Purple, pungent and pricey—and here growing in Napier's Esk Valley in all its splendour.
Purple, pungent and pricey—and here growing in Napier's Esk Valley in all its splendour.
Among the main attractions of New Zealand’s thermal centres are the distinctive deposits of silica known as sinter. These provide as much of a drawcard for tourists as do the effervescent hot springs and geysers that give them birth. They are remarkable, persistent, chemical sediments, deposited wherever hot water discharges at the surface. And the sinters come in an endless variety of colours and shapes. A hundred and fifty years ago the Pink and White Terraces with their innumerable warm water basins were a wonder of the world. Today a slick of delightfully tepid water floods the wide expanse of Waiotapu’s Primrose Terrace whose subtle pastel yellows provide a contrast to the bittersweet apricots lining Champagne Pool. A virginal white sinter provides a sublime backdrop for Pohutu Geyser at Whakarewarewa. And at Orakei Korako the fabled Golden Fleece is displayed above the slime‑ greens and rust-browns of Emerald and Rainbow Terraces. Irrespective of colour and shape all these encrustations are formed of silica. The admiring tourists may not appreciate is that these deposits are not inert, like so much poured concrete. They are a dynamic environment. Not only do they continue to grow and evolve as long as their hot spring parents discharge, but from the moment they are deposited they commence to undergo a series of profound changes. Surprisingly, these continue long after the parental springs have ceased to flow. In Coromandel and Northland, ancient sinters mark the sites of long dead hot springs. These deposits are harder, denser and less porous than those forming today. Some contain cavities lined with quartz crystals never seen among their younger relatives. Clearly changes have occurred after the sinters were deposited. Further, today’s hot springs contain microbes that thrive in the heated environment. Their bodies provide one of several substrates that the silica encrusts, with their fossilized remains giving clear evidence that hot springs were responsible for our ancient sinters. For a decade or more NASA has been bankrolling extensive studies of sinters. The hot spring environment is extreme when compared with most others on our planet. The life that flourishes in such extreme environments provides scientists with the best available model of what they might encounter on planets where conditions are literally unearthly. Consequently, microbe-associated sinters give some idea of the sort of evidence scientists need in their quest for ET. However, if hot spring deposits change their character with time then any fossil evidence that microbes were involved may be modified if not erased. The sinter deposits of northern New Zealand contain pollens, silicified plants and even ancient snails that show them to have been deposited tens of thousands and even millions of years ago. They testify to a long and protracted history of hot water discharge at the surface in this part of the North Island. As such they provide a key to understanding any changes that have occurred. In the past couple of decades Auckland University students have been major players in unravelling what they have discovered is the ever-changing Kiwi sinter story. The parental waters of our sinters are generated at depths of 5–7 km in the shattered basement rocks that make up New Zealand. At these depths and pressures the water becomes heated well above its surface boiling point of 100°C. When it reaches 175°C it can dissolve silica—and other constituents—from the surrounding rock. The product is a nearly neutral solution rich in alkalis such as sodium and potassium. Where this solution discharges on the surface a hot spring forms. As it cools, the dissolved silica is deposited. However, the deposition is not a simple process such as occurs when sugar or salt precipitate from a hot saturated solution on cooling and the exact mechanism is not fully understood. Nonetheless, the nature of the initial silica is well-documented. It is always a non-crystalline opal or hydrated silica, commonly referred to as opal-A. It deposits on all available surfaces, in and about hot springs. In the 19th century at Yellowstone National Park in the USA, bottles, coins and ornaments were placed in hot spring discharges so that they would become coated with opaline silica and provide tourist knickknacks. In New Zealand silica sinter encrusts manmade structures about pools, lines pipes in geothermal plant, and, of course, envelopes microbes living in the springs—killing them in the process. Both a silicified weta and a partly silicified mouse have been recovered from springs at Orakei Korako. The opal settles on the surfaces as microscopic spheres that accumulate in clumps and layers. In part, the structure of the deposit is determined by the flow rate of the discharge and the amount of silica being deposited. For example, at Primrose Terrace a slow steady flow over a wide area gives rapid cooling and causes the build-up of numerous thin layers. At the Wairakei power station a large volume of spent hot water is flushed through the discharge drain at high speed. Here, innumerable long waving filaments of silica coat microbe strands growing from the drain walls. The first recognition that sinters change with time came from US Geological Survey scientist Dr Donald White, working at Steamboat Springs in the USA in the 1950s. White’s long-time friend, Auckland’s Dr Pat Browne, long believed a similar story was waiting to be told of New Zealand sinters. In the 1990s one of his students, Rina Herdianita, confirmed this to be the case. She established the essential aspects of the process and, importantly, demonstrated that the changes are widespread and systematic. Although noncrystalline opal-A is the first-deposited silica, in sinters several thousand years old it has changed to a form known as opal-CT that contains traces of two types of crystalline silica: cristobalite and tridymite. In sinters older than 50,000 years, Rina Herdianita found all opal had long gone and that these sinters consist solely of fully crystalline quartz and an unusual crystalline silica called moganite that just happens to be very rare elsewhere on our planet. Work by other research students at Auckland verified Rina Herdianita’s results and New Zealand sinters are now the best-documented in the world. In particular, the researchers showed that along with the changes in mineralogy, the outward appearances of the opaline microspheres that make up the sinters alter, as do the physical and chemical properties of the sinters themselves. -A forms in moderately hot (60°C) to warm (30–45°C) thermal waters as distinct tiny glassy spheres, less than 0.05 µm in diameter. These coagulate together and grow in number and size to produce the tiny microspherical masses 2–10 µm across that encrust all available surfaces. (1 µm is 0.000001 m or one millionth of a metre.) A second generation of micro-spheres can grow atop the first, modifying the appearance of the original sinter and in-filling gaps. Elsewhere, when the rate of silica deposition is greater than the rate of decay, biological matter like branches, leaves, wetas and mice that fall into the springs become silicified. From the moment they form, the opal-A microspheres slowly but steadily change. Apart from some slight increase in size, initial changes are largely internal. However, some thousands of years down the track, and often well after the hot springs have stopped flowing, tiny thin platelets of silica begin to sprout from the surface of each sphere. These grow in size and number to eventually appear as interlocking packets of plates that coat the roughly spherical masses of silica. It is in these plate-encrusted spheres that small amounts of cristobalite and tridymite are found. Commonly the opal-CT microspheres are larger than the original opal-A spheres which they replace. Their odd appearance has led to them being christened lepispheres. Occasionally the transformation goes a little differently. In one ancient sinter found alongside Hossack Road west of Te Kopia (by the Paeroa Range, south of Rotorua) and studied by Greg Holland, the opal-CT spheres resemble balls of yarn, their platelets seemingly woven from loosely-packed threads. Some of these lepispheres have intergrown in a ying-yang arrangement. the original sinter remains hot after the spring has stopped flowing as, for example, where it is heated by steam, both Rod Martin and Bridget Lynn found the change from opal-A to opal-CT can occur within a few hundred years or less. This is happening presently in several of the steam-heated older sinter masses at Orakei Korako and Te Kopia. On the other hand, opal-A can persist and in a 40,000 year-old sinter just south of Lake Omapere in Northland, Dion Pasters found 99.9 per cent of the mound to still consist of opal-A. Two types of quartz crystals occur in those ancient sinters that are either in the process of changing or have totally changed to quartz. Typical examples include the 20,000,000 year-old Kuaotunu sinters of Coromandel studied by Zowie Newton, and a 100,000 year-old Atiamuri sinter. In places the quartz appears as elegant doubly-terminated interlocking microcrystals 5–20 µm long. Frequently these crystals are arranged in spherical clusters that mimic the original microspheres and their lepisphere successors but have now enlarged to 50 µm in diameter. Elsewhere, clusters of quartz crystals, 30–50 µm long, grow perpendicular to the walls of cavities. Moganite is intergrown with the quartz and is indistinguishable from it except by x-ray analysis. This same association was found by Wendy Hampton in Northland’s ancient Puhipuhi sinters, once mined for mercury, and tinted bright pink-red by the toxic metal. Although sinters formed by near-neutral alkali-rich waters dominate in New Zealand’s thermal areas, silica also deposits from waters that are highly acid as at Rotokawa, Tikitere, Wairakei’s Geyser Valley and Waimangu. Kristen Cook and Clem Teece found that opal-A again forms first but with time it too changes first to opal-CT and then quartz. Regardless of the nature of the parent water, all these changes in mineralogy and appearance arise when the silica of a sinter becomes mobilised within films of moisture that coat surfaces within a sinter mound. This process alters the sinter mass itself. The density increases, pores become in-filled, and the relatively soft spongy original sinter mass is replaced by a hard brittle compact rock. At Puhipuhi, Wendy Hampton found these changes to be something of a never-ending story. Not only were the quartz crystals in this ancient sinter chemically eroded, but Wendy Hampton found tiny opal-A microspheres perching atop the etched quartz crystal surfaces demonstrating that, despite appearances, the rock-hard silica of this sinter is remarkably mobile; it is dissolving and redepositing in the present day through the normal processes of weathering and in the total absence of any hot springs. As these different processes advance the original texture of the sinter alters. This can occur in varying ways and to varying degrees. Ultimately there is a blurring of any distinction that may once have existed between original textures considered to have resulted from silica deposition about microbes and those with no obvious microbial associations. In effect the sum of all changes in mineralogy and microsphere appearance results in the erasing of the original sinter textures. Until the details of these changes are fully understood, the usefulness of sinters to those seeking extraterrestrial life must remain equivocal. The situation is not helped through the role played by microbes in the original deposition of silica proving to be more passive than active. The microbes are little more than one of many available substrates on which the silica settles. No evidence has been found that they actively seek out silica in the way a snail does calcium. While a snail employs the selected elements to preserve its life, the silica that settles on a microbe kills and entombs it.
Auckland is a thirsty city. It has always been that way. Whether water is required for washing the car, watering the garden, taking a shower or just a making a cuppa, Auckland’s demand seems insatiable.
Far too many folk know I was once a geologist. For such folly, I am often quizzed about this or that rocky feature. Some striking roadside formations in Rodney County are a recent case in point. A certain editor must have had too much time on his hands some months back. He found himself on Rodney Road, a dead-ender running west towards Mount Tamahunga off the summit of the Leigh–Pakiri road, 100 km north of Auckland. The views from here are fabulous, taking in Whangarei Heads and the Hen and Chicken Islands to the north, Little and Great Barrier Islands and the Coromandel across the water to the east, and the islands and headlands of greater Auckland’s eastern reaches, with the Hunua Ranges beyond, to the south. A picture-postcard spread of Whangateau and Tawharanui Regional Park occupies the southern foreground. The white sands of Pakiri glisten below to the north. Near the end of the road, the editor reported finding, “large rounded rock masses...piled up in the paddocks and along the road side...shaped like squat bananas... formed in light coloured rock”. The largest he claimed to be over 3 m high. They were quite unlike the well-known spherical mega-concretions of Silverdale that resemble giant marbles, he declared. So what were they? My partner and I had planned a weekend away from the computers and the Leigh hinterland offered as good an escape as any. With the weather hot and sunny it was a grand occasion to go forth and examine the nature of the editor’s rocks. However, although 37 years chasing students around lower Northland had left me with a reasonable knowledge of its highways and byways, Rodney Road had passed me by. Confirming we had the right location was not straightforward, given that a crucial signpost had been vandalised in the way of today’s world. It was a typical Kiwi metal road, the sort that used to be described as Grade III in my youth, when I walked such stony trails. Some nice homes occupied the first kilometre or two, and we noted some interesting, partly grassed road cuttings containing large rock masses worthy of later inspection. We were perplexed when it seemed we may have missed our target rocks, but then we swung round a corner, drove up a slight incline and there they were—like mushrooms sprouting after summer rain. Over many years I had learned not to rush to judgement and to refrain from making geological identifications from a car window. We stopped. We got out. We browsed. We mused. We photographed. And then we had lunch. Some of the rocks were oval, others amygdaloidal, one or two crudely cylindrical—the editorial bananas. A few were spherical but most were discoid. Sizes varied. The most curious feature was that all were perched on edge in a most unnatural-looking manner. At the end of the road one shattered example lay inside a gateway. Its interior showed it to consist of very coarse, crudely layered, muddy sandstone containing numerous large pebbles. A second boulder, intact, was perched alongside on its narrow base, surrounded by freshly dug earth. Scars on its surface from steel hawsers and chains were clearly visible. Mystery solved. It had recently been erected. We were looking at standing stones, or menhirs; that is, blocks of stone removed from their natural location and erected on another spot. As we wandered back up the hill we examined others. Clearly, a local landowner, with a lot of time on his or her hands, had been indulging in a little free-stand landscaping. Over a dozen boulders had been heaved upright and each balanced in a relatively small hole, in a manner similar to that in which the European megalith builders of 3000 years ago stood their monuments. On one vacant lot for sale, a foreshortened avenue of stones extended either side of the gate. Finally, on the road out, our eyes now wide open, we noted two recently erected small stones near the front entrance of an up -market property. The source of the stones is the hills themselves. The parent rock of much of the area, including the main ridge and presumably Mt Tamahunga, is 20-million-year-old Waitemata sandstone, laid down in very thick, coarse beds. This rock is relatively impervious to water, but the beds have been broken into large blocky slabs by ancient movements of the earth’s crust. Water percolates down the fractures that separate the blocks and initiates weathering on and along the blocks’ surfaces. Gradually the blocks’ outer surfaces alter. Clay minerals form, their presence causing the outer layers of the blocks alternately to shrink and swell as the supply of ground water decreases and increases with seasonal rainfall. This slight movement is sufficient for the outer altered skin to crack and expose fresh rock beneath, allowing the alteration process to start anew. As the years go by, more and more altered layers develop about each block, like so many onion skins. The process is known as spheroidal weathering. Where erosion of a slope occurs, blocks may become exposed on the surface. The outer layers now slough off under the influence of sun and rain. The relatively unaltered inner rock remains. For obvious reasons these residual masses are known as core stones, and it is just such core stones, in their wide variety of shapes, that have provided an outlet for someone’s creative urges. Those wanting to see the various stages of core-stone production could do worse than visit Buckleton Beach, 20 km south on the northern side of Kawau Bay. At its south-western end, rocks in the intertidal zone and the cliffs consist of fractured, coarse Waitemata sandstone. Wave-cut sections through the blocks show all the stages of spheroidal weathering. Visitors to Rodney Road can inspect large, onion-beskinned blocks of the same coarse sandstone in roadside cuttings, but this isn’t an entirely safe place for recreational viewing. The road is narrow and local vehicles travel fast. Auckland history buffs may recall that in the 1840s and ’50s, buildings throughout the region made use of sandstone won from Mahurangi—the same sandstone of which Rodney’s menhirs consist. The chimney and pump house of the Kawau Island copper mine were constructed from this material, as were the window lintels and sills of St Andrew’s Church in Auckland’s Symonds Street. After a few months’ exposure to the elements, these structures began to degrade. The church’s lintels and sills required patching within a few years, then coating and, in some cases, finally replacing with concrete. A similar degradation awaits Rodney’s standing stones, but for the moment they provide a grand echo of the past—and leave me in profound admiration of those ancestors of mine who built the stone circles of Avebury and Stonehenge. Not only do the sarsen stones of these prehistoric structures consist of extremely hard quartz sandstone that has withstood the elements for millennia, but there was nary a JCB or D8 in sight to assist the ancient master builders transporting and manhandling them. Each stone mass was dragged into place and erected by hand—along with much sweat, blood and tears. The average weight of each Ave-bury sarsen is 40 t. Recently a 100 t monster has been located buried near the main circle. Readers who opt to inspect Rodney’s new menhirs may like to estimate their weight while they picnic and contemplate the magnificent views on offer.
Born in 1835 to poor parents in Scotland, Andrew Carnegie became the richest man of his age. His family migrated to Al‑legheny, Pennsylvania, in 1848, and he immediately started work, at age 13, in a textile mill, earning $1.20 a week.
Rotokawa, 10 km north-east of Taupo on the route to Broadlands, is among the hottest of New Zealand’s thermal areas. It is also one of the most dangerous. On still days poisonous gases collect in depressions, setting invisible traps for unwary birds and rabbits and any too-casual humans.
Travel down State Highway 5 and it’s hard to miss Tirau. The township’s big sheep and matching dog cause many a visitor to do a double take. Those tourists alighting for photo opportunities find a town bedecked with corrugated critters. For this Tirau cheerfully blames Nancy and John Drake. They started it all a decade ago. The Drakes were school teachers. During a spell in the Wairarapa, Nancy learned the art of spinning and became passionate about wool. Her knitwear clothed her family and she hankered after a shop in which to sell her excess product. When John abandoned teaching some years later, they packed a campervan and went in search of a shop site on the main North Island tourist routes. Towards the end of their protracted and somewhat frustrating quest they purchased an empty section in the middle of Tirau. Nancy was prepared to bring on a Skyline garage and start selling immediately. John, however, comes from the slow-but-steady school. He counselled building a full-sized shop in which she might sell a range of woollies and sallied forth to find the cheapest building they could afford that would be big enough. A barn was the answer. For the price of an ordinary house, they could get a kit-set barn complete with its own flat. However, they realised that few travellers would stop for a barn alone. They needed something that would cause potential customers to back up. Much brainstorming led to the idea of “a very big, eye-catching sheep—in white”. But how to make a barn look sheepish? John tried and rejected both sprayed concrete and polystyrene. Both produced results that more closely resembled a corrugated caterpillar than a sheep. Then one serendipitous day, a young architect suggested trying corrugated iron. He even steered John in the direction of a redundant manual iron roller. John was now set on a whole new tack. He built models from balsa and paper and finally figured how it might be done. However, when he sought council approval he found himself hamstrung by the lack of plans and specs. Fortunately the South Waikato District Council was keen to capture new businesses and their building inspector went out of his way to help. Among other things, he ruled that the head was merely a façade and there were no regulations covering façades—just so long as they didn’t fall down. And so in January 1994 the Drakes began construction, their trusty campervan doubling as builder’s wagon and sleeping quarters. The Big Sheep is essentially a kitset barn with a head grafted on to the front. No local builder would touch the project. Builders in Tirau are a prudent bunch, allergic to projects that lack plans or specs. Fortunately, local chippy Keith Carver was between jobs. Despite reservations, he offered to help out. And so it came to pass that Keith climbed ladders and hammered while John sawed and schemed. Building the head was somewhat hit-and-miss. Seven prefabricated barn quarter-arcs, arranged in a semicircle, provided the basic structure of the head. On them a 100 x 50 mm wooden framework was hung. John worried that the nails needed to restrain the sections of iron needed for the head could result in unsightly streaks of rust. Serendipity again kicked in. He discovered that roofs in Rotorua use corrugated aluminium and was able to scavenge what he needed from a local scrap merchant. Suddenly John’s dream was a reality, although for a short time Keith argued that the structure more closely resembled a baboon than a sheep. However, the day the nose was clipped in place Tirau suddenly took on a decidedly sheepish look. The total cost had been a little over $100,000. The sheep proved an instant attraction. People stopped. They looked. They came. They spent money. Nancy had insisted on opening her shop before the head was added. For a while she seldom made $300 a month. On one occasion she sold nothing for five days in succession. But the day the head was finished it all turned around. She found herself with a $1000-a-day business on her hands. The Drakes never looked back and nor has Tirau. With the sheep finished and a proven success, John advocated building more corrugated iron structures about town. The city fathers—and a few mothers—were reluctant. They had hoped to revitalise their township with gentile, olde-worlde antique shoppes. But one day Tirau needed new public toilets. The township asked to use a piece of John and Nancy’s property. The Drakes agreed and offered a peppercorn lease, provided the toilets were built in a style that would complement the sheep and incorporated an information centre. The upshot was Tirau’s dog, the construction of which became a voluntary community project. The dog was driven by Henry Clothier, one of the olde-worlde antique shop owners. He twisted the arm of his son, engineer Stephen, to co-ordinate the building. John produced plans for a dog of similar parentage to the sheep but engineer Stephen wanted something more refined. He enlisted his bull terrier as a model but regrettably this failed to provide the required sheep-doggy appearance. Modifications were called for, such as the droopy ear tips. It was duly completed in July 1998. The dog led to Stephen being asked to undertake other corrugated commissions. Gradually Tirau has been transformed to Corrugated City. Even the street lamps now bear the town’s logo of a cabbage tree, the ti, punched in corrugated metal. The effectiveness of John’s design is shown by Nancy’s wool shop never having to advertise. From day one journalists flocked to Tirau. The sheep and the dog, and now the mantis, moo-loo mouse and a bevy of other corrugated critters have become regular features of travel mags and TV programmes. Today the Drakes have retired to Milford, leaving the shop in the capable hands of daughter Sally. John retains one not-so-secret ambition. He would like to build a giant corrugated pink pig. He is looking for a sponsor and a community wanting a sure-fire tourist attraction.
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