Dialogue: Be cautious with Auckland's volcanic heritage

It is time to stop the destruction and start the restoration of Auckland's volcanic heritage, writes BRUCE HAYWARD*.

Plans to plough a motorway through the Mt Roskill volcano reserve, within a stone's throw of the foot of its scoria cone, have raised public concern about the protection of Auckland's volcanic heritage.

Certainly this proposal is a significant issue for Mt Roskill, but it is one of the less offensive developments nibbling away at Auckland's irreplaceable volcanic landforms.

Others have been the erosion of Mt Eden by grazing cattle and the successful campaign by Epsom residents to prevent a large house being built on the crest of Mt St John.

Soon there will be more controversy over Transit New Zealand motorway plans affecting three other volcanoes. A proposed East Tamaki link road to the Southern Motorway cuts through the side of the proposed reserve for the Waiouru volcanic tuff ring and severs the reserve's vital landscape links with the Tamaki Estuary.

As in the Mt Roskill proposal, the route for the Eastern Motorway will slice through reserve land close to the base of Mt Wellington's cone. This motorway will also be built through the Orakei Basin's volcanic crater and tuff ring, like the adjacent railway line.

But these are insignificant compared to the destruction that has been wreaked in Manukau City.

The last vestige of the Otara Hill scoria cone was removed last year. Its site is rapidly disappearing beneath factories. The last traces of the volcanic cone of Maungataketake (also known as Elletts Mt) will also be gone in a few months. The last remaining scoria from Wiri Mt, too, is close to exhaustion, apart from the small remnant that has been saved because it hosts New Zealand's most important lava cave.

It is too late for these and the other lost cones of Manukau - but not too late to save the remains of the outstanding Puketutu Island volcano and Papatoetoe's Crater Hill, both of which are being quarried.

Puketutu is the Browns Island of the Manukau. Many believe the charitable trust that owns Puketutu should bequeath it to Auckland as a regional park or set it up as a privately owned farm park.

Crater Hill is a large explosion crater surrounded by a relatively high tuff ring of volcanic ash.

Auckland, and particularly Manukau City, has lost a great deal of its volcanic character. Now is the time to start restoring some of our lost heritage.

We have the technology and we have large volumes of cleanfill requiring disposal sites. All that is needed is public and political support for a phased programme to rebuild some volcanoes.

The sites of some of our quarried and damaged cones are now so alienated by houses, factories and other developments that restoration could not be considered. But other damaged or destroyed volcanoes are candidates for restoration.

The small scoria cone that once was the centrepiece of the Mangere Lagoon explosion crater could be restored to its former glory at little extra expense.

Instead of creating an artificial bird roost in the centre of the lagoon as proposed, the natural slopes of the original gently sloping scoria cone should be rebuilt and could equally well provide the roost for the waders when the tide comes in.

Another volcanic cone that could be considered for immediate restoration is Otuataua, within the Otuataua Stonefields Historic Reserve, south of the Mangere sewerage plant.

Manukau City is quarrying the last valuable rock from inside the core of this former cone. The natural lower slopes of Otuataua's cone remain untouched but inside these, the plan is to landscape the old quarry as an artificial crater with a new central mound built in the centre, imitating the raised volcanic plug of nearby Mangere Mountain.

When protection is inevitably attained for Puketutu Island and Crater Hill, both will be prime candidates for restoration of their former cones. Material removed from the decommissioned sewerage ponds at Mangere is being used to build artificial hills and to reclaim parts of the harbour. Surely this material could be put to good use in restoring the lost volcanic heritage of nearby Mangere Lagoon, Puketutu, Otuataua, and Maungataketake.

Restoration need not be restricted to Manukau City. As the future of the Mt Wellington quarry is being formulated, now is the time to consider reconstructing the tiny Purchas Hill scoria cone. Options for the future of the giant quarry at Three Kings should include filling in the hole and reconstructing all the removed scoria cones.

Auckland is not only built on an active volcanic field, but much of the city was built out of the aggregate resources supplied by the volcanoes. Now, as those resources are quarried out, protected in reserves or unavailable beneath subdivisions, Auckland still has an insatiable appetite for aggregate.

Greywacke from the Hunuas can supply certain needs and artificial slag from the Glenbrook steel mill could replace some of the specialist needs for porous rock, but there will still be a demand for quality volcanic basalt aggregate.

The next nearest source is from the 80 older volcanoes in the extinct Franklin volcanic field. We must ensure that quarrying here does not have the same impact on the volcanic heritage as it has in Auckland.

The best volcanic cones (for example, Pukekawa, Onepoto and Pokeno) and volcanic craters (east Pukekohe, Kellyville and Onewhero) must be zoned for protection.

The destruction of Auckland's volcanoes should have ceased decades ago. But in achieving this, let's not repeat Auckland's mistakes in our other volcanic fields.

* Bruce Hayward is an Auckland geologist and former member of the Conservation Authority.