Auckland's volcanoes are powered by a 'hot spot' deep beneath the surface.
The hot spot is a zone of magma fed by a plume of hot rock rising up out of the mantle, perhaps as deep as a hundred kilometres below the Earth’s surface. In this hot spot, temperatures are high enough so that the rock begins to melt.
Every few hundred or thousand years, a batch of magma breaks off from the hot spot and forces its way up towards a weak place in the Earth’s crust right here under Auckland.
The first sign of an impending eruption is ground tremors, as the batch of magma rises. Volcanic earthquakes and tremors make different seismic patterns from non-volcanic earthquakes. The next time we see a volcanic earthquake show up on our monitoring stations, we can make a good guess at what will follow.
- Volcanic earthquakes (up to M 5) will damage buildings as the magma rises and breaks its way through to the surface.
- The formation of the volcano’s crater, cone or debris ring will destroy all infrastructure close to the vent.
- Close to the vent, shock waves will break windows and flatten weak buildings.
- Base surges (lethal explosive ground-hugging mixtures of steam and rock fragments) will, within minutes, envelop areas several kilometres from the vent if the magma comes into contact with water.
- Fire-fountains of lava will set nearby buildings ablaze. Fires could spread underground through buried service networks.
- Lava flows could travel up to 11 kilometres from the vent. Everything in their path will be burned, crushed or buried.
- The ash from the eruption column will halt air traffic, bury crops and cause severe distress to those with respiratory problems.