Roger Hanson: The rise and fall of the Southern Alps

The Southern Alps are eroding away almost as quickly as the tectonic plates are pushing them up.
STEVE CULLEN/SUPPLIED

The Southern Alps are eroding away almost as quickly as the tectonic plates are pushing them up.

The Southern Alps are among the fastest rising mountains in the world. In his book "The Rise and Fall of the Southern Alps", geologist Glenn Coates explains why.

"The Pacific Plate is literally being rammed up and over the Australasian Plate along a huge crack in the crust of the Earth. At the same time, the pressure is squeezing the rim of the Pacific Plate upwards".

The huge crack in the crust is the Alpine Fault and the uplift rate of the Southern Alps is 10-20mm per year – that's more than 10kms in a million years. But, incredibly, the Southern Alps have been wearing down almost as fast as they have been rising.

If it wasn't for this erosion, the Southern Alps would be colossal, about six times their present height – that would make Mount Cook 2.5 times the height of Everest. Since their formation about half a million cubic kilometres of rocks have been eroded from the Southern Alps.

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The breakdown of rocks and minerals through contact with the atmosphere and water is called weathering; this can be chemical or mechanical.

Chemical weathering occurs through the reaction of rainwater and oxygen with certain minerals to form soft oxides such as clays.

However, chemical weathering is not the main mechanism for erosion in the Southern Alps for two reasons. Firstly, in the Southern Alps chemical weathering is very slow due to the low temperatures.

Secondly, one of the main constituents of the rocks in the Southern Alps is quartz. Quartz is particularly resistant to breakdown through oxidation.

Mechanical weathering is the main mechanism; in this, rainwater enters the rock interstices and during the frigid alpine nights, freezes. Water on freezing expands by 4 per cent and this expands the rock structure.

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During the warmer daytime temperatures the ice in the cracks melts, the rocks contract and over time the rock starts to fracture. Eventually, the rocks break apart and form loose scree.

Weathering is a powerful process - if the Southern Alps were not rising and only being weathered, then within half a million years the South Island would be as flat as a pancake.

Eventually the rocks are transported to the sea by, rock avalanches, glaciers and rivers – it is only a question of time. Coates uses the analogy, glaciers are like excavators and rivers are like dumper trucks.

There are dozens of glaciers in the Main Divide of the South Island and they never stop working – they grind their way through the mountains, breaking up the bedrock and carrying it away.

Rivers are less relentless but get busy after major rainfalls, a common occurrence in one of the wettest regions on Earth.

A spectacular means of breaking up rocks and moving them substantial distances is a rock avalanche. Anyone in New Zealand on 14 December 1991 must surely remember the announcement that Mount Cook had suddenly become 10 metres shorter after a huge avalanche shed 14 million cubic metres of rock and ice from its summit.

Forty six substantial rock avalanches have been identified in the ranges between Arthur's Pass and Mount Cook alone.

A good example of how rock is cycled by the Earth is provided by a type of rock found in the Southern Alps called schist. Millions of years ago granite sand was washed off mountains on Gondwana and deposited into the sea.

The sediments of sand were buried over a period of 200 million years on an ocean floor far away from New Zealand and under hundreds of metres of overburden were crushed into silt and cemented into a grey mass.

This collection of mudstone, sandstone and some less common rock types is called Torlesse rocks. They were subsequently rafted on a tectonic plate to form the bedrock of New Zealand.

Some was subducted deep into the Earth's mantle and under huge pressure and temperature became schist. Later tectonic upheavals pushed these rocks upwards, completing the cycle to form new mountains in New Zealand.

This rock is busy eroding now and in the distant future will once again be recycled and become part of a new mountain chain, an example of the ever changing face of New Zealand.

 - Stuff

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