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Auckland Volcanic Field

mt eden

Mt Eden, Auckland.

The Auckland Volcanic Field is monogenetic meaning each volcano usually only erupts once. The field is still active and there is no way to predict where or when the next ‘bubble’ of magma will rise to the surface and create a new volcano. The size and length of each eruption depends on how big the ‘bubble’ of magma was.

If the basalt magma mixes with water (seawater or groundwater) super heated steam blows it apart. This causes a pyroclastic eruption that produces fall and flow deposits and has created the low rings of pyroclastic rock (called tuff) around the craters of many Auckland volcanoes such as Lake Pupuke.

When the magma has no contact with water, lava can fountain out less explosively and build a cone of tephra. Basalt tephra is called scoria so the cones are commonly called scoria cones (e.g. One Tree Hill).

To monitor the Auckland volcanoes there are currently 9 seismographs operated by GNS Science’s GeoNet programme.

Rangitoto Island

Rangitoto means ‘Bloody sky’ and is thought to refer to the serious injury of a Māori chief during a battle.

For a one page fact sheet / poster on Rangitoto volcano click VolcanoFactSheets Rangitoto.pdf (535.59 kB)

Rangitoto is a volcanic island in the Hauraki Gulf visible from most parts of Auckland City. It is the most recent, largest and least altered volcano in the Auckland Volcanic Field. This is made up of around 50 small volcanoes that have formed over the last 250.000 years. Rangitoto makes up nearly 60% of the total volume of material erupted. It was formed by at least 2 eruptions 600-700 years ago and is now about 260 m above sea level and 5.5 km wide.

Rangitoto Island

Rangitoto Island

Roads and tracks allow visitors to walk over lava fields and through lava caves (tubes left behind by the passage of liquid lava). Vegetation varies from ‘raw’ lava fields to scrub and sparse forests including the largest pohutukawa forest in NZ.

It is an intra-plate or hot spot volcano (these occur away from plate boundaries and are not related to subduction). A mantle hot spot exists about 100 km below Auckland. When rock is melted by this extra heat, it will separate from the surrounding solid rock and rise to the surface. The melted rock is basalt magma which has a low viscosity (flows easily) and may rise to the surface at speeds of up to 5 km/hour.

Rangitoto consists of scoria cones on top of a broad ring of lava flows. A moat like ring around the summit is due to subsidence of the mountaintop as underlying lava flows cooled and shrank. When it erupted 600-700 years ago over an unknown time span, the sequence of events was likely to have been:

  • a violent pyroclastic eruption as cold sea water met molten rock, creating an explosion crater and a tuff ring.
  • ongoing fire fountaining built scoria cones once water could no longer reach the magma
  • lava flows from the base of the scoria cone