3. Geology

The Wairakei Geothermal Field is at the edge of the Taupo Volcanic Zone. This zone is where some of the most violent volcanic action has occurred on Earth. A million years of this has formed rock layers that can trap lots of hot water and steam, close to the surface.
The 186AD eruption of Taupo is compared with other recent eruptions. However the Taupo eruption 26 000 years ago was eight times larger than this, but is not shown on the diagram - Image: Contact Energy Ltd.
thin ground sign
A common warning in the Taupo Volcanic Zone - Image: Heurisko Ltd.
The Taupo Volcanic Zone extends across much of the central North Island. This is the famous Pohutu geyser 8Okm away from Wairakei - Image: Heurisko Ltd.

The Main events of the Taupo Volcanic Zone have been

  1. About a million years ago a large lake formed near Taupo. The lake bed was made of mud that later became mudstone.
  2. The lake was destroyed 26 000 years ago by a huge volcanic eruption
  3. 1820 years ago another big explosion formed what is now Lake Taupo.

This was is important in forming these rock layers:

  1. On top is a mudstone rock that traps steam and water
  2. Layers volcanic rocks which allow hot water to flow through them
  3. Pools of magma that heat the volcanic rocks.
  4. Many fault planes which allow the steam and water to move up and along.

The Resistivity Boundary

Geologists use many tools to find geothermal water. One very useful tool is Resistivity.

Resistivity measures how hard it is for electricity to flow through something. Very high numbers mean there is no salty, hot water underground.

So where the Resistivity rises, the hot underground water stops. This is known as the Resistivity Boundary.