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Geoff Chapple is no stranger to the coun­try’s topography. He spent more time than he probably cares to remember finding a route for what would become the 3000-kilometre-long Te Araroa hiking trail—a grand traverse of New Zealand that wends from Cape Reinga to Bluff through an interconnected series of shorter tracks and walkways.

Seven days into a trek across the Richmond Range in Marlborough, Chapple caught sight of spectacular rust-red hills “glowing like a sunrise”. Informed by a geolo­gist that they were “seabed rock that lay on the land like a wrecked ship”, he decided to find out more.

In fact, that Richmond encounter sparked a larger enterprise, of which Terrain is the tangible result. Subtitled ‘travels through a deep landscape’, the book is the record of what Chapple calls his “year of geology”— a 12-month dance with Te Araroa in the company of an articulate medley of ‘rock’ people, a jeweller here, a sculptor there, and almost everywhere the interpretive sonority of geologists.

Chapple’s earlier published guide to walking Te Araroa looked outward, at vegetation, land­scape, the built environment. Terrain largely directs its metaphoric gaze downward and back, picturing the work of geological agents over unimaginably vast stretches of time.

The narrative begins at Cape Reinga, on what Chapple calls “old oceanic floor”, and immediately geology is to the fore. A gravel path from the lighthouse is identified as red chert, trucked from a quarry on Karamuramu Island in the Hauraki Gulf. Flagstones are traced to a limestone quarry near Whangarei. Boulders in a rock wall are identified as gabbro, dolerite, and basalt, probably from Kaitaia. The point being that geology is every­where. It is the stuff with which we shape our world.

But it is also a thing of unimaginable power, size and duration. Chapple’s bigger purpose is to peel back the veil of familiarity and, with the help of some of the country’s leading geologists, report what is really going on beneath our feet.

Two broad themes emerge:

Scientific theory is in a constant state of becoming, and we are living on the back of something that has a frightening capacity for destruction and a capricious disposition.

James Hector, the first director of the New Zealand Geological Survey, was blind to Northland’s allochthon—a vast, distantly formed intrusion of rock that began moving in 23 million years ago—because the evidence didn’t mesh with what he had been taught. An older stratum of rock appeared to sit above a younger one, but he simply left the offending layer out of his 1877 report and got on with the job of mapping coalfields. Northland’s low topography, vegetation and relentless weathering had made a mess of the clues and discouraged theoretical leaps. The allochthon was not conclusively acknowl­edged until the 1980s.

The conceptual struggle was reprised further south as the long-cherished notion of the New Zealand geosyncline came under threat from an upstart theory of terranes. A terrane is a fragment of crustal material that has broken from one tectonic plate and attached itself to another with an entirely different geological history. Again, the new understanding was slow to supplant the old, partly because, as Chapple says, terranes couldn’t just be talked into existence, they had to be painstakingly “proved” through fieldwork.

Science is also steadily recalibrating the ways in which we might meet our end. These include the Wellington Fault, a major rupture of which is estimated to occur roughly every thousand years so that, in the words of geologist John Begg, “one genera­tion in 20 has to pick up the pieces”. Less well known is the Plate Interface Fault, 25 kilometres under the capital, which appears to be locked—and accumulating strain. What happens if and when that coiled spring releases is anybody’s guess.

Then there are the volcanoes. Auckland sprawls above a 50-kilometre-wide hotspot roughly aligned with the city’s volcanic field. But the Taupo Volcanic Zone is in another league entirely. Mangakino darkened the skies and chilled the air for distant Homo erectus a million years ago. Okataina did the same for Homo sapiens and Homo nean­derthalensis 62,000 years ago. And British volcanologist George Walker calculated that Taupo’s biggest eruption, 25,500 years ago, pushed 530 cubic kilometres of magma into the sky.

With the current odds of a rhyolitic eruption in the Taupo zone in any one year at just 900 to one, a question arises: When do you give a warning? After an earth­quake swarm? A rise in lake level? And when do you give the all-clear to return?

“Whenever scientists talk about giving warnings about this or that, have at the back of your mind, what are they warning against,” geologist Colin Wilson tells Chapple. After the first Christchurch earthquake, people were told the largest aftershocks would be about a magnitude smaller. No one said: it will be channelled on to another fault that will rip Christchurch apart with one of the highest peak ground accelerations ever recorded.

“That is what’s missing from the whole hazards and warnings thing. A real appreciation of the cussedness of Mother Nature,” says Wilson.

Terrain may lack an index, take anec­dotal diversions that are not always relevant and, unavoidably, deliver explanations that require concentration, but Chapple is not short on arresting ideas.

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