The Ring of Fire
The central North Island is part of the Pacific Ring of Fire – a term used for the rim of the Pacific Ocean, which has earthquakes and active volcanoes.
Volcanic rocks poke through the greywacke sandstone. Volcanoes have been a feature here for many millions of years, and in the north of this ecoregion there are ancient, weathered, volcanic mountains such as Pirongia in the western Waikato.
The current volcanic activity began around 1.6 million years ago, and is driven by the Pacific Plate sliding under the Australian Plate under this area. Volcanism is concentrated in the Taupō Volcanic Zone, which stretches from the Bay of Plenty south to Mt Ruapehu.
This is a highly volcanically active area by world standards. It has three frequently erupting cone volcanoes: Whakaari (White Island), Tongariro/Ngāuruhoe, and Ruapehu. There are also two large calderas – Okataina and Taupō. These are lake-filled hollows that have regularly blasted out huge flows of pumice and ash (ignimbrite). Taupō has erupted 28 times in the last 27,000 years, most recently around 232 CE.
Ruapehu is the tallest North Island mountain, and has the only permanent ice and snow in the North Island. Besides the volcanic peaks, there are greywacke mountains from East Cape down to the Kaimanawa Range, and rugged sandstone and mudstone hill country in the west.
To the west are Mt Taranaki (Mt Egmont) and older, worn-down remnants of volcanoes. Eruptions have spread tephra (volcanic ash) across the landscape. Much of the Taranaki region is cloaked in massive ignimbrite deposits, metres thick, formed from ground-hugging hot-ash flows.
Even outside the main volcanic zone, the soils are mostly formed from volcanic eruptions. The flood plains of Waikato have deposits of recent pumice, and deeply weathered clay soils from older volcanic eruptions. Friable, nutrient-rich soils derive from the andesitic cone mountains such as Taranaki, Tongariro and Ruapehu.
In sharp contrast are the deep, nutrient-poor pumice soils of the major valleys and the shallow, infertile soils of the greywacke ranges.
The taller cone mountains are subject to lahars – vast landslides of volcanic debris. Repeated lahars have helped build broad aprons of coarse deposits around the volcanoes, and create a hummocky landscape.
Mild lowlands, cool uplands
The lowland coastal districts of the Bay of Plenty and Taranaki share the mild, moist, winter-wet climate of the Northland ecoregion.
In contrast, the higher volcanic plateau and adjacent mountains are cool and wet. The northern warmth-loving plants gradually give way to more cold-tolerant, southern species. Kauri and many other trees stop growing naturally around latitude 38° south. Others, such as kohekohe, are only found close to the coast.
Before human clearances began, the flood plains were covered with dense forests of kahikatea, rimu and mataī, now reduced to tiny remnants.
On deep pumice soils and lower mountainsides, the main vegetation is conifer–broadleaf forest, with subcanopies of tawa and kāmahi.
Higher up, there is a mixed forest of conifer–broadleaf and beech, and then mountain beech, silver beech and cedar forest dominate. The higher ranges and the taller volcanic peaks have alpine vegetation, mostly dispersed from the older and richer alpine flora of the South Island.
On the Hauraki plains and in the Waikato basin, there are enormous raised bogs dominated by the tall jointed rush (Sporodanthus traversii) and mānuka.
Vast ignimbrite eruptions – the most recent being the Taupō eruption, about 200 AD – have had a permanent effect on life forms. They incinerated all vegetation and formed thick tephra deposits, on which new soils have formed. Tall conifer forest thrived, but many less mobile animal species such as land snails were displaced.