Geothermal Power at Wairakei

Geothermal Power at Wairakei Michael P. McIntybe* New Zealand, in common with most other developed nations, is suffering an electrical energy shortage; foreign petroleum supplies are evanescent, domestic ones nonexistent, and ultimately there are physical limits or ecological barriers to expanding outputs based upon local coal, running water, or nuclear energy. But in New Zealand yet another and more esoteric source, geothermal power, has been utilized for nearly two decades and is currently undergoing significant enlargement. As an early entry into the geothermal power field the New Zealand experience may prove instructive in evaluating the world picture of geothermal power prospects and problems. The Geologic Setting Sited as it is atop the active southwest Pacific Tonga-Kermadec volcanic ridge, New Zealand would seem a reasonable prospect for geothermal exploration. Actually, present and potential developments are confined to a single region of the North Island, the 30-milewide Taupo Graben1 which extends for 150 miles northeastward from the massed volcanoes of Ruapehu, Ngauruhoe and Tongariro to White Island, 20 miles offshore in the Bay of Plenty (Figure 1 ). Vulcanism within and marginal to the Graben, continuing almost uninterrupted for perhaps one million years, is distinctive for the violence of its eruptive pattern (Figure 2) and hence the° Dr. Mclntyre is Professor of Geography at San José State University, San Jose 95192. 1 The Greater Taupo Graben is in reality a complex mosaic of blocks. At its northern end are three sharply defined lesser grabens: Rotorua, Tarawera, and Whakatane. See the geologic map in L. I. Grange, "Geothermal Steam for Power in New Zealand," New Zealand Department of Scientific and Industrial Research Bulletin No. 177, Wellington, 1955. 65 66 ASSOCIATION OF PACIFIC COAST GEOGRAPHERS While Island KAWERAU · Mt. TaraweraX •ROTORUAI BROADLANDS a¡K*'o BWAIRAKEI TAUpo I Kfflí·] Mt. Tortgariro Mt. Ngauruhoe ? M. fluapenu Figure 1. Maps of the New Zealand geothermal belt (bordered by dashed lines), showing its location on North Island (A) and selected features within the belt (B). YEARBOOK · VOLUME 38 · 197667 Figure 2. Natural steam plumes witliin the breached crater of White Island. Source: New Zealand Information Service. character of the éjecta.2 Typical was the eruption in 1886 of Mt. Tarawera, which blew away its crest and inundated a sizable district with coarse scoria and ash. The region is nearly devoid of dense flow lavas; even the "solid" rock consists either of breccias or massive ignimbrites (formed by incandescent clouds of ash welding upon deposition). More prevalent still are thousands of feet of unconsolidated "pumice" comprising ash, cinder, and scoria from repeated showers of minute solid éjecta.3 2 L. I. Grange, "The Geology of the Rotorua-Taupo Subdivision," New Zealand Geological Survey Bulletin No. 14 (Wellington: Government Printer, 1937), pp. 1-37. 3 J. D. McCraw, "The Geologic History of the Waikato River Basin," The Waters of the Waikato, Proceedings of the University of Waikato Extension Seminar, September 9-11 (Hamilton: University of Waikato, 1971), pp. 12-19, 21-22. 68ASSOCIATION OF PACIFIC COAST GEOGRAPHERS As a result, the general character of the strata is one of extreme porosity, allowing access of the abundant groundwater (the entire region receives a minimum precipitation of 40 inches per annum) to the heated rocks below, and providing a huge holding reservoir or aquifer for hot water and steam. The precise details of water movement and heating remain uncertain despite increasing numbers of drill cores for analysis and for gravity, seismic, and resistivity studies.4 However, the gross outline is one of a thin stratum of impervious lacustrine mudstone sandwiched between perhaps 200 feet of coarse and highly permeable pumice at the surface, and an essentially bottomless mass of tuff, pumice, and breccia. This last, the Waiora formation , is the active steam zone, and when the capping mudstone layer is faulted, as it is on a large scale in many parts of the Taupo Graben, geysers, fumaroles, and hot springs manifest themselves at the surface.5 Initial Utilization That anyone would choose to live in the Taupo Graben, with its smoking volcanoes, frequent tremors, and steaming fissures, seems dubious. Yet, to the ancient Maori this was a region of the gods and people who resided here were singularly blessed.6...