Geology of New Zealand

Exposed rocks at Franz Josef Glacier, New ZealandIf there was one place on the planet that you had to choose as a showcase for the wonders of the geological world, New Zealand would probably be it. There are volcanoes, glaciers, active fault lines, sheer granite cliffs, geysers, limestone country, precious metals, great uplifted mountains and myriad rock types – and that doesn’t even scratch the surface - no pun intended!

Geologically-speaking, New Zealand - as the spectacular country of snow-capped peaks and volcanoes that you’ve seen filling the cinema screen in movies like ‘The Lord of the Rings’ - was only really born about 5 million years ago. That’s very young on the geological timescale. Having said that, there was quite a long gestation and New Zealand’s oldest rocks, which can be found near Nelson at the top of the South Island, were formed during the Cambrian through to the Devonian periods, 540 to 360 million years ago. The oldest sedimentary rocks that contain fossils are found in the Cobb valley near Nelson. These rocks contain now-extinct animals called Trilobites, and are about 510 million years old – about one eighth the age of the oldest rocks on Earth found in countries like Australia, Canada or Greenland.

For a long time, several chunks of the Earth’s crust that now make up New Zealand were part of the super-continent of Gondwana (named after the Gond region in India). That massive landmass included what we know today as India, South America, Antarctica, Africa, Madagascar and Australia. We know that these now separate lands were once together because geologists have found the same fossil animals and plants in the rocks of those places. In fact, one of our ACTIVE NEW ZEALAND people, Brett Black (a former geologist), lays claim to finding one of only a handful of fossil leaves known as Glossopteris that tie New Zealand to the other continents once part of Gondwana. Also, rocks similar to those found in parts of Australia and Antarctica can be found on the West Coast of the South Island.

Layered sedimentary rocks, Punakaiki's famous Pancake RocksMuch of New Zealand, however, is made up of recycled rocks – kind of a geological second-hand store. When Gondwana was in its hey-day, between about 300 and 200 million years ago, there were a bunch of large rivers running to the ocean along the coast of what is now Antarctica and Australia. Those rivers carried enormous amounts of sediment out onto the ocean floor off the coast of Gondwana. That sediment was ‘squashed’ and heated up under some big mountains around 180 million years ago and became the metamorphic rocks that would later make up much of New Zealand’s South Island and some of the North Island.

Those mountains from 180 million years ago aren’t the ones you see in the photos you see on our website or in our free brochure though. They didn’t last because around 120 million years ago Gondwana began to break up, with New Zealand parting from Antarctica and Australia about 80 million years ago with the opening of what is now the Tasman Sea. As New Zealand parted ways with Gondwana it slowly began to sink into the ocean and until around 25 million years ago only a few scattered islands remained. All of New Zealand’s birds and reptiles – there weren’t any mammals (except for a couple of wayward bats who probably wished they’d stayed in Australia with all their mates) – were contained on that ‘Moa’s Ark’ of small islands.

Submerged New Zealand also provided a vast area of warm shallow sea that may have played an important habitat for the global evolution of whales, dolphins and penguins to take place. Nowadays, a vast number of these animals are found as fossils in limestone rocks around the country, including North Otago where people on our Weka cycling tour have the opportunity to ride amongst the intriguing formations of 25 million year-old limestone.

The tropical paradise didn’t last though. Around 20 million years ago the tectonic plates around New Zealand began to shift and a plate boundary broke its way through the middle of our submerged continent. The islands and surrounding submerged rocks started to rise from the ocean and form low-lying hills and lakes. For a time, there were some pretty substantial lakes and the sediments from those lakes are now found in Central Otago. The Otago Central Rail Trail, that people riding their bikes on our Weka cycling tour pass along, has many railway cuttings that expose those sediments. Amongst them geologists have found a wide variety of fossil tropical plants, birds and reptiles, including palm leaves, wading birds, coconuts and even a crocodile!

This warm lake-filled land persisted until about 5 million years ago when there was another shift in the way the tectonic plates were passing each other and they went from just sliding past, to pushing towards each other as well. Like a massive, super-slow car-crash, the Pacific Plate and Indo-Australian plates started colliding and several things happened to shape the New Zealand that we know and love today. In the North Island the Pacific Plate started sliding underneath the Indo-Australian Plate and this resulted in the many volcanoes, geysers and hot springs that you’ll explore on our Kauri trip.

Volcano on New Zealand's North IslandSome of the volcanoes in the North Island, like Mounts Ruapehu, Tongariro, Ngauruhoe, Taranaki and White Island are obvious because they have a pretty typical volcano shape, or some steam pouring out the top. However, some of the North Island’s more powerful volcanoes are more subtle in appearance. The best example of this is the Taupo Volcano. If you look at a map of New Zealand you’ll notice that the North Island has a large lake right in the middle – that’s Lake Taupo. The lake is actually a very large water-filled volcanic caldera. A caldera is a cauldron-like volcano, usually formed by the collapse of the ground following an eruption. They appear in many volcanic regions around the world; including Yellowstone National Park (you might recall the Yellowstone Caldera being shown in a glamourised eruption via some clever CGI footage in the movie ‘2012’).

The Taupo Caldera has been erupting for about 300,000 years, but made its mark, geologically, about 26,500 years ago when it produced the largest known volcanic eruption on Earth in the last 70,000 years! The volcano erupted over 430 km3 (100 miles3) of pyroclastics (hot bits of rock flying through the air). In some places around the North Island the deposits from that eruption are over 200m (650 feet) thick.

The South Island is very different from the North Island. For a start, there aren’t any active volcanoes. Here the two tectonic plates have gone partly head-to-head in a titanic struggle, with the Pacific Plate being pushed up to create the majestic Southern Alps. As the Earth’s climate has cooled in the last couple of million years the peaks of the Alps became snow-capped and glaciers formed in many of the valleys between the peaks. As the Southern Alps continue to push up they also bring up warm water from down in the Earth’s crust and that results in some wonderful natural hot springs where weary hikers can rest their bodies, such as the Copland Hot Pools that you’ll visit on our Winter Rimu trip.

Of course, to create big mountains (the non-volcanic kind) you need big fault lines, and the South Island has plenty of ‘em. Along the foot of the mountains on the West Coast is the tectonic plate boundary – it’s dominated by the Alpine Fault, which is New Zealand’s equivalent to the San Andreas Fault in California. The Alpine Fault forms the longest natural straight line anywhere on the planet - over 300 miles or 500 km long! Earthquake geologists think large earthquakes (about magnitude 8 or bigger) happen on the Alpine Fault about every 100 to 350 years, with the last large earthquake being about 295 years ago. The plates either side of the fault pass each other at a rate of up to 30 mm per year and the Pacific Plate (the Southern Alps) push upward at about 5 mm per year. So, if there’s a big earthquake on the Alpine Fault, say every 250 years, then the plates will move sideways about 8 metres (26 feet) and upwards about 1.2 metres (4 feet). That would be amongst the highest fault slip movements recorded around the world. It’s not all about the Alpine Fault though; other faults around New Zealand can produce large earthquakes too. This was tragically demonstrated by the series of earthquakes that Christchurch has experienced in recent times, including the devastating February 22 earthquake in 2011 that resulted in the loss of 181 lives and widespread damage within Christchurch city. 

So, New Zealand has a pretty interesting geological history, but lucky for us we happen to have stumbled across the place at a time when there’s a lot going on – and that means plenty of spectacular, weird and interesting places for you to visit when you come down to our part of the world. It looks just like Middle Earth, but with less Hobbits.