Our volcanic past exposed

20:58, Mar 28 2014

The sunken forest on Waitara's Airedale reef, north along the east beach, has fascinated me from childhood. The mudslide weaving its way out on to the reef and out to sea took all the trees down in its path; many huge stumps appear cut off at ground level, an ancient forest flattened. It's a grizzly reminder of a turbulent time in Taranaki's volcanic past.

Of course this period mudslide can be seen all along the coastline in North Taranaki, from Bell Block to as far as Onaero Beach and beyond, so what do the geologists say? New evidence in recent years suggests the last eruption on Mt Taranaki (Egmont) was in 1800, and not the Tahurangi ash eruption in 1755. However, the mudslide on the North Taranaki coast was not hundreds of years ago, but 100,000 years ago.

According to geologists, the forest growing in the Airedale Reef area of Waitara was destroyed when a debris avalanche known as the Okawa Formation, originating from the Pouakai Volcano, swept across the former volcanic ring plain, journeying down a former river valley.

The fossil forest of the reef was podocarp hardwood with rimu and beech, which was growing during the warm interstadial period 100,000 years ago. This warm period was followed by a cold stadial period when grass shrub land, with some cedar forest, grew very different vegetation than that we see today.

Later there was a change in the climate to a warmer period and a forest grew which was similar to the earlier fossil forest of the Airedale Reef. This forest phase was perhaps around 80,000 years ago.

What is amazing is that the trees and debris that can be seen here today are 100,000 years old. This wood, preserved in peat, can be dried and burnt still, or turned on a wood lathe - quite incredible.


We must realise that all those years ago the land stretched way out on to the end of the reef and beyond, so what we're looking at is what coastal erosion has exposed on the floor of the beach we see today.

The coastal erosion I've seen in my lifetime gives some indication of what has gone into the sea in the past. Certainly old maps clearly show this quite clearly.

Fishermen using fishing ledges along the coast will know exactly what I mean.

There is a lot of pollen preserved in the cliff section at the Airedale. Scientists have found more than 10,000 pollen grams per cm3 in places. Analysed to study the plant communities from the period of time the avalanche occurred, it also allowed research into climate variations through time, as different species appear and disappear up through the cliff section on the beach, from the base to the top.

Rimu and tree fern indicated a lush podocarp forest that grew in warm and wet conditions; a layer above this represents a species in a sub-alpine shrub land that grew in a cooler climate. Each layer gives information on the climate; quite amazing when you consider how long ago this was.

Podocarp trees boast a lineage that stretches back to a time when New Zealand was part of the super continent of Gondwana.

There are 17 podocarp tree species here, belonging to the coniferous families. The best known are rimu, kahikatea, miro, matai and totara. Few remnants remain of areas dense in undergrowth of shrubs, ferns and tree ferns, left over from an ancient forested time.

A podocarp forest is of course a mixture of tall and short trees with an understory of shrubs, plants and ferns. Both light-loving and shade-tolerant species; soil and climate playing a major role in what species will dominate in a forest.

Mounds of the Okawa formation are concentrated in a 2.5km belt that extends north-east from Inglewood. A major portion of the mudslide deflected north-east for seven kilometres along the Inglewood fault scarp before the main bulk of it entered the Maunganui River valley and was channelled for a further 18km, heading for the coast.

A portion of the avalanche which did not enter the Maunganui River continued 4km along the Inglewood fault scarp to the present course of the Waitara River.

A smaller portion of the avalanche that surmounted the fault scarp at Inglewood was channelled northwards along the Waiongana stream valley. When the mudslide emerged from the valley, it spread laterally as a broad lobe across three extensive uplifted marine terraces.

Rocky volcanic mounds can be seen throughout the Waitara and Waiongana areas, and many were quarried during early road- making in the area. Pukekohatu Pa in Waitara was quarried for the railway in 1874-75, a huge amount of stone taken out from what was known as Pewa's pit.

The Pa site we see today is only part of the original fortification.

More of these rocky mounds are seen in the Waiongana area and were used by early Maori as fortifications and urupa.

On the main road to Inglewood from Waitara, the Sentry Hill Redoubt site was quarried for road use, leaving a huge ugly hole, which I'm sure fascinates the curious who drive past.

Pre-European Maori knew of the preservative qualities of peat. Under threat of invasion from another tribe, wooden and stone taonga (treasures) were concealed in swamps. Swamp excavations in the 1960s, not far from the sunken forest on the Airedale Reef, revealed artifacts such as garden stakes, weaving sticks, fern root beaters, fish hooks, bowls, ko (digging sticks), many of which had the footrests still lashed to them, and canoe prows, etc, in remarkable condition. They were all more than 200 years old at that time.

Taranaki's first volcanic centre survives today as the heavily eroded Sugar Loaf Islands (Nga Motu) and Paritutu Rock. These represent the oldest volcanic activity on the Taranaki Peninsular, and are thought to be between 1.7 and 1.74 million years old.

This volcanic area was followed by the Kaitake range, which was active approximately 575,000 years ago. Pouakai was the last active area, 230,000 years ago.

The last eruption on Mt Taranaki was more than 250 years ago. Earlier volcanic centres have been covered by lava deposits from Taranaki's eruptions. We live in a district rich in not only colonial history, but a violent ancient volcanic history.

So pick a low tide, a nice day, and take a stroll along Waitara's east beach to the Airedale reef, which is a leisurely walk, to what is known as Taniwha Point, and view the sunken forest.

It is truly amazing and one which geologists continue to use as a reference for their studies.

Due to a production error at the Daily News end, the wrong caption was used for a photo printed as part of Graeme Duckett's previous feature, The Roaring Twenties. The photo described as Cook and Lister was actually Harry Hawkes barber shop.

Taranaki Daily News