Geography

Two million years ago a relentless and endless battle began between the Pacific crust and the Indian-Australian crust. The resulting rapid mountain building created one of the world's most spectacular alpine faults, the Southern Alps. The Pacific plate is literally being rammed up over the Australian 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 at an extreme rate of about 20mm per year.

However, the Southern Alps are eroding almost as fast as they have been rising. What remains of them today is tiny compared with the total amount of rock uplifted and eroded away. Around half a million cubic kilometres of rock is missing. If no erosion of the mountains had ever taken place, the Southern Alps would extend more than 20 kilometres into and beyond the stratosphere.

From 25 to 15 million years ago most of New Zealand was still covered by ocean. Areas of land projecting above the sea were mainly Torlesse rocks. As the plates began to collide, the New Zealand crust came under pressure and the Alpine Fault was formed. The Haast Schist were still well below the surface along the line of the fault.

Between 15 and 25 million years ago Gondwana rocks of the Australian Plate were carried north along the Alpine Fault and brought alongside Torlesse rocks. Chlorite-grade schist came to the surface. As the crust thickened under pressure, new areas of land were pushed above sea level and the Southern Alps were born.

From 5 million years ago to the present day the rate of uplift accelerated, pushing garnet-ogioclase schist above the surface. About 20km of uplift has taken place along the Alpine Fault.

Information and diagrams supplied courtesy of Glen Coates and Geoffrey Cox. For more information see 'The Rise and Fall of the Southern Alps', Canterbury University Press, (2002) or visit www.kahupublishing.co.nz