. The long gap since the last large earthquake suggests that a major quake of magnitude 8 or more on the Richter scale is overdue.