Cone or stratovolcanoes
These are ‘picture-postcard’ volcanoes – conical structures up to several hundred metres high, built by many eruptions of andesite lava over tens of thousands of years.
Eruptions at cone volcanoes can be explosive (forming tephra or pyroclastic deposits) or quiet (forming lava flows). The eruptions often switch between these two styles of activity during a single episode. Because of the layered sequences, or strata, of lavas and pyroclastic deposits, cone volcanoes are also called stratovolcanoes. Sometimes the term ‘composite cone’ is used to indicate their layered nature.
Eruptions from cone volcanoes are generally small to moderate, with small eruptions occurring every 10–50 years. Taranaki, Ruapehu, Tongariro (including Ngāuruhoe), and Whakaari (White Island) are New Zealand's largest and most frequently active cone volcanoes.
Too many volcanoes?
Models of volcanoes are often exhibited at school science fairs – almost always large cones, with smoke or chemicals to simulate eruptions. Unfortunately they hardly ever win prizes because they are too common.
The different shapes of these four cone volcanoes reflect variations in the size and type of eruption, the number of different vents that have erupted at each volcano, and sometimes partial collapse by landsliding of the cone during an eruption, earthquake, or storm.
Cones and ring plains
The classical, almost symmetrical cone shape of Taranaki (Mt Egmont) is due to recent eruptions coming from one main vent at the summit. The multiple peaks of Tongariro volcano are due to many vents being active at about the same time, forming several overlapping cones. Ngāuruhoe dominates the landscape, but it is simply the most recently active vent that has erupted many times over the last 2,000 years.
Although the central cone is the most obvious part of a stratovolcano, most are surrounded by a gently sloping plain (ring plain) formed of landslide deposits from giant avalanches, debris flows called lahars, and tephras from explosive eruptions. A lahar may be associated with a volcanic eruption, but may also be caused by the collapse of the unstable steep slopes or melting of snow near the top of a volcanic mountain. Many cone volcanoes like Ruapehu have crater lakes. A large body of water at high altitude can result in unexpected and disastrous lahars and floods downstream, as happened in the 1953 Tangiwai disaster.
Rather than a single mountain or crater, a volcanic field is an area containing many small, isolated scoria cones and craters. Each cone or crater has formed usually in a single, small eruption of basalt magma, which may have lasted a few days to a few months at most.
Existing craters or cones in volcanic fields are unlikely to erupt again. Instead the next eruption will form a new cone of scoria or ash, or explosion crater or small lava flow. Sometimes craters may overlap within the same cone as can be seen at One Tree Hill (Maungakiekie) in Auckland. The Auckland volcanic field has about 48 cones and craters. The most recent eruption occurred about 1400 AD, forming Rangitoto Island.
Basalt volcanic fields are also found in Northland – for example, near Kaikohe, Bay of Islands, and Whāngārei – where there is a wide range of ages dating from about 60,000 years to about 10 million years. The Bombay–Pukekohe–Pokeno area of South Auckland contains an extinct field (0.5 million to 1.5 million years old), and basalt volcanoes in the western Waikato date from 1.5 million years to nearly 3 million years ago. Much older volcanic fields are found in the South Island, for example in Otago.
Eruptions in volcanic fields
The fluid basalt magma means eruptions from volcanic fields are small, and mostly lava-forming. Most explosive eruptions are relatively weak, throwing scoria out to no more than several hundred metres from the vent. If the magma encounters water as it is rising to the surface, steam explosions may excavate wide shallow craters (such as occurred at Ōrākei Basin and Lake Pupuke in Auckland).
Even the smallest eruption from the Auckland volcanic field would cause major disruption to the city, and dust-size volcanic ash is likely to spread over a large area.