I am standing at the edge of a volcanic crater, just a careless step away from plunging fifty metres into a lake of incandescent orange lava that boils and hisses. The crater punctures the top of Pu`u 0`o, a cone that sits like a boil on the flanks of its great volcanic parent, Kilauea, on the Big Island of Hawaii.
From 1983 to 1986 Pint 0`o was the site of thundering lava fountains which rattled window panes 15km away in the appropriately named village of Volcano. Those and more recent eruptions fed lava flows which have engulfed over 40 square kilometres of the surrounding landscape.
Fall-out from the lava fountains built Pu`u 0`o up to a height of 255m, but at times it seemed as if the fiery convulsions would rip the mountain apart. Comparatively quiet now, the lava still sounds like muffled ocean waves crashing on to cliffs. Acidic yellow fumes spiral up from the lava lake, leaving sulphurous shadows on the blackened land that stretches away on all sides.
Watching this spectacle reminds me that this is how much of Auckland's landscape once appeared, for Pu`u 0`o is a living relative of the volcanoes which have shaped Auckland, and helps to tell the story of what those volcanoes were once like. It also serves as a warning of what will almost certainly happen again in New Zealand's largest city.
Auckland truly is a city of volcanoes. Climb any one of its volcanic peaks and you can see at least half a dozen more floating above the suburban sprawl like green bubbles. Scattered among these landmarks are many less conspicuous cones, and some volcanic sites which are not cones at all—a number of eruptions simply blasted a hole in the earth's surface, leaving a broad crater with its contents strewn in a thin mantle over the surrounding topography.
Auckland's volcanoes are all relatively small, but what they lack in size they make up for in number. There are no fewer than 48 within a 20km radius of central Auckland. They are spread from the dark waters of Lake Pupuke in the north to the quarried stump of McLaughlin's Mountain (Matukutureia) at Wiri in the south. On average, only 3km separates one volcano from the next, and together they comprise the Auckland Volcanic Field.
The largest of the volcanoes is the youngest. Rangitoto, its handsome symmetrical form now almost emblematic of Auckland, took its place as sentinel of Waitemata Harbour only 600 years ago. Its arrival was certain to have been witnessed by the Maori, whose footprints, hearths and food scraps are preserved under layers of Rangitoto ash on the neighbouring island of Motutapu.
Until recently, most geologists considered that outbursts from the Auckland Volcanic Field have occurred sporadically for the last 100,000 years. However, a new study at the University of Auckland, using the technique of thermoluminescence dating, has estimated that rock samples from the Pupuke volcano are up to 141,000 (+1- 10,000) years old.
Thermoluminescence dating is based on the fact that low levels of radioactivity which are naturally present in rocks cause electrons to move about, and eventually to become trapped in defects in mineral crystals within those rocks. When rock samples are heated, the trapped electrons are released, emitting a pulse of light. The intensity of the emission is a measure of how long the crystals have been accumulating electrons, and from this data the age of the rocks can be estimated.
Like all dating techniques, the method has inherent uncertainties, and geologists do not yet know how reliable it is for Auckland's volcanic rocks.
Most Aucklanders probably avoid wondering too much about the goings on in the infernal kitchen beneath their feet. Far better to worry about what's cooking for dinner than what's cooking, and perhaps ready to boil over, under your own back yard.
The earth's interior has been hot since the planet began to form an estimated 4.7 billion years ago. At that time, masses of swirling gas and dust were accumulating into a dense sphere. High temperatures were generated by the collisions of impacting material as the primordial earth grew.
Later, gravitational compression of the newly formed planet produced more heat, and the earth's temperature increased still further as it differentiated into layers. Heavy iron sank to form the core, and lighter materials rose upward to settle in layers near the surface.
Radioactive decay helps to maintain the earth's high internal temperature, but, while the core remains extremely hot, the earth is thought to be slowly cooling down—losing heat towards the surface, mostly by convection.
The earth's solid outer shell, called the lithosphere, is made up of two layers, the crust on top and the upper mantle beneath. These layers are made up of relatively lightweight material which floats on yet another layer called the asthenosphere. This layer usually extends from about 50km to 250km below the surface. It is very hot, and plastic flow of the rock within it allows the lithosphere above to move over it.
The lithosphere is not one continuous shell, but is divided into atleast 15 giant sections called tectonic plates that slowly move about, jostling and bumping at the edges. When two tectonic plates collide, one often subducts or dives under the other and into the asthenosphere. Here the high temperatures melt the surface rock which, being less dense, may then ascend again to erupt as volcanoes.
This is one way that much of the world's volcanism is generated, especially along the edges of continents. For example, the minefield of volcanoes in the central North Island, which forms the New Zealand portion of the "Pacific rim of fire" was laid by the Pacific plate subducting the Indian-Australian plate (see Earth, fire and water, Issue 7).
Similar circumstances 15-20 million years ago gave rise to the Coromandel and Waitakere Ranges. Although within sight of the Auckland Volcanic Field, these volcanoes are long extinct and are geologically separate from it.
At the time of their eruption, the Auckland area was submerged in a deep marine trough. Over millennia, sediments settled out from the sea, and debris avalanches flowed down the submarine slopes of the island volcanoes. The sediments piled up as layers of sand and mud on the seabed. Compressed, hardened and raised from the sea, those sediments form much of the present landscape around Auckland's volcanoes.
The Auckland Volcanic Field, some 400km from the edge of the Indian-Australian plate, is considered to be unrelated to the volcanic activity which occurs at a plate boundary. Instead, the volcanoes are spawned from what geologists call a hot spot"—an extra-hot zone about 100km below the city, near the boundary of lithosphere and asthenosphere.
This subterranean oven, which is thought to be fuelled from plumes of hot material rising from deep within the lower mantle, causes partial melting of the upper mantle. The molten rock or magma rises from there up through the denser surrounding rocks to form a plume or diapir. When enough magma forms, it can break away and rise all the way to the surface, much like a blob of oil in a bottle of salad dressing.
The type of magma that feeds Auckland's volcanoes is known as basalt. It has a high melting temperature and is very fluid, so it rises fast, perhaps making the trip within a week. It usually cools to a hard, dark, fine-grained crystalline rock.
Much of what scientists know about the nature of basalt volcanoes has come from the close study of Hawaiian volcanoes, especially Kilauea, which erupts frequently. There, a very large and prolific hot spot has existed for as long as 80 million years.
While the Hawaiian hot spot has remained more or less stationary, the Pacific plate has moved slowly across it, collecting a trail of volcanoes on its surface. They extend nearly 6000km from the newest Hawaiian volcanoes like Kilauea in the southeast to the eroded Emperor seamounts in the northwest.
Auckland's hot spot is very small in comparison, and its volcanoes are termed monogenetic because each one results from a single, separate batch of magma.
The Auckland Volcanic Field is part of the Auckland Volcanic Province, which includes two other volcanic fields further south: one near Pukekohe and Tuakau, and the other south of the Waikato River mouth at Ngatutura. Those volcanoes were active from roughly 2 million to 0.5 million years ago, and could have been spawned from the same hot spot which has since moved northward.
Another area with recent basalt volcanoes very similar to those in Auckland is the Northland Volcanic Province, which has been active from over 2 million years ago near Whangarei, to 1300 years ago between Kaikohe and the Bay of Islands.
For the 100,000-plus years that Auckland's volcanoes have been bursting on to the scene, the surrounding landscape has been in a state of constant flux—mostly the result of changes in global climate. For example, for most of the last 120,000 years the sea level has not been as high as it is now. The familiar harbours surrounding the city were instead the "Manukau" and "Waitemata" Rivers. Several of the volcanoes which are now harbour inlets (Orakei Basin, Panmure Basin) or islands (Puketutu) actually erupted on to dry land.
As climate changed, so did the prevailing vegetation. At times, Auckland was swathed in luxuriant forest like that of present-day Northland. In the warmest periods, stately kauri, attended by great rimu, rata, totara, and kahikatea, were to be found towering above ranks of broadleaf trees throughout the isthmus. Moa probably stalked the forest floor, cropping the undergrowth.
In cooler epochs, forest types now found in the South Island probably re‑placed those that preferred more tropical climes. Around 20,000 years ago, scattered beech forest amidst grass and scrublands reigned around Auckland. Sea levels were at their lowest then, so terrestrial vegetation could spread across the harbours and even out past Great Barrier Island on the floor of what is now the Hauraki Gulf.
The changing mosaic of vegetation was complicated further by the volcanoes that periodically ruptured the landscape, leaving blackened scars smoking amidst the verdure. After a slow reinvasion, much like what can now be seen on Rangitoto, forest would eventually thrive again on the new soils of the fresh volcanic land.
Rock forest with puriri, titoki, mahoe and puka eventually developed on the volcanic cones. Although it has been altered by residential properties, the forest on the eastern slopes of Mount Eden (Maungawhau) is the last surviving example of this original volcanic vegetation.
Volcanic destruction sometimes served to preserve a record of these ancient forests. At Ihumatao, north of Auckland International Airport, a forest was caught and buried by tuff from the Maungataketake volcano about 27,000 years ago. The forest has since been excavated by wave action from the Manukau Harbour. There in the mudflats lie the remains of huge kauri, and other tree trunks still stand upright in the eroding cliffs.
Forest was recorded in a different way at Takapuna Beach. There, an ancient forest was engulfed by Pupuke lava flows which surrounded trees, then solidified before the trunks burned away. The result is a ghost forest of "lava tree" stumps and tree moulds that you can stroll through at low tide. Nearby, at Thorne Bay, a 2m-diameter kauri tree and other forest debris have left burnt impressions in the solid rock.
Charming little Brown's Island (Motukorea) is in many ways typical of Auckland's volcanoes. Today, it is a collection of bright green, curvaceous hillocks, bathed by the dappled blue and teal waters of the Waitemata Harbour. Several thousand years ago—geologists' estimates vary from 30,000 to 8000 years—it was the site of a violent eruption brought about by a batch of 1200°C magma squeezing upwards from thehot spot below.
If the later date is accepted, the eruption would have thrust Motukorea into the waters of a shallower version of today's Waitemata Harbour, in the manner described below. (An eruption 30,000 years ago would put the volcano on dry land.)
When the rising magma reached groundwater buried deep in the layers of sedimentary rock beneath the harbour, the water flashed to steam causing a huge build-up of pressure. The pressure soon became too great to be contained by the surrounding rocks, and a violent phreato-magmatic explosion tore through the earth's surface.
In the ensuing paroxysms, hot, shattered lava and sedimentary rock was blasted high into the air from the volcano's throat. A dirty mushroom cloud of steam and volcanic gases laced with bolts of lightning towered above the harbour. At its base, white clouds billowed outwards as water pouring into the vent boiled to steam. Many explosions in rapid succession sent out waves which churned the calm waters of the harbour into a frenzied maelstrom.
Expanding steam, caught within the rocks thrown from the volcano, disintegrated them into fine dust and gravel. As the eruption continued, saturated ejecta falling back down jetted horizontally outwards (like the shock wave from an atomic explosion), forming a basal surge around the vent. Larger pieces of rock shot out from the volcano to splash into the sea or be caught up in the surges of finer material.
Many chaotic eruptions pulsated and rocked the adjoining vents which formed over the next few days or weeks, as the supply of magma and water varied. Successive blasts of pulverised rock called tuff (with the consistency of wet concrete) built a low rim of consolidated ejecta extending up to 1km away.
This crescent-shaped ridge or tuff ring is now prominent on the east side of the island where it grew most. Among the shattered sedimentary rock which was redeposited in the tuff ring, there is also fragmented basalt and larger chunks of rock that were caught in the frenzied surges. Fossil seashells, torn from their resting place under the harbour bed by the blasts, are also visible.
As the eruption continued, sea water was excluded by the tuff ring, and the aquifer either dried out or was sealed off by solidified magma. The explosions became fewer and less violent as the source of steam diminished, and the volcano entered a new phase in its growth.
Driven by rapidly expanding gases in the magma, lava flows and Strombolian lava fountains replaced the phreatomagmatic eruptions. Lava is the term for magma once it has reached the surface. Named for the Italian volcano Stromboli, Strombolian lava fountains pulsate with frequent small explosions throwing frothy lava fragments from the vent.
Rough, solidified pieces of this lava froth are called scoria, and are riddled with gas bubbles. Scoria piled up around several small vents on Brown's Island to build scoria cones. When fountaining was profuse, lava sometimes stayed molten, and continued to flow upon landing. At other times, degassed lava poured gently from the vents without fountaining.
In some places, lava flows breached the cones, carrying scoria away with them, and leaving horseshoe-shaped cone remnants. In the last stages of eruption the summit cone with its elegant crater was built, smothering some of the smaller, earlier cones.
Perhaps only a few weeks after its boisterous arrival at the surface, the batch of magma ran out. There stood a brand new island, blackened and steaming in the middle of the harbour.
The presence of groundwater or surface water (such as the sea) in the path of rising magma was a major factor affecting the evolution of Auckland's volcanoes.
If the eruption site was "wet" the volcanoes got off to an explosive start. The resulting phreatomagmatic eruption left a wide crater surrounded by a more or less circular tuff ring. Such volcanic craters, called maars, usually formed in low-lying areas where the rocks were saturated with water. Several of the volcanoes, including Panmure Basin, Lake Pupuke, Onepoto, Orakei Basin and Pukaki got no further than this. Most of the debris erupted from a maar is not of volcanic composition at all. Rather, it is "country rock", the rock already present before magma intruded.
Not all phreatomagmatic eruptions let rip in low-lying areas. The most explosive of all Auckland's volcanoes, Three Kings (Te Tatua-aRiukiuta), must have encountered a large aquifer in the ridge where it erupted. It started with multiple phreatomagmatic blasts followed by lava effusion that filled the craters it had just excavated.
If it had happened today, the fallout from this centre would have thickly blanketed houses and streets within an egg-shaped area more than 6km across from Mount Roskill to Remuera. Close to the vent, everything would have been flattened and buried by tuff up to 50m deep.
Later, lava fountaining built the three "kings" and several other scoria cones. A lava flow from the volcano flowed 10km to the banks of the ancestral "Waitemata River". (Now that the sea level is higher once again, that flow forms Te Tokaroa, or Meola, Reef, and nearly bridges the Waitemata Harbour.)
Apart from the presence of water, the violence of eruptions is determined by the fluidity and gas content of magma. Gas-rich and viscous rhyolite magma, near one end of the fluidity scale, can produce cataclysmic eruptions, burying thousands of square kilometres in pumice and ash. Eruptions of Lake Taupo, the last about 1800 years ago, were of this sort.
Auckland's basalt magma is fluid and relatively gas-poor. Rapid and continuous discharge of this sort of magma is known as Hawaiian-style lava fountaining. Spraying a hail of small incandescent fragments, it produces gravel-like deposits called basalt lapilli. Strombolian lava fountaining, like that at Brown's Island, occurs when there is less gas and the lava is more viscous—but not as viscous as silica-rich rhyolite magma. Deposits from both types of eruption contribute to Auckland's scoria cones.
Some volcanoes, like the Domain and Mount Richmond (Otahuhu), managed a bit of fountaining activity and are left with tiny scoria cones within a maar. A few, including the now quarried Little Rangitoto (Maungarahiri), in Remuera, had no initial explosive upheaval. They erupted only a minor scoria cone followed by some lava. Others accumulated large cones or combinations of cones which have become prominent landmarks in the city. Mount Wellington (Maungarei) is the largest individual scoria cone (8.5 million cubic metres). The cone stands astride a low tuff ring.
One Tree Hill (Maungakiekie) is one of Auckland's most complex centres. While building a cluster of scoria cones, it produced lava in profusion. An estimated 260 million cubic metres of lava spread north as far as the Domain, and south nearly to what was then the "Manukau River".
The flow of lava from One Tree Hill was so great that two of the craters were never able to form complete circular scoria cones. As the erupted scoria fell, it was immediately swept away by the fiery torrent, and the cones remained horseshoe-shaped.The cones of Mount Hobson (Remuera), Mount Victoria (Takarunga) and Mangere Mountain, by contrast, were breached late in their eruptions by similar outpourings of lava.
Lava flows issued from 30 of Auckland's volcanoes. They poured downhill into valleys, sometimes overflowing them and completely masking the earlier topography. Rangitoto's gentle lower slopes were created by outpourings of lava from the crater and flanks of the central cone.
Most of Auckland's volcanoes formed within a short time, probably taking up to a few years—though it is entirely possible that even a Mount Eden-sized cone could have formed in a matter of days.
Subterranean tunnels connect Auckland's volcanic peaks? Are alien monsters using the network in a heinous plot to destroy the planet? Maurice Gee wrote of such possibilities in his book Under the Mountain, which screened as a television series in 1981. Fantasy, certainly, but within its lava flows Auckland has the next best thing: lava tubes.
Lava tubes are the arteries that supply a lava flow. Liquid lava flowing from a volcano rapidly forms a river as the flow edges cool and raise levees. Gradually, the cooling surface of the river congeals and a crust develops, especially in downstream parts. Once formed, the crust thickens from the underside as cooling lava adheres to it.
within a lava tube, and still a searing 1100°C or more, lava can travel large distances—much further than a surface flow of the same volume.
When the lava supply to a tube system stops, and if the lava within it is still sufficiently hot, it continues to flow, draining the tubes and leaving empty tunnels. Often the ceiling collapses without the support of liquid lava, leaving channels in the surface lava.
Dozens of lava tubes or caves have been found in Auckland, most often during excavations for buildings or roads. Several lava caves are noted as prehistoric Maori burial sites. One cave at Crater Hill in Mangere even concealed an underground press used for printing subversive literature during World War II.
Auckland's known lava tubes are quite short, because of collapses or lava blockages. The longest and best preserved is the 290m Wiri lava tube that once carried lava from a small scoria cone called Wiri Mountain or Matukutururu.
Win lava tube has excellent examples of lava flow patterns and other features that give it high scientific and educational importance, but it has been under threat of quarrying for many years. The owners, New Zealand Railways Corporation, have recently agreed that it should be protected, and negotiations are under way to ensure that it will be.
Bejewelled with volcanic monticules and set between two harbours, the site for Auckland has long been admired for its natural beauty. Best remembered of the city's earliest settlers, Sir John Logan Campbell wrote excitedly that the view from the summit of Mount Hobson (Remuera) surpassed that even from the Acropolis in the ornate Greek city of Corinth.
For hundreds of years before Campbell's time, Maori tribes fought for occupation of the district, rich as it is with fertile volcanic soils for gardening, coastal waters for fishing and hills for pa sites. The city is still known by the Maori name for the isthmus, Tamaki-makau-rau, "Tamaki of the hundred lovers"--a popular place indeed.
In Maori mythology, Auckland's isthmus sustained its volcanic wounds in a battle between the mystical forest-dwelling Turehu people of the Waitakere and Hunua Ranges. During the conflict, which came to be known as Pakurangarahihi, a tohunga from the Hunua side caused the sun to rise prematurely, blinding the Waitakere combatants. Many were brutally dispatched in the resulting confusion. When night fell once again, the Hunua warriors pressed on towards the Waitakere Ranges, but didn't get far.
A Waitakere tohunga invoked the volcano deity Mataoho to intervene, and he promptly threw the whole isthmus into convulsions of volcanic activity. The Hunua warriors were quickly put to rout by great explosions, tides of lava and billowing ash. However, fires started by the volcanoes swept towards the forests of the Waitakeres, so a tohunga had to call upon rain to quench them. To this day, Auckland's swarm of volcanic features, together with high rainfall in the Waitakere Ranges, remind us of these events.
Not surprising, then, that the Maori who occupied the land much later were keen to keep Mataoho happy. Offerings to placate him were made in the crater of Mount Eden, which has the shape of an enormous food bowl.
Regardless of how they came forth, the volcanoes were not the last marks to be made upon Auckland's landscape. The Maori who chose to make the district their home, starting perhaps 1000 years ago, were quick to clear the forests and shape the volcanic land to their liking.
What we know of these people comes from a melding of archaeological information with oral traditions (like the stories of Mataoho) and tribal lore passed on by kaumatua (elders).
Tribal stories recall Titahi, chief of the Ngati Awa, as a famous builder of the impressive pa on Auckland's volcanic cones. The collective name for the pa was "Nga Whakairo a Titahi"—the carvings of Titahi.
By the seventeenth century, many cones had been sculpted, their natural curves transformed to flights of terraces. All the earthmoving was done by hand, aided only by stone and wooden tools. It possibly took generations to produce the final form of each pa.
Terraces were built primarily as house sites, with food storage pits, hearths and open ground nearby. Gradually, as the population of a pa increased, terraces were added to accommodate new families. The family of the chief lived near the summit of each pa.
The defensive role seems to have come later in the development of terrace systems. The growing population of Tamaki-makau-rau, and increasing attacks by tribes from other districts, led to the construction of palisades and defensive ditches to discourage invaders.
Carbon dates suggest that cone pa were in general use in the 15th century. Population of the district reached a peak about the 16th and 17th centuries, with most of the populace coming from a cluster of tribes known collectively as the Waiohua.
In total, Maori of Tamaki-makau‑rau built 33 pa on volcanic cones. They ranged from the massive earthworks of Maungakiekie to a few terraces on Motukorea, which, like most pa, was not occupied constantly. It is, apart from Rangitoto, the least modified of all Auckland's volcanoes.
Cleared of the original forest, the lava stonefields surrounding the pa were intensively utilised. Radiating like a cobweb from each pa was an array of stone walls, delineating a garden system.
Stone was also cleared from the soil and gathered into heaps. Rather than simple clearance structures, the heaps were used to warm soil placed in and around them to gain an extended growing season for crops brought from warmer climates.
Kumara, yam, taro and other vegetables were grown in the fertile volcanic soils. During fallow periods, the gardens became smothered in bracken. Over the years, segments of the garden array were cultivated in sequence, sometimes taking a generation to complete the cycle.
Some of the stonefield garden sites were huge. It has been estimated that Maungakiekie had 1000 hectares of garden to support its population. House sites were scattered among the gardens, so the walk to work was not too long for the inhabitants.
The stonefields were probably settled before the cone pa. The earliest carbon date obtained for Maori occupation in the volcanic field suggests people were living at Wiri Mountain (Matukutururu) from about 905-1405AD.
The majority of Auckland's pa were abandoned after a series of conquests by invading tribes decimated the local people. In the 1700s, Ngati Whatua from the Kaipara drove the Waiohua people from the Tamaki isthmus. In the 1820s, they in turn were routed by musket-toting Ngapuhi raiders from the north, and when European settlers arrived soon after, the isthmus was virtually deserted.
The following decades saw further massive changes wrought upon the landscape, and these led ultimately to the modern city of Auckland. From the hills of regenerating manuka and bracken, not long vacated by Maori gardeners, new farms were cleared and ploughed. The settlers, too, made good use of basalt rocks cleared from the lava fields for stone walls, some of which still border suburban sections.
In the mid-1800s the view from Mount Eden was predominantly rural, but it was not long before the site of the city which had been surveyed from that summit in 1840 was thick with houses and roads.
The growing city was hungry for construction materials. Basalt scoria was used for early roads, and lava for some of the first permanent buildings. Many of the scoria cones were scarred by quarrying, and a few removed altogether for railway ballast.
Mount Smart (Rarotonga) was quarried from a cone over 50m high down to its roots by the Railways Department. During the last stages of excavation a stadium was thoughtfully constructed out of the rock pile. On Mount Albert—a cone decapitated in the period of railway, and later motorway, construction sports fields were built in the quarry pits.
A few of Auckland's historic buildings were built of stone hewn from the basalt lava flows. St Andrew's Presbyterian Church in Symonds Street was the first to be built from that material (taken from flows at Newmarket) in about 1850.
The Melanesian Mission at Mission Bay and Kinder House in the suburb of Parnell were constructed of lava taken from the foreshore of Rangitoto Island. Near St Andrews the defensive wall of Albert Barracks was also built from blocks of lava. It was a conspicuous but unsightly feature of early Auckland, and had mostly been demolished by 1871. Part of the structure still stands in the grounds of the University of Auckland.
Another use of the lava flows that, through the centuries, has been a focal point for settlement around Auckland is the supply of water. The lava flows are very porous, so rainwater falling on the surface rapidly percolates downwards until it reaches a non-porous layer. In Auckland, that is the old surface of sedimentary rocks that was buried by the lava flows.
Imprisoned beneath the lava flows, the pre-volcanic topography of ridges and valleys still exists, and, like rivers on the surface, the groundwater follows the buried valleys. At the far end of lava flows, where such valleys are close to thesurface, the water can emerge as springs. Upstream, the aquifer can be tapped by sinking a well into it.
Western Springs, at the foot of lava flows from Three Kings and the contiguous Mt Eden and Mt Albert flows, was one of Auckland's earliest municipal water supplies. The springs were dammed and excavated to form a reservoir lake. The massive steam-driven pump still on display at the Museum of Transport and Technology pumped water up to a series of reservoirs from 1877 until 1928.
At Onehunga, water for municipal supply is extracted from four wells sunk into One Tree Hill lava flows. An estimated 27,300 cubic metres of water per day flows through that aquifer, reaching it from as far as five kilometres away through lava flows which fill an ancestral river valleys beneath the contemporary terrain. Many of the industries that used to operate in Onehunga, such as wool scourers and tanneries, were attracted there by the opportunity of cheap water from the ground.
The municipal water supply in Auckland has benefited from volcanoes in another way. Water from Western Springs and early Onehunga wells was pumped to reservoirs constructed on Mount Eden and One Tree Hill respectively. Gravity was then, as now, an inexpensive method of distributing water to houses and factories.
Most of the cones support one or more reservoirs, forming nodes in the region's bulk water supply network. Most go unnoticed, except by the few observers who detect a peculiar angularity in the outlines of some of the cones. Others are distinctly obtrusive, like the concrete "crown" topping (and incidentally, saving from quarrying) the last remaining of the Three Kings.
All the cones bear marks from Auckland's growth. Quarries at Wiri extract rock products selectively to make the best use of them. Hard basalt from lava flows is used predominantly as base material in road construction, but also as road sealing chip, concrete aggregate and railway ballast. The major use of porous scoria is for drainage and soakage purposes.
The construction of the airport and Mangere sewage ponds required huge quantities of fill that was obtained by flattening several South Auckland cones.
The volcanic cones remaining within the city are centrepieces for its parks—havens from the urban fray. The same cannot be said for those in Auckland's hinterland, or for the lava fields which are still regarded as sources of rock rather than scientific, historic and aesthetic resources. Of all the volcanoes, Brown's Island alone escaped quarrying, but it does carry a reservoir on its shoulder.
Over the city of Auckland there hangs a question, perhaps triggered in people's minds by the way the morning mist clings eerily like smoke to the summit of Rangitoto: Will there be another eruption?
There is no doubt among geologists that the answer is yes. Given the long record of intermittent eruptions for the Auckland Volcanic Field, there is no reason to think they won't continue. Since all previous eruptions have come from completely separate magma batches, a future eruption is likely to burst out somewhere between existing centres.
Ian Smith, a geologist at the University of Auckland, has studied the mineralogy and chemistry of volcanic deposits to find out about the nature and depth of their source. I asked him what he thought would happen next in the Auckland Volcanic Field. "I believe the field is hotting up, and I'd predict the next eruption to be Rangitoto-sized."
Rangitoto contains 58 per cent of the total volume ever erupted by the Auckland volcanic field, so it is far larger and, Smith says, different from all the other volcanoes. "Subtle changes in its mineralogy suggest that the magma source was shallower, and more melting occurred than in previous eruptions. Rangitoto marks a fundamental change in the Auckland field, and future eruptions will probably be bigger and more frequent. But we need the next eruption to find out for sure."
Predicting where and when that next volcanic outburst will occur is no easy matter. Seismometers are being set up at key locations around Auckland by Civil Defence and the University of Auckland to monitor movement of the magma.
Geologists estimate that Aucklanders would receive, at best, two weeks' warning of an eruption—enough time, hopefully, to save the city from becoming a latter-day Pompeii.