John Roughan is an editorial writer and columnist for the New Zealand Herald.

Auckland: Fields of fire

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.

Aerial view of One Tree Hill. Photo / NZ Herald
Aerial view of One Tree Hill. Photo / NZ Herald

About 80km below the volcanoes of Auckland there is thought to be a "hot spot" in the Earth's crust - a zone where hot magma can rise closer to the surface an occasionally produce a small eruption - small by geological measurement, but devastating for land and life around it.

It seems to be a moving zone. Half a million years ago it was a little further south, leaving a similar field of about 80 small volcanoes, now largely eroded away, in the Franklin district.

Its eruptions started in Auckland perhaps a quarter of a million years ago. Until then the isthmus had been a lowland of rolling hills and valleys eroded out of the underlying sandstone.

None of the Auckland eruptions have been more than a fraction the size of Taupo. Together they have spewed out about 4 cu km of magma over a few hundred thousand years, compared to Taupo's 70 cu km, some of which flowed as far as Auckland.

Fully half of the lava from Auckland's volcanoes came from the most recent eruption, Rangitoto, just 600 years ago.

One of the first eruptions was on the site of the central city today. It erupted from the side of what is today Albert Park and its lava flowed down the slopes of Victoria St to Queen St and followed the valley floor to the sea.

Another early eruption created the crater in the Domain, and another blew on the North Shore, leaving Lake Pupuke.

Not all eruptions build lava cones; some leave wide craters surrounded by tuff rings that become lakes or, if the tuff ring is breached, fill with seawater.

The formation on the surface depends on whether the hot magma meets water on its journey up through the crust. If it meets ground water the eruption will be more violent, exploding from the surface and leaving a wide crater indented in the ground and throwing out ash which builds up around the crater as a tuff ring.

Those that did not meet water on the way up erupted as a "fire fountain" of lava that cooled over a smaller area to form a cone.

Two more volcanoes, also on the North Shore, were of the more violent kind. One left the Onepoto basin in Northcote and the other caused the "Tank Farm" crater that is today a mangrove lagoon alongside the northern motorway.

Around that time Mt Albert erupted on the isthmus and Puketutu Island near Manukau.

Auckland's most violent eruption, Three Kings, started 28,000 years ago. The initial explosion created a crater 1,200m in diameter, Auckland's largest. The highest part of its tuff ring forms Landscape Rd.

In subsequent eruptions at Three Kings, lava flowed from many vents and built up five corners, all but one of which have been quarried to oblivion, and only the highest survives disfigurement by a water tank on top.

After Mt Wellington, 10,000 years ago, the Auckland field was practically dormant until Rangitoto rose from the sea.

Rangitoto, 10 times bigger than any before - bigger than all the others put together - is the only volcano to have erupted from the sea bed.

Earlier eruptions, such as North Head and Puketutu that appear today to have risen from the sea may have occurred during a cold phase in the world's climate when sea levels dropped far below those of today.

These ice ages, when much of the world's water was trapped in northern hemisphere ice sheets, have occurred about 30 times in the past two million years, creating temperature cycles of 40,000 to 100,000 years.

The world has been in a warming phase for the past 18,000 years but the sea level did not return to its present level until 7200 years ago.

The climate cold and the sea level much lower when Mt St John erupted, and when Meola Reef was formed by lava, it would have been on dry land. The sea level was still well below its present level when Mt Eden and Mt Wellington were formed.

The Waitemata was a forested river valley and the Hauraki Gulf a region of hills.

The highest of them have just their hilltops above the sea today as islands.

- NZ Herald

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