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.

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The Wairakei Geothermal Field is at the eastern edge of the Taupo Volcanic Zone. This zone is the result of a million years of some of the most violent volcanic action recorded on this planet. This has resulted in a unique series of rock layers that can trap large volumes of hot water and steam, close to the surface.

1. Sedimentation (about a million years ago) in the large lake that extended from Taupo to present day Reparoa. The lake bed filled with muddy sediments that became mudstones.
2. The lake was destroyed about 26 000 years ago when a massive volcanic eruption blew 800 cubic kilometres of rock into the earth's atmosphere.
3. In 186 AD (1820 years ago) another massive explosion blew a further 100 cubic kilometres of rock into the earth's atmosphere, from what is now Lake Taupo.

A common warning in the Taupo Volcanic Zone - Image: Heurisko Ltd.

This sequence is important in producing the following rock formations:

1. A mudstone cap rock from the Taupo-Reparoa lake that now traps steam and water.
2. Layers of porous volcanic rocks such as Andesite and Rhyolite which allow water to flow through them. These rocks are typically 20% porous which allows the hot water and steam to be held in them.
3. Large pools of magma that lie close beneath the rocks and heat them.
4. A series of fault planes which allow the steam and water to move up and along to the North East. These fault planes are the result of stretching of the crust in the Taupo area.

The Resistivity Boundary

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.

Geologists use many techniques to decide where geothermal water occurs underground. Of these techniques resistivity measurements are particularly useful.

Resistivity is a measure of how hard it is for electricity to flow through a substance.

• The less the resistivity, the greater the flow of electricity.
• The greater the resistivity the less the flow of electricity.

Because hot underground water contains many dissolved salts electricity will flow easily through it and will therefore have a 'low ' resistivity.

Where the resistivity increases it marks the outer limit of hot water in a geothermal field. This change is known as the Resistivity Boundary.