“Birth and death. Like us, geothermal features begin and end, moving through cycles of their own. We draw towards them, lured by change, beauty, and an unusual cast of the familiar – water, rocks, and heat. We search them for answers to mysteries in our own lives, like birth and death.”

Susan F. Hodgson – 1995

1.1 Introduction

Geothermal energy – earth heat – can be found anywhere in the world. But the high-temperature energy that is needed to drive electric generation stations is found in relatively few places. The purpose of this opening chapter is to provide the geologic framework within which high-temperature geothermal resources can be understood, both with regard to their occurrence and their nature.

Readers who are unfamiliar with the rudiments of earth science may wish to consult any of the standard texts on the subject, e.g., Refs. [1–4]. Those interested in the history of geologic thought, dramatic geological events, and of ancient geothermal energy usage will find fascinating reading in Refs. [5–8]. W.A. Duffield provides an excellent, brief introduction to modern geologic theory of volcanoes in a beautifully illustrated book [9]. In selecting general texts on geology, one must be aware that any book written before 1970 will not include the most recent thinking on the structure of the earth and the dynamic mechanisms that give it its life. We refer to the theory of plate tectonics, now universally accepted, and which provides us with the basic tools to understand the origins of high-temperature geothermal resources.

1.2 The earth and its atmosphere

In 1915 A.L. Wegener (1880–1930) put forth a highly controversial theory of continental drift in the first edition of his book The Origin of Continents and Oceans [10]. Although he elaborated on it in later editions of his book in 1920, 1922 and 1929, the controversy persisted. His theory was motivated by the observation that the continents, particularly South America and Africa, seemed to be pieces of a global jig-saw puzzle that had somehow been pulled apart. He reasoned that all land masses were once connected in a gigantic supercontinent he named “Pangaea”. He posited that the now separated continents floated and drifted through a highly viscous sea floor. This part of his theory was later proved incorrect but the basic notion of drifting continents was right. Wegener’s problem was in identifying correctly the forces that ripped apart the pieces and in fact keeps them moving.

Studies that began in the 1950s and continued into the 1960s matched the ages of rocks found along the northeastern coast of South America and the northwestern coast of Africa [11]. The correlation of rock ages ran from Recife in Brazil to Trinidad off the coast of Venezuela on the South American side, and from Luanda to Sierra Leone on the African side. Oceanic research also showed that new land was being created on either side of the mid-Atlantic ridge, the so-called “sea-floor spreading” phenomenon [12]. By dating these deposits, earth scientists were able to confirm the movement of the vast plates that constitute the crust of the earth. Continents are part of the crust and have been in constant motion since the beginning of the earth some 4.5 billion years ago.

An excellent animation of this motion starting about 740 million years ago can be viewed at the web site of the University of California at Berkeley’s Museum of Paleontology [13]. From this animation it is clear that Pangaea existed as a supercontinent for only a blink of geological time, around 200 million years ago, having itself been formed from the collision of several land masses beginning in the Precambrian era.

While there is no controversy today over the theory of plate tectonics, there remains much uncertainty about the detailed structure of the inner earth. A great deal of research has gone into exploring and characterizing the earth’s atmosphere but only one or two projects have aimed at probing the depths of the earth. One of them, Project Moho, intended to drill through the thinnest part of the oceanic crust (about 5 km thickness) to enter the mantle. In 1909 Croatian scientist A. Mohorovičic (1857–1936) had observed, at a certain depth, a discontinuity in the velocity of seismic waves caused by earthquakes. He deduced that this represented a boundary between the generally solid crust and the generally molten mantle. This interface has become known as the Mohorovičic Discontinuity (or simply the Moho) in his honor [1]. However, Project Moho was halted in 1966 apparently for lack of funds and produced no results.

Another deep drilling effort, the Salton Sea Deep Drilling Program, ran from 1984–1988 with funding from the U.S. Dept. of Energy but failed to achieve much [14, 15]. One well was drilled to a total depth of 10,564 ft but suffered a collapsed liner at 6380 ft. Although this was later repaired, the deepest measurements were taken at 5822 ft and indicated a temperature of roughly 290°C. Neither the depth nor the temperature was particularly remarkable given the state of geothermal drilling at the time. At the conclusion of this effort, the following problems were cited as serious barriers to any future deep drilling program (to say, 50,000 ft): extremely high temperatures in the well, loss of control of the orientation of the well, lost circulation of drilling fluids, and fishing for equipment lost downhole.

Currently there is an international consortium of eleven countries called the International Continental Scientific Drilling Program [16] that funds projects to give insight into earth processes and to test geologic models. So far the deepest proposed well-drilling project received by the ICDP is for a 5000 m well in China; that well was reported to be at a depth of 3666 m on October 23, 2003 [17].

Thus our knowledge of the planet earth beyond a depth of a few kilometers is based on indirect evidence. What we accept as the model for the earth’s inner structure is burdened with uncertainty, particularly the temperature as a function of depth. Table 1.1 summarizes the model of the earth and its atmosphere; shown are the distances from the surface of the earth to each significant layer, the temperature thought to exist there, and the density. The crustal thickness is for continental areas; oceanic crusts are much thinner, about 7–10 km on average. The wide spread in the temperatures at the deepest levels reflects the speculative nature of these estimates.

These layers are usually depicted as concentric spheres, much like the inside of a golf ball, in ultra-simplified schematics. However, the interfaces are likely so irregular and the boundaries so fuzzy that such a representation is misleading.

Sometimes the analogy is drawn between the earth and a chicken’s egg, with the earth’s crust compared to the shell of an egg. Relating the thickness of the earth’s crust, 35 km for continental regions, to its diameter, roughly 12,700 km, we get a ratio of 35/12,700 or 0.00276. If we apply the same ratio to an egg with a diameter of say, 50 mm, we would find a shell thickness of 0.138 mm or 0.0054 in. In fact the shell of an egg is about 1/64 in or about 0.016 in. Thus an egg’s shell is about three times thicker proportionally than the crust of the earth. Put in other words, if the earth’s crust were in proportion to the shell of an egg, it would be about 100 km thick instead of 35 km.

Table 1.1 Data on the earth and its atmosphere from various sources; distances are not shown to scale.

Since the temperature at the base of the crust is about 1100°C, the temperature gradient between the surface (assuming a surface at 10°C) and the bottom of the crust is 31.1°C/km or about 3.1°C/100 m. This is usually taken as the normal conductive temperature gradient. Good geothermal prospects occur where the thermal gradient is several times greater than normal. The rate of natural heat flow per unit area is called the normal heat flux; it is roughly 1.2 X 10–6 cal/cm2·s, in non-thermal areas of the earth.

The earth’s crust is composed of various types of rock which contain some radioactive isotopes, in particular, uranium (U-235, U-238), thorium (Th-232) and potassium (K-40). The heat released by these nuclear reactions is thought to be responsible for the natural heat that reaches the surface. Table 1.2 lists three rock types and their radioactive constituents.

These basic ideas are enough for us to move on to explore how the motion of the tectonic plates creates the conditions favorable for the exploitation of geothermal energy.

Table 1.2 Radioactive elements in common rocks in the earth’s crust.

1.3 Active geothermal regions

The relative motion of plates, of any size, gives rise to several possible interactions. These are shown in Fig. 1.1.

When a plate comes under compression, it can...