Aotearoa; Journeys into the Long White Cloud; Part 2; Dynamic Earth; a land forged from fire and ice

The imagery conjured up by Tolkien’s map of Middle Earth depicts a tame and comfortable homeland of rolling English pasture and oak woodland, appropriately named The Shire, bounded by mystical extremities formed of wilder, primeval forest, fiery volcanic landscapes and mountains gripped in severe glaciation. It’s not hard to forgive Peter Jackson and the production team from the Lord of the Rings trilogy for the temptation of bringing this fictional world to reality with Aotearoa. The New Zealand landscape took centre stage in the cast list of these big screen films; the North Island Volcanic Province starring as the fiery slopes of Mount Doom, the Southern Alps as the Misty Mountains high and the arid grasslands of Canterbury High Country featuring as the wild Kingdom of Rohan.

But what of the story of the real-life Middle Earth. As with the story of it’s human occupation, New Zealand is a land of relative youth by geological standards. In the latter stages of the Mesozoic period (80 million years ago), with the breakup of the great southern sub-tropical continent of Gondwana in full swing, a fragment of continental crust jettisoned off on an eastward trajectory. This rifting, and the gradual wearing down of former mountainous continental terrain created a low lying, often submerged landscape with extensive coal measures and limestones being deposited in a shallow sea environment. This lonely fragment of the supercontinent gradually assumed it’s present day position in the Western Pacific (DOC, n.d.). Very recent research work by a collaborative international team of scientists led by New Zealand’s GNS Science has led to a remarkable advance in an otherwise long-accepted geological principle. This 5 million km2 piece of continental crust, 94% submerged but with the large islands of New Zealand and New Caledonia protruding above the waves at the present day should be recognised as Earth’s eighth continent; Zealandia. It is only through committed and extensive sea bed drilling and seismic surveys that this remarkable discovery has been made, and in a most convincing way that turns a the (mis-)concept of a continent being predominantly formed of present day land above sea level on it’s head. In the paper, Zealandia: Earth’s Hidden Continent, Mortimer et al. (2016) make the case for Zealandia as a continent through the relative elevation compared to the surrounding abyssal plains of the Pacific, the geological assemblage of it’s basement rocks, the structure and thickness of crust (being between 10 and 30 km and always thicker than the average for oceanic crust at 7 km) and its separation from the Australasian continent through the existence of the Cato Trough, whilst only 25 km across, being 3600 m deep and formed of oceanic crust (Mortimer et al., 2016).

So once thought of as an isolated fragment of the paleo-supercontinent Gondwana, but thrown in rather awkwardly to the Australasian or Oceania geographical regions consisting of such an inconvenient splattering of islands littering the South Pacific, New Zealand as part of the continent Zealandia suddenly finds itself even more critically connected to it’s biogeographical cousin New Caledonia out on the western extremity of the new continent.

After a relatively passive period lasting almost 60 million years, things suddenly came to life 24 million years ago on the eastern edge of Zealandia. Earth processes started to re-elevate the land surface and fabricate the modern day New Zealand archipelago. Renewed movement along the Pacific/Australian plate boundary created magma and the formation of the volcanic provinces forming Northland and the Coromandel peninsula. This volcanism continued with the volcanic hills of the Otago peninsula erupting up out of the Pacific plate at around 13 million years ago followed by the Banks Peninsula of Christchurch at 10 million years ago. In the present day, these processes are still active with the subduction zone focussing the activity under the Central North Island leading to the ongoing formation Volcanic Province of Taupo, Tongariro and Taranaki. Isolated hotspot shield volcanoes have even popped up in the Auckland region in very recent times (Auckland Museum, 2009). This is displayed most strikingly by the coarse and exposed basalt boulders, scant vegetation and poorly developed soils of Rangitoto Island. This perfectly shaped volcanic island erupted out of the Hauraki Gulf just 600 years ago and was observed by early Maori settlers from the adjacent Motutapu island (Jamieston, 2004).

Putting aside the scattered distribution of mountains of volcanic origin, the spine of New Zealand’s South Island, the Southern Alps/Kā Tiritiri-o-te-Moana are one of the youngest and most active Alpine mountain ranges in the world. Throughout the last 3 million years, compression, faulting and crumpling along a 500 km margin between the Pacific and Australian plates has created South Island’s Alpine fault and resulted in an average uplift rate of 7 mm per year (University of Otago, n.d.). If it were not for equally as enthusiastic weathering and erosive forces, the mountains would stand proud at 20,000 m above sea level (GNS Science, n.d.). The net result, a solid wall running parallel to the fault line and the general orientation of South Island, culminating at 3724 m atop Aoraki/Mt Cook; the Cloud Piercer. This is a land of active Earth; frequent Earthquakes and landslides regularly occur with most spectacular and catastrophic consequences.

A transect through these mountains from west to east reveals a remarkable geo-climatological phenomenon. Covering the short coastal plain and cloaking their western slopes is dense temperate rainforest. Moving up through impossibly steep slopes of greywacke rock, there is a rapid transition to the pristine white of the heavily glaciated basins, and faces of the Southern Alps. The moisture laden air of the Tasman Sea is slammed violently by the roaring forties against the 3000 m wall of the mountains dumping anywhere up to 10,000 mm of precipitation annually, firstly as rain but increasingly snow. The mountain landscape has, as a result, been pasted thick with glacial ice. To the east, the mountains rapidly give way to the undulating plains of Canterbury, gradually descending to the Pacific coast. The strong orographic forces create an incredibly pronounced rainfall gradient. At Mount Cook Village, just 40 km as the crow flies from the Tasman Sea, average annual rainfall can be just 2500 mm and on the dry, grassy plains of Canterbury further east, below 500 mm (NIWA, 2016).

The Southern Alps have been, and continue to be a landscape shaped by the current ice-age. Geomorphological evidence from the New Zealand landscape points to four main glacial periods through the Pleistocene epoch, matching the global trend dictated by the more widely documented evidence from the northern landmasses (McClintock, 1966). It was the activity during and following the most recent Otiran glaciation, culminating around 20,000 years ago at the Last Glacial Maximum, that put the finishing touches to the New Zealand landscape of today. Continued tectonic uplift, supplemented by the isostatic readjustment of the land surface elevation through the Holocene epoch of the last 10,000 years created the final outline of New Zealand, consisting of three main and many subsidiary islands. The isolated volcanic provinces and the great fold mountain chain of the Alpine fault bridged by lands of rolling hills and sedimentary basins. In some cases these sediments are a product of Holocene Earth surface processes themselves. The case example being the low-lying sedimentary plain upon which the City of Christchurch is built. This is the result of sediment from the Southern Alps having been deposited as a delta-like plain, building continuous land between the mountains and Banks Peninsula (formerly an island). However some of the rolling sedimentary hills of the north island take us right back to where it all began; the limestone exposures of Waitomo Caves for instance having been deposited in the shallow seas of the Zealandia continent 30 million years ago as it drifted eastward across the Pacific (Waitomo Caves, 2013).

Aotearoa; Journeys into the Long White Cloud continues with Part 3; Predator Free; the battle for New Zealand’s native Forest and Bird

References:

Auckland Museum, 2009, ‘Zealandia the ancient continent of New Zealand – Auckland Museum’ (online video), accessed 20 January 2018, <https://youtu.be/S_Ohu8KHkBs>.

DOC, n.d., ‘Gondwana’, Department of Conservation Te Papa Atawhai, accessed 7 February 2018, <http://www.doc.govt.nz/Documents/about-doc/concessions-and-permits/conservation-revealed/gondwana-lowres.pdf>.

GNS Science, n.d., ‘The Geology of New Zealand’, GNS Science Te Pū Ao, accessed 13 January 2018, <https://www.gns.cri.nz/Home/Our-Science/Earth-Science/Regional-Geology/The-Geology-of-New-Zealand>.

Jamieston, A., 2004, ‘Rangitoto, Island volcano in the city of sails’, New Zealand Geographic, accessed 20 January 2018, <https://www.nzgeo.com/stories/rangitoto/>.

McLintock, A.H., 1966, ‘Glaciation’, from An Encyclopaedia of New Zealand, edited by A.H. McLintock, originally published in 1966. TeAra – the Encyclopedia of New Zealand, accessed 11 February 2018, <https://teara.govt.nz/en/1966/geology-new-zealands-geological-history/page-11>.

Mortimer, N., Campbell H.J., Tulloch, A.J., King, P.R., Stagpole V.M., Wood, R.A., Rattenbury, M.S., Sutherland, R., Adams, C.J., Collot, J., Seton, M., 2016, ‘Zealandia, Earth’s Hidden Continent’, GSA Today, v. 27, pp. 27-35, DOI: 10.1130/GSATG321A.1.

Ref 4: University of Otago <http://www.otago.ac.nz/geology/research/structural-geology/alpine-fault/nz-tectonics.html>.

NIWA, 2016, ‘The Climate and Weather of Canterbury’, NIWA Taihoro Nukurangi, accessed 20 January 2018, <https://www.niwa.co.nz/static/web/canterbury_climatology_second_ed_niwa.pdf>.

University of Otago, n.d., ‘Tectonic setting of New Zealand: astride a plate boundary which includes the Alpine Fault’, University of Otago, Department of Geology Te Tari Tātai Arowhenua, accessed 20 January 2018, <http://www.otago.ac.nz/geology/research/structural-geology/alpine-fault/nz-tectonics.html>.

Waitomo Caves, 2013, ‘Waitomo Caves Geology’, Waitomo Caves, accessed 11 February 2018, <http://www.waitomo-caves.com/about-waitomo-caves/Waitomo-caves-geology/>.

One thought on “Aotearoa; Journeys into the Long White Cloud; Part 2; Dynamic Earth; a land forged from fire and ice

  1. Pingback: Aotearoa; Journeys into the Long White Cloud; Part 1; There and Back again…* – DARREN AXE IML FRGS

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