Auckland's continental journey

By John Roughan

As Auckland merges to create a supercity, the Herald looks back at how Auckland has changed over the years. Click here to view the full series.

Lion Rock at Piha stands near the edge of a giant undersea volcano. Photo / Paul Estcourt
Lion Rock at Piha stands near the edge of a giant undersea volcano. Photo / Paul Estcourt

Auckland owes all its natural distinctions - its mild climate, volcanic landscape, scenic coastline - to its global position. And like all places on the planet it owes that position to the heat circulating deep in Earth's molten cauldron.

This year a geological research project found the oldest fossils yet discovered in the Auckland region, in rocks at Island Bay on the northeastern coast of Waiheke Island.

The microscopic fossils are 250 million years old.

When they were set in stone New Zealand was not where it is today. There was probably no land at all above the sea at this spot on the globe. The rock on which New Zealand lies was carried here like a raft on the Earth's moving crust.

The oldest rock in the Auckland region, greywacke that is visible today on Waiheke, Motutapu, Ponui and in the Hunua Ranges, was formed thousands of kilometres to the southwest, on the edge of the vast ancient continent geologists call Gondwanaland.

"Gondwana" was a
southern hemisphere land mass made up of the continents of Australia, Antarctica, Africa, South America and India. New Zealand was a small part of a continent, now drowned, that geologists call Zealandia.

Over millions of years the forces that form continents such as Gondwanaland also split them apart. The primary force is heat. When the solar system formed as a fiery ball of chemicals in space, orbiting a continuing nuclear explosion, the sun, the surface of the Earth eventually cooled and solidified to form a hard but brittle outer skin of rock called the lithosphere.

The lithosphere is about a hundred kilometres thick, and within it is embedded the Earth's crust. Below it is a pliant material called the mantle, and below that is the superhot liquid rock, mainly metallic, of the Earth's outer core.

The lithosphere is a very effective seal to the Earth's hot interior
but the seal is not perfect. Hot magma from the mantle is always finding its way up through cracks in the brittle lithosphere to set
within it or erupt on the planet surface and form crust.

As crust spreads it forms plates and where the edges of the plates meet, one of them is forced down, or "subducted". Water is squeezed from the descending crust and reacts with rock above to form hot magma
that can rise through cracks in the crust to erupt on the surface.

Magma eruptions in cracks under Gondwanaland began to break the supercontinent apart and large chunks were carried away on spreading crust. India and Madagascar were the first to go. Then Africa sheared off, creating the Atlantic Ocean. It was followed by South America.

Our slice of continental crust, "Zealandia" - 10 times larger than New Zealand is today - began to break away from the eastern margin of what remained of Gondwanaland (Australia-Antarctica) about 125 million
years ago. Zealandia completed its separation about 80 million years ago, opening the gap that became the Tasman Sea.

Continuing eruptions of fresh crust in the mid-Tasman Sea caused sea fl oor spreading and rifted the continent of Zealandia to the northeast. As it moved, it slowly subsided and most of Zealandia went below the sea.

The crust below Zealandia ceased to expand more than 50 million years ago, leaving a largely submarine land mass where New Zealand is now.

But Zealandia kept on slowly sinking until about 23 million years ago when the unpredictable geometry of heat currents within the Earth's mantle suddenly changed.

Mid-Tasman eruptions ceased and the plate on which Zealandia was riding was re-fused with the Australian plate. The pressure of the full Australian plate grinding against the Pacific plate has been occurring under New Zealand ever since.

Written with the assistance of Dr Hamish Campbell of the Crown Research Institute GNS Science.

- NZ Herald

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