The Rangitoto Eruption: Birth of an Iconic Island

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Rangitoto Island seen from Auckland

Rangitoto Island dominates the skyline north and east of Auckland, New Zealand’s largest city. A mere seven hundred years ago, Rangitoto did not exist.

Scientists are unsure exactly when or over what timeframe the land that would become Rangitoto rose from the sea in spectacular fashion. They know it is easily the youngest of the 50 or so vents in Auckland’s volcanic zone, its formation a very recent occurrence in the geological timeframe. There is archaeological evidence that humans witnessed the cataclysm. So it had to be recent, given that Polynesian people have only settled Aotearoa/New Zealand within the last eight centuries.

The Land is Occupied, the Stage is Set

Sometime in the 15th Century: Europeans are beginning the great voyages that would lead to colonisation of the farthest corners of the earth, while China’s status as the most advanced civilisation is on the wane. In what would become the New World, North America is still the preserve of people who had roamed the land for aeons. They would not be calling it their sole domain for much longer. Down under in Australia, the story is the same.

To the east of the Australian continent, the Polynesian voyagers were finally in reach of what would later be branded the youngest country on earth. The Maori were just starting to settle their new land, just starting to establish a society that would become distinct from those of their tropical homelands.

The narrow isthmus that lies between two natural harbours and on which the city of Auckland would later be built is the creation of a volcanic field active for many thousands of years. Auckland is not on a tectonic plate boundary, and so the nature of this volcanic field is different from the Taupo Volcanic Zone to the south, where the release of the earth’s magma at the surface is the direct result of subduction. Under Auckland there’s a mantle ‘hot spot’ – a weakness in the earth’s structure that allows pressurised material to well up from deep below the crust. Other examples of such inter-plate hotspots are those beneath the Hawaiian island chain and Yellowstone National Park.

Witnessing the Rangitoto Eruption

Back in the 15th century, the people who called themselves Ngati Whatua were building their fledgeling communities on this fertile piece of land, with its equitable climate and easy access to the rich bounty of the sea. Securing the high ground in case of war, they lived and worked on the extinct volcanos that dotted the isthmus. Their view of the harbour and sea beyond was spectacular. In the distance lay the islands of the Hauraki, the nearest being the non-volcanic Motutapu, from where the Ngati Tai of Tainui could look back at the mainland.

These people could only have stood in awe when the earth began to tremor and the sea boiled, as its bed was suddenly rent. Superheated molten lava rose from 100km below to hit the water then break the surface in a massive series of explosions, as it burst forth into the air. The Maori populations were sparse, and there was of course no written language. It would be up to legend to give this new piece of land a very appropriate name – ‘Bloody Skies’.

A New Island Forms in a Flash

As the magma continued to flow and then cool, Rangitoto Island grew in stature, until it rose some 280 metres above sea level. Eventually, its top collapsed in on itself to form the distinctive rim that makes the island symmetrical, with virtually the same profile regardless of the direction of view.

For the Ngati Tai, the fishing grounds between their island and the sandy mainland beaches to the west had turned into a massive landform. Around 60% of all the lava ever released in the Auckland Volcanic Zone was deposited on the landscape in the creation of this brand new island.

A Prominent Part of the Auckland Landscape

Today Rangitoto Island is an Auckland icon that stands guard over the northern approaches to the city. While its neighbouring islands were all developed for pastoral farming, there was no such opportunity here on this mountain of jagged scoria. But despite its harsh terrain, Rangitoto soon became home to a variety of native vegetation. It is now all the more distinctive for the forest cover that, along with its geology, sets it apart from its neighbours.

Rangitoto is the youngest of Auckland’s family of volcanoes. They are known as monogenetic, meaning they normally only erupt once, and then become extinct. So Aucklanders go about their business without fear of an existing cone bursting back into life. They are reminded however, that there will surely be additions to this family sometime in the future.

Sources:

  1. Arc.govt.nz, Volcanoes of Auckland
  2. Gns.cri.nz, Auckland Volcanic Field
  3. Researchcommons.waikato.ac.nz, Role of Tephra in Dating Polynesian Settlement and Impact, New Zealand