Posted on Tuesday April 4, 2017
Mountain ranges around Wellington are the wrinkles created when the Pacific Plate bulldozes its way under the Australian Plate, with the marine sediments it is carrying scraped off and piled up to create more New Zealand. But the process is messy, with the crust cracking and sliding and heating up in patches. And once piled up, they get weathered and eroded down again, and moved around. And you can see signs of all those processes in Wellington, a short walk from a bus.
From Wrights Hill we saw the Wellington fault trace, and the Rimutaka Range which have been created by uplift on the Wairarapa fault. In the harbour the fault line is a few hundred metres out to sea, but can be seen clearly in Upper Hutt (there is a labelled section of fault scarp in a park there). It then runs up Tinakori Road through Premier House, and then through Zealandia and into Cook Strait heading for Seddon. The bends in the fault are associated with intense shaking and lots of slips – e.g. at Maungaraki, Premier House.
We also saw patches of schist rock where the greywacke (marine sandstones brought in from deep water) had been altered by intense forces.
At Worser Bay we saw pebbles that have been brought from the Wairarapa Rivers by long-shore sediment movements. And maybe, or maybe not, bits from the Wahine, smoothed by the sea and dropped on the beach.
At Princess Bay we saw micro-fossils – the traces left by micro-organisms (foraminifera) that burrowed into the sediments before they became rock. These date from around 210m years ago. And we looked at how some of the rocks had been tilted and altered by pressure and heat (essentially natural fracking), resulting in a tortured form and veins of quartz.
From the bus we could see the bigger wrinkles known as the Kaikoura Ranges, and the sea doing its perpetual work of rearranging rocks. In the other direction is Baring Head – the flat top of which is only 100,000 years old, uplifted by earthquakes. The same surface is below Petone.
And at Owhiro Bay quarry we explored some glorious examples of fault lines – long cracks cutting across the layers of grey sandstone and darker siltstone, with the layers displaced by up to 5m, and curved at the fault itself where some rock was slower to move during the earthquake.
And throughout we were given stories about the rocks, the way the earth works, past earthquakes and the science. And there were lovely scones and muffins at Bel Mondo. So another perfect day out (and it didn’t even rain on us after all).