Auckland’s 49 volcanoes are different from most other volcanoes in New Zealand.
The Auckland volcanic field owes its origin to the presence of a region of hot rock known as a hot spot or plume located about 100km beneath the city.
In this hot spot, temperatures are high enough so that the rock begins to melt. When enough molten rock has accumulated (probably about 5 to 10%), it separates from the solid residue and rises toward the surface.
This melted rock is basalt magma and one of its important characteristics is that it has a low viscosity (flows easily) so that it can force its way through the overlying crust quite quickly (speeds of 5 kilometres per hour have been estimated).
Each volcano in the Auckland volcanic field has been fed from a deep source and each time there has been an eruption it has been of a new batch of basalt magma.
An important aspect of this style of volcanism is that there is no crustal magma reservoir present between eruptions so there is no source of heat to drive geothermal systems as there is in the central North Island.
The lack of surface activity in the Auckland region leads to the mistaken impression that the field is extinct whereas nothing could be further from the truth.
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www.gns.cri.nz/what/earthact/volcanoes/nzvolcanoes/aucklandprint.htm