By Sam Bain
When geologists visit the Leapfrog offices they often end up chatting with our geo’s about the spectacular geology that can be found here in New Zealand. We can usually recommend some interesting outcrops to visit when travelling our country. Today I thought I would try to give an overview of this geology. Given the tectonic complexity of this area, it will be a very simplified description but hopefully it will provide a starting point for those who are interested. In the long term I hope to look in more detail at some specific sites.
New Zealand is a section of Zealandia, a much larger submerged continental landmass. Zealandia extends a significant distance east into the Pacific Ocean and south towards Antarctica. It also extends towards Australia in the north-west. This submerged continent is dotted with topographic highs that sometimes form islands. Some of these, such as the main islands (North and South), Stewart Island, and the Chatham Islands, are settled. Other smaller islands are eco-sanctuaries with carefully controlled access.
The New Zealand landmass has been uplifted due to transpressional tectonics between the Australian and Pacific plates (these two plates are grinding together with one riding up and over the other).
To the east of the North Island the Pacific Plate is forced under the Australian Plate. The North Island of New Zealand has widespread back-arc volcanism as a result of this subduction. There are many large volcanoes with relatively frequent eruptions. There are also several very large calderas, with the most obvious forming Lake Taupo. Taupo has a history of incredibly powerful eruptions, with the Oruanui eruption approx. 26,500 years ago ejecting 1170 cubic km of material and causing the downward collapse of several hundred square km to form the lake. The last eruption occurred c.232CE and ejected at least 100 cubic km of material, and has been correlated with red skies seen at the time in China and Rome.
The subduction direction is reversed through the South Island, with the Australian Plate forced under the Pacific Plate. The transition between these two different styles of continental collision occurs through the top of the South Island. This area has significant uplift and many active faults. As you can imagine, large earthquakes are frequent occurrences here. The most powerful in recent history, the M8.3 Wairarapa earthquake, occurred in 1855. This earthquake generated more than 6m of vertical uplift in places, and caused a localised tsunami. Fortunately casualties were low due to the sparse settlement of the region. Recently, the area has been rattled by the M6.5 Seddon earthquake, but this caused little damage and no injuries. New Zealand’s capital city, Wellington, is situated bang in the middle of this region.
The subduction of the Australian Plate drives rapid uplift in the centre of the South Island (approx. 10mm per year). This uplift forms the spectacular Southern Alps. These roughly divide the island, with a narrow wet strip to the west and wide and dry plains to the east. A significant amount of the movement between the two plates is accommodated by lateral sliding of the Australian Plate north relative to the Pacific Plate. The plate boundary forms the nearly 800km long Alpine Fault. This fault has an estimated rupture reoccurrence interval of ~330 years, and last ruptured in 1717 along 400km of its length. Worryingly, it passes directly under many settlements on the West Coast of the South Island and shaking from a rupture would likely affect many cities and towns throughout the country.
The rapid uplift and high erosion rates within the Southern Alps combine to expose high grade greenschist to amphibolite facies rocks, including the gemstone ‘pounamu’ (jadeite). Geologists visiting the West Coast can easily access high-grade metamorphic rocks and mylonites associated with the Alpine Fault, and in certain places can stand astride the fault trace of an active plate boundary.
To the south of New Zealand the Australian Plate is subducting under the Pacific Plate, and this is beginning to result in back-arc volcanism. The youngest (geologically speaking) volcanism in the South Island occurred in this region, forming the Solander Islands (<2 million years old). This region is dominated by the rugged and relatively untouched Fiordland, an area of flooded glacially carved valleys with little human settlement. Fiordland is a New Zealand must-see, with many visitors heading to Milford Sound to take a day-trip aboard a boat through the fjords.
The Leapfrog offices are in Christchurch, on the east coast of the South Island. The city itself is mostly built on a plain of alternating gravel outwash from the Southern Alps and marine sediments from periodic marine transgression. Two large eroded volcanic cones form the Port Hills and Banks Peninsula to the immediate south of the city.
Since the end of 2010, several large (M7, M6.3, M6.4, M6.2) and shallow (all <7km) earthquakes have occurred immediately beneath Christchurch. These have resulted in 185 deaths, widespread destruction of buildings and significant liquefaction. These earthquakes are releasing distributed stress in the Pacific plate from the ongoing collision with the Australian plate to the west and north of the city. Leapfrog HQ has had to move numerous times due to earthquake damage. The earthquake sequence gradually tailed off, with very little activity occurring in the last 6 months. As shown below, the earthquakes have significantly altered the city, forcing the demolition of many commercial and residential buildings.
Whew, that is a lot of geology to cover in one post! The geological complexity that shapes New Zealand’s beautiful and varied scenery has the drawback of living with some significant geological risks. However, it also means that New Zealand geology students have easy access to great field examples for most topics. As I said earlier, I will look to talk about some specific areas in New Zealand in the future. If you would like to know about a certain area then let me know. There are a few geologists working here at Leapfrog and between us I am sure we can tell you something about it. Even better, come visit Leapfrog and then take some time to explore New Zealand!