Volcanic formation

It is thought that Auckland's volcanoes first began to appear between 60,000 and 140,000 years ago, starting with the eruptions of the Domain and Albert Park. The largest and most recent eruption was Rangitoto, about 600 years ago.

The Auckland volcanic field is expected to have a total life of approximately one million years, so potentially there is a lot of life left in the field yet.

There is limited data available to determine the precise ages of past eruptions. This makes it difficult to determine when the next eruption is likely to occur. However, we do know that the frequency and magnitude of eruptions seems to be increasing with time. Of the approximately 50 eruptions in the Auckland field, 20 have occurred within the last 20,000 years and of these, 18 occurred between 20,000 and 10,000 years ago. The graph above shows the relationship between the volume of deposits and time.

Each of Auckland's volcanoes has an eruptive history all its own. However, a general sequence of events can be put together which is fairly typical of past eruptions in the Auckland volcanic field, and of what we might expect from other eruptions in the future.

1. Hot magma is forced through fissures to the surface from depths of around 100 km.

2. As magma comes into contact with surface water, it causes violent explosions. Magma gases add to the blast. The explosions throw ash and shattered rock high into the air, which land to form a tuff ring or rim around the vent.

3. If there is no more magma after the first violent eruptions, then a broad crater called a maar forms, with a low rim around the edge. Several of Auckland's volcanoes, including Panmure Basin and Orakei Basin developed no further than this stage.

4. If magma keeps rising, a lava lake may form in the crater.

5. Rising magma leads to lava bursting from one or more vents (fire fountaining), building scoria cones. Smaller lava flows may emerge, but are held within the tuff ring. The Domain, Taylor Hill and Mangere Lagoon reached this stage. A lava lake may fill the crater then develop a crust. Subsequent eruptions blast holes through the crust, and build cones upon it (eg Three Kings).

6. As the eruption continues, scoria cones grow larger and eventually bury the tuff ring. Lava may break through a cone or burst out (breach) at a lower level, flowing into surrounding valleys (eg Mt Eden, Mount Smart and Mount Mangere).

7. The stream of lava can sometimes be so great that it weakens the cone, which collapses into the flow. A horseshoe-shaped crater is left behind (eg Mount Victoria, Mount Hobson and One Tree Hill). If the flow continues, a large sheet of lava may cover the surrounding area, forming a 'shield' volcano (eg Rangitoto).

The formation graphics have been sourced from a poster, "Volcanic Auckland," produced by and available from: New Zealand Geographic, PO Box 8900, Symonds Street, Auckland. Phone 0800 800 735, email subscribe@acpnz.co.nz

Allen, S.R., and Smith, I.E.M., 1994, Eruption Styles and Volcanic Hazard in Auckland Volcanic Field, New Zealand, Geoscience Reports of Shizuoka University, No. 20, pp. 5-14.