IN AD 186, one of the most powerful volcanic eruptions in the earth's history sent pumice and a stream of hot gases from the North Island of New Zealand more than 50 kilometers (31 miles) into the stratosphere. Today, New Zealand is mining the heat that powered this eruption and earlier ones from the same source.
The "mining" is actually in the form of bores sunk into reservoirs of hot water below the surface. Once the hot water rises to the surface it boils, creating steam that powers turbines. This geothermal energy provides New Zealand with 7 percent of its electricity. About half the power is produced at the Wairakei geothermal field, about a four-hour drive south of Auckland.
"There is a close relationship between geothermal activity and volcanism," says Patrick Browne, associate professor at the Geothermal Institute at the University of Auckland.
The relationship takes place because New Zealand sits on the intersection of two large drifting slabs of earth. Only 50 miles off New Zealand's east coast, one mass, the Pacific plate, is sliding southwestward. Above it, the Indo-Australian plate is forcing its way eastward.
As the earth's surface stretches and compresses, the dense hot rocks of the earth's outer mantle are rising from their normal depth of 15 miles to less than eight miles from the surface. The volcanism is taking place as the Pacific plate enters the asthenosphere, or mantle, and melts.
"This stage of the volcanism is relatively recent - it began about 1.8 million years ago," estimates Mr. Browne. The volcanic activity in New Zealand began after the Pacific plate shifted direction.
At the same time, New Zealand usually has abundant rainfall. The rain seeps through rock, which has been fractured by earthquakes, until it accumulates in aquifers. These form reservoirs that are heated by the hot rocks.
As the water sinks deeper into the ground, it comes under increasing pressure. Water under pressure boils at a much higher temperature. Thus, it is hot but not boiling in its reservoir. In the case of Wairakei, the water is about 240 degrees C. (554 F) at 1,000 feet below ground. But as the water rises to the surface in a bore hole, a natural spring, the boiling temperature is lowered. Within the bore the water begins boiling, creating steam, as it nears the surface.
The Wairakei field has been running since 1958 and is expected to continue producing electricity at least until the year 2015. "The economic life of a typical geothermal field is determined more by the life of the power station than the duration of geothermal activity," says Browne. The power stations usually fall apart before the geothermal activity stops.
AFTER mining the heat for so long, New Zealand has developed a considerable expertise, which it shares. As part of the country's foreign aid, the Geothermal Institute trains 30 students each year from such countries as Kenya, Ethiopia, the Dominican Republic, Panama, El Salvador, the Philippines, and Indonesia. These countries sit on top of volcanic belts and are increasingly finding heat mining attractive.
Geothermal energy provides 19 percent of the Philippines' power needs, 40 percent of El Salvador's requirements, and 10 percent of Kenya's electricity. In the United States, "The Geysers" provide a significant amount of electricity for northern Californians.
Importantly, geothermal is a "base load" source - it runs all the time. In fact, it is considered damaging to a field's health to shut it down. "It creates pressure fluctuations," explains Browne. The geothermal power is a real boon to New Zealand, which is currently experiencing a power shortage, because a drought is drying up the reservoirs that provide hydroelectric power.
Geothermal-energy production does not involve burning hydrocarbons or adding pollution to the atmosphere. Some of the older fields do release hydrogen sulfide (a rotten-egg smell) and carbon dioxide. In the newer fields, however, those gases are condensed and collected.
The Kiwis are also using some new techniques. For example, they are now injecting waste water into the aquifers to extend the life of the fields. This also stops pollutants from entering the river system and helps to alleviate any subsidence.
New Zealand is also producing steam in two stages - once at the well head and then a second time on the way to the power station.
"This way you get more steam and less water," says Browne. And, finally, the heat miners are using waste water, which is 170 degrees C. (347 F), to heat iso-butane, which boils at a lower temperature. The butane condenses and is liquefied, so it remains in a closed circuit. The steam from the iso-butane spins the turbines.