John Roughan: We're a nation built on shifting rock

A road damaged in the Christchurch earthquake. Photo / Sarah Ivey.

When I come back I'm going to be a geologist. They understand this world. The rest of us think we live on solid rock, geologists know we don't.

They are going to explain the two events that will keep 2010 in New Zealand's memory: the Christchurch earthquake and the Pike River disaster.

The quake could take them a while, since they didn't know the fault existed, but Pike River is probably less of a surprise to them. They know the fault the mine went through. We hear the the coal seam was a fractured reservoir of carbon and gas. We will all learn a lot more geology next year.

I learned a little this year. When the Herald decided to publish a history of Auckland, we wanted to understand how its landscape came to be. The Auckland Museum put me in touch with Dr Hamish Campbell, of GNS Sciences.

In a fascinating hour he told me one thing that was not particularly relevant to Auckland's story. But it is so generally relevant to human existence that I have to pass it on.

And since I'm clearing the desk for the end of the year (no paper next Saturday) let me tell you now.

We all know a little about plate tectonics - that the earth's crust is the hard outer edge of giant convection currents of molten rock that rise from the planet's depths and move across the surface until one plate edge meets another plate edge, when the rock in one of them descends under the other, liquefying again as it heats.

But I'd always wondered why the flowing hard rock isn't flat? It's a fairly important question, because if the crust was flat the land we live on wouldn't exist.

Crust, explained Dr Campbell, comes up from the earth's depths in two forms, granite and basalt. They are the same stuff but different in density, "like milk and cream", he said.

Basalt is the milk. It's heavier and more viscous. It forms most of the ocean floor. Granite is like cream. It is lighter and lumpy. Granite is the continents. The Auckland Museum exhibits lumps of granite and basalt that you can lift. The weight difference is remarkable.

Milk and cream. The analogy helps many things fall into place. It explains why one plate descends under another, sending up volcanoes of basaltic rock as it heats.

It might also explain why New Zealand's plants and birds are unique and there were practically no animals here until humans brought them.

Back when much of the globe's continental cream had coagulated into a giant geologists call Gondwana, New Zealand's base rock was pushed up from the sea floor against Australia.

When a crack in the crust carried us away, there must have been more milk than cream in our make-up because we were stretched thin and probably went entirely below water.

We lost the snakes, crocodiles and many other creatures that survive on fellow travellers from Gondwana.

Milk and cream may also explain a kink in the plate edge under New Zealand that has produced the geology of Canterbury and the West Coast.

Undersea relief maps show New Zealand to be the highest point of a continent that runs from New Caledonia to sub-Antarctic islands. The continent has been broken in two by the boundary between the Australian and Pacific plates.

Off the east coast of the North Island the continent meets deep ocean, and there the Pacific plate is descending. But down south, the reverse is happening; the Pacific plate is bearing Canterbury, Otago, Southland and an undersea continental shelf many times larger. It meets deep ocean off Fiordland and down there, the Australian plate is descending

But in between, along the South Island's Great Alpine Fault, neither plate is descending. The rock is moving horizontally in different directions. Geologists measure the pressure this builds and had been expecting a major release.

They say Canterbury's magnitude 7 rupture on September 4 was too far from the Alpine fault to be the big one. But it was big enough to be probably the major geological event of our lifetime.

It will rank in the national memory with the Napier earthquake nearly 80 years ago and the Tarawera eruption 50 years before that. These intervals are nothing in geological time.

The science deals in time beyond imagination. Western minds with a historical bent can grasp the 2000 years of the Christian era. With an effort of imagination the mind can conceive maybe four times as many years BC.

That would take us back to the eruption of Mt Wellington, the second most recent volcano in Auckland. Nearly 10,000 years elapsed between it and Rangitoto 600 years ago.

Geology says 10,000 years was not an unusual interval between eruptions in the Auckland field. Have a happy Christmas.