Earlham College - Geology 211

It is nearly impossible to visit New Zealand and not become interested in geology. To learn how the beautiful landscapes were created is not to destroy a mystery, but to open a door to wonder and awe. We, as humans, tend to think of the world as solid, stable, and unchanging. Any change that happens occurs so slowly that we cannot see it with our naked eyes and thus forget about it easily. But New Zealand is one of those rare places where the geological processes are still so dynamic that one is constantly met with a reminder that our world changes and grows, just like we do.

Geographically, New Zealand is two main islands and several smaller islands scattered around which cover on area of 1600km from north to south. To give a frame of reference, New Zealand’s land mass is only slightly larger than Colorado. New Zealand has many rivers and lakes due to the large amounts of rainfall they receive. Currently, New Zealand can be found between 34 degrees South and 47 degrees South putting it right in the middle of the ‘Roaring Forties’ latitude and explaining the large amounts of wind New Zealand receives. The moisture-filled air usually comes from the Tasman Sea to the West and thus the west side of New Zealand receives more rain than the east side. However, the Southern Alps in the South island create more of a barrier and thus more of a rain shadow than the volcanic chain in the North Island. Lonely Planet: Tramping in New Zealand likes to remind its readers that New Zealand is a maritime climate, not a continental climate and as such the weather is subject to change with amazing rapidity – a fact to which any tramper or visitor to New Zealand can easily attest (DuFresne, 23)!

Geologically, New Zealand is amazingly active as it sits on two tectonic plates. The North Island and some parts of the South Island are located on the Australian Plate while the rest of the South Island is on the Pacific Plate. Currently, the Pacific plate is being subducted underneath the Australian Plate which, in the case of the North Island leads to many active volcanoes. The volatility of the North Island is also influenced by the fact that the crust is only 15-20km thick as opposed to the normal 35-45km (Ross, Rotorua). In the South Island, however, subduction is not possible because continental land mass is too light and buoyant to subduct. As a result, the two continental crusts push together and result in orogenesis which has created the Southern Alps which uplift as much as 10mm per year (Tourism NZ). In spite of the massive amounts of weathering inflicted on the Southern Alps by wind and rain, the mountains continue to grow faster than they can be eroded.

Although there is much controversy involved in the exact dates and order of New Zealand's geological formaion, it is accepted as fact that New Zealand was once part of Gondwanaland, a hypothetical super continent which was also composed of Australia, Antarctica, Africa, India, and South America. Gondwanaland began splitting apart approximately 160 million years ago during the Jurrasic Period and New Zealand broke off from Gondwanaland about 85 million years ago during the Cretaceous Period. The oldest rocks in New Zealand have been dated at approximately 500 million years old. There is evidence that, contrary to today's geological situation, from the late Cretaceous to the late Eocene New Zealand was situated on a passive boundary and had a very low altitude meaning that much of its land was covered in shallow sea basins. In the late Eocence, however, the faults formed and became active leading to the rapid crustal uplifting which began in the Pliocene and continues until today (Cooper, 293-294).

Cooper, Alan and Roger A. Cooper. "The Oligocene Bottleneck and New Zealand Biota: Genetic Record of a past Environmental Crisis". Proceedings: Biological Sciences, Vol. 261, No. 1362 (Sep. 22, 1995), 293-302. JSTOR. Accessed 2005 March 28.

Cotton, C. A. "The Structure and Later Geological History of New Zealand". New Zealand Geomorphology: Reprints of selected Papers 1912-1925. New York: Hafner Publishing Company, 1955.

DuFresne, Jim. Lonely Planet: Tramping in New Zealand. Victoria: Lonely Planet Publications Pty Ltd, 2002.

Marshak, Stephen. Essentials of Geology. New York: W.W. Norton and Company, 2004.

University of Otago. date unknown. "Tectonic setting of New Zealand: astride a plate boundary which includes the Alpine Fault". http://www.otago.ac.nz/geology/af/alpinefault.htm Accessed 2005 April 11.