GEOLOGICAL SETTING OF GEOTHERMAL SYSTEMS: A PERSPECTIVE ON GEOTHERMAL SCIENCE IN THE 1960S AND 1970S

Geothermal science in 1960 in the United States was in its infancy. We knew that some major hot-spring systems were associated with young volcanic rocks, Yellowstone being the premier example. But each geothermal system seemed unique, and the mere existence of a young volcanic field did not guarantee that there was an associated geothermal system. Furthermore, some small volcanic fields (e.g., the Salton Buttes) accompanied huge geothermal systems, and some high-temperature geothermal systems (e.g., Steamboat Springs and Beowawe) had no obvious igneous source.

Development of the plate-tectonics paradigm in the 1960's provided the essential framework for geothermal exploration and resource assessment. But it was not until the 1975 UN Geothermal Symposium in San Francisco that this paradigm was widely accepted as a guide to geothermal exploration. In a general sense, by 1975 we had learned that high-temperature geothermal systems were associated with plate boundaries (particularly subduction zones and spreading ridges) and hot spots.

In great part stimulated by the 1973 energy crisis, extensive scientific investigations in the 1970s allowed plate-tectonic principles to be applied in finer detail and showed that there were specific plate-tectonic settings for geothermal systems. Most large geothermal systems indeed were associated with young volcanic fields, particularly those with silicic rocks. Yet it remained perplexing why some subduction zones (e.g., the Tohoku volcanic arc in Japan) supported major geothermal systems, whereas other subduction zones (e.g., the Cascade volcanic arc) supported few geothermal systems. It also remained a mystery why many high-temperature geothermal systems in conspicuous extensional and transtensional environments had no identified igneous heat source. Whether these systems are driven by deep circulation of meteoric water along faults or driven by a cryptic igneous source at depth is still a problem ripe for modern geophysical investigations.