GEOLOGICAL SURVEY CIRCULAR 519 

Geothermal Energy 

Geothermal Energy 

By Donald E. White 

GEOLOGICAL SURVEY CIRCULAR 519 

Washington 1965 
United States Department of the Interior 

STEWART l. UDALL, Secretary 

Geological Survey 

William T. Pecora, Director 

First printing 1965 

Second printing 1966 

Free on application to the U.S. Geological Survey, Washington, D.C. 20242 
CONTENTS 

Page Page 

Abstract--------------------------- 1 Hydrothermal systems of composite 

Introduction------------------------ 1 type---------------------------- 9 

Acknowledgments------------------- 2 General problems of utilization ----- 10 

Areas of "normal" geothermal Domestic and world resources of 

gradient ------------------------- 2 geothermal energy--------------- 12 

Large areas of higher-than-"normal" Assumptions# statistics, and 

geothermal gradient--------------- 3 conversion factors--------------- 14 

Hot spring areas-------------------- 4 References cited___________________ 14 

TABLE 

Page 

Table 1. Natural heat flows of some hot spring areas of the world--------------------- 5 

III 

Geothermal Energy 

By Donald E. White 

ABSTRACT 

The earth is a tremendous reservoir of heat, most of which 

is too deeply buried or too diffuse to consider as recoverable 

energy. Some large areas are higher-than-"normal" in heat 

content, particularly in regions of volcanic and tectonic activity. Recovery of stored heat from these large areas may 

be economically feasible in the future but cannot compete in 

cost now with other forms of energy. 

Certain hot spring areas, commonly near active or recently 

active volcanoes, are cischarging heat at rates per unit 

area of 10-1,000 times the "normal" (1.5 x l<.i6 calories 

per square centimeter per second) heat flow of the earth; 

some of the largest and hottest areas have been explored 

for geothermal energy. These areas are characterized by 

high permeability, at least locally on faults, fractures, and 

sedimentary layers; this high permeability permits fluid 

circulation, most of the total heat flow being transported 

upward in water or steam. The circulation has produced 

reservoirs of stored heat closer to the earth's surface than 

is normally possible by rock conduction alone. Local nearsurface thermal gradients are typically very high, but the 

gradient decreases greatly, and even reverses, at greater 

depths in any single drill hole. 

Some other geothermal systems are here considered as a 

composite type. Near-surface caprocks are low in permeability, and temperatures in these rocks are therefore controlled dominantly by conduction. Beneath the insulating 

caprocks, permeable reservoir rocks may permit convective 

transfer of fluids and heat. Natural hot springs are small 

and relatively unimpressive in a composite system because 

little fluid escapes. Temperatures near the surface are low 

but increase steadily with depth, in contrast to hot spring 

systems that have vigorous near-surface circulation and, 

therefore, high near-surface temperatures. 

Some undiscovered geothermal systems may be composite 

in type. Because the near-surface permeabilities are so low 

that no fluid escapes, the principal evidence of such a system will be abnormally high geothermal gradients and heat 

flows distributed over a relatively large area. 

Geothermal energy stored in the upper 10,000 feet of 

many hot spring systems is 1,000-10,000 times the annual 

natural heat flow at the surface. Most of these hot spring 

systems, therefore, have appreciable age and stability relative to human activity. E:aergy is being recovered from some 

commercially developed hot spring areas at rates of five to 

more than 10 times their rates of natural heat flow prior to 

development. Such overdrafts, in at least some systems, can 

continue for many years, the excess heat being suppli-ed 

from the heat reservoir. Eventually, depending on the characteristics of each individual system, the effects of sustained overdraft must become evident. 

Present world utilization of geothermal energy is about 

1 million kw (kilowatts) or 7.5 x 1015 cal/yr (calories per 

year), and this amount can probably be increased 10-100 

times for at least the next 50 years. Potential reserves to 

depths of 3 km (kilometers) recoverable at or near present 

costs (l percent of total assumed recoverable) £.re estimated 

to be 2 x 1019 cal; resources to depths of 10 km recoverable 

at much more than present costs are estimated to be 1 x 1022 

cal. Of the total world resources of geothermal energy, probably 5-,..10 percent occurs in the United States; the areas of 

highest potential are in the Western States. 

INTRODUCTION 

Geothermal heat supplies only a very minor fraction of present domestic and world 

use of energy. In the United States. a geothermal steam powerplant generating 28,000 

kw (kilowatts) of electricity (McNitt 1963 

p. 14) is now operating at The Gey~ers i~ California, and many other areas are being 

actively explored; hot spring water is also 

used locally for space heating. Total utilization of geothermal energy in the world today 

is roughly equivalent to a production capacity of 1 million kw, and approximately half of 

this total has been developed in the past 15 

years. In the future, geothermal energy is 

expected to be of considerable local significance, and of a much greater total quantity 

than now. but it is not likely to rank as one 

of the major sources of energy. 

This brief survey of geothermal energy is 

hampered by a serious lack of reliable data. 

Scattered bits of information, generally consisting of a few temperature measurements, 

are available for individual wells; heat flow 

1 
2 GEOTHERMAL ENERGY 

has also been estimated or measured semiquantitatively in a few thermal spring systerns. 

In this report, four types of thermal systerns are considered; each is gradational into 

at least one of the other types: 

1. Areas of "normal" geothermal gradient 

and heat flow ... 

2. Large areas of higher-than-"normal" 

geothermal gradient and conducted heat flow. 

3. Hot spring areas, characterized in their 

upper parts by convective transport of most 

of the total heat flow in circulating water or 

steam. 

4. Composite hydrothermal systems involving in their upper parts convective and 

conductive heat transfer of types 2 and 3. 

Near the surface heat is transferred largely 

by conduction through rocks of low mass 

permeability that permit little or no discharge of water or steam. Temperatures 

immediately below the insulating layers are 

too high to be explained by conductive heat 

flow alone; circulating fluids are the indicated 

major agents of heat transfer. 

Types 3 and 4, grouped together as the hydrothermal types, are particularly important 

for geothermal energy because they provide 

heat reservoirs relatively near the surface 

and because they also insure a natural fluid 

for transferring heat from the reservoir to 

the powerplant. The importance of this natural fluid is considered in a later section of 

this report. 

ACKNOWLEDGMENTS 

The author is indebted to many individuals. 

Particularly noteworthy have been contributions of Gunnar Bodvarsson of Iceland and 

James Healy and John Banwell of New Zealand. Arthur H. Lachenbruch of the U.S. 

Geological Survey has been very helpful in 

many ways, specifically suggesting the concept of volumetric specific heat. The author's 

opportunities to study relationships at depth 

in many different geothermal systems were 

made possible through generous cooperation 

of domestic companies and individuals, but 

the author takes responsibility for the interpretations contained in this report. 

AREAS OF "NORMAL" GEOTHERMAL GRADIENT 

The earth as a whole is a tremendous reservoir of thermal energy but most of this· 

energy is too deeply buried or too diffuse tc 

justify consideration as an energy resource. 

Calculations of internal earth temperatureE' 

and of total quantity of heat are very different, depending upon assumptions concernin~ 

the origin of the earth, the total amount anc~ 

distribution of radioactivity, and other factors (Gutenberg, 1951; Birch, 1955; Jacobs, 

1956; Verhoogen, 1960; Wyllie and Tuttle, 

1960; Clark, 1961; Ringwood, 1962). Nevertheless, the general order of magnitude of 

thermal energy within the earth at depthE' 

now accessible by drilling, or that may become accessible, is of some value to the 

purpose of this report. 

The global average heat flow is about 

1.5 x 10-6 cal/cm2 /sec (hereafter referred 

to as 1. 5 heat-flow units); the calculated 

continental average from sparse data ir 

slightly higher (1.65 units) (Lee and MacDonald, 1963). 

The heat stored above surface temperatures in the outer 100 km (kilometers) of the 

earth is about 2 x 1028 cal.!/ This amount o:V 

energy is equivalent to the heat lost by conduction at the surface of the earth for nearl~ .. 

100 million years, to solar radiation re~ 

ceived by the earth for 10,000 years, to 

2 x 1022 kwhr (kilowatt hours); or to the heat 

content of 3 x 1ol8 short tons of coal. 

Wells have been drilled to depths of abou~ 

8 km and drilling can now attain a maximun1 

of about 10 km. Heat stored under the surface of the earth to a depth of 10 km is about 

3 x 1026 cal. Heat stored under the United 

States to the same depth is about 6 x 1024 cal. 

which is equivalent to conductive heat transfer from the United States for nearly 1 i million years, or to solar radiation received by 

the United States for approximately 200 

years; this heat is also equivalent to about 

5 x 1018 kwhr or the heat content of 9 x 10 14 

short tons of coal. 

The heat content of rocks in areas of 

"normal• geothermal gradient is an extremely low-grade source of energy. In th~ 

preceding calculation the heat is assumed to 

1 See section on • Assumptions, statistics, and conversion 

factors•. 
LARGE AREAS OF HIGHER-THAN-•NoRMAL• GEOTHERMAL GRADIENT 3 

be dispersed through about 100 million cubic 

kilometers of rock underlying the United 

States. The average heat content available 

in excess of surface temperatures is about 

60 cal/ cm3 of rock or approximately 20 cal/ 

gram in the outer 10 km of the earth. This 

is only 0.3 percent of the heat content of 1 g 

(gram) of coal. and less than 0.01 percent of 

the heat equivalent of fissionable uranium 

and thorium in 1 g of average granite. 

LARGE AREAS OF HIGHER-THAN-"NORMAL • 

GEOTHERMAL GRADIENT 

Major regional differences have been found 

in the U.S.S.R. (Khitarov. 1956, 1959. 1961). 

Kraskovskiy (1961) concluded that the heat 

flow from large areas of old crystalline rocks 

of the world is only about 0.9 unit. Heat flow 

has been determined recently at 39 localities 

in the islands comprising Japan and at 12 

places in the surrounding seas (Horai and 

Uyeda. 1963). The average heat flow on the 

islands of Japan is 1. 55 units and at sea 1.48; 

both of these figures are very close to the 

global average of about 1.5. Heat flow is 

higher ~2.00 units) than the Japanese average 

on the concave (western) side of the arc formed 

by the Japanese islands. and relatively low 

(<1.00 unit) on the Pacific Ocean side of northeastern Honshu. The Precambrian shield of 

Australia has an average heat flow of 1.0 unit. 

and areas surrounding the shield average 

about 2.0 units (Howard and Sass. 1964). The 

Hungarian basin. which is composed of Tertiary and older rocks. is an isolated geothermal high (about 2.4 units) surrounded by areas 

of nearly "normal" heat flow (Boldizsar. 

1964). Bodvarsson (1957) estimated a heat 

flow of 4. 7 units in those parts of Iceland that 

lack Quaternary volcanism. 

Prior to 1964. very little was published on 

regional variations in North America. Garland and Lennox (1962. p. 246) plotted the distribution of all values published prior to 1962. 

Heat flow from the Great Lakes region and 

the Precambrian Shield averages about 1.0 

unit for four localities. Seven localities in 

western North America have an average of 1.4. 

The structural trench of the Gulf of California extending northwestward from Mexico 

into the Imperial and Coachella Valleys of 

California is an example of a large area that 

has higher-than-"normal" heat flow. Von 

Herzen (1963) found a mean of 3.1 units for 

13 measurements in the Gulf of California; 

this mean is about 2 times the global average. 

Throughout the Imperial Valley abnormally 

high temperatures have been reported in all 

deep oil-test wells for which data are available. At least 10 wells from 4,000 to more 

than 13.000 feet deep have been drilled southeast to west of Salton Sea (not including geothermal exploration wells). Reported temperatures in the oil-test wells range from 

only slightly higher to about 3 times more 

than temperature increases expected from 

an average geothermal gradient. The sediments of this basin area are largely finegrained siltstones and claystones, and their 

thermal conductivities are probably somewhat lower than average. Nevertheless, a 

large area about 60 miles long and 2 5

1

' miles 

wide within the United States probably has 

an average heat flow at least 1! times 

"normal". 

A large "hot spot" on this broad geothermal 

high lies immediately southeast of Salton Sea 

and is a composite hydrothermal system, a 

type discussed in a following section. 

Although volcanic eruptions are now rare 

in the United States, except in Hawaii and the 

Aleutian Islands. many areas in the Western 

States have had extensive volcanic activity in 

the last few million years. Each of these 

areas is a likely place for much stored hea:t 

in excess of "normal," and some areas may 

still have molten magma at depths of less 

than 1 0 km. The available heat of granitic 

magma in excess of the mean surface ternperature is about 7 x 1017 cal/kms (See footnote 1), which is the heat equivalent of about 

100 million tons of coal. Many old magma 

chambers, now cooled and exposed at the surface by erosion, are t~ns to hundreds of cubic 

kilometers in volume; the chamber supplying 

the Steamboat Springs, Nev., thermal system 

is calculated to have a minimum volume of 

50 km3 (White, 1957a, p.1642) and an original 

heat content equivalent to at least 5 billion 

short tons of coal. 

Brown and Silver (1956) suggested that 

granite may someday be mined for the energy 

of its contained uranium. Average granite is 

very unlikely ever to be mined for its uranium 

content, perhaps 4 ppm (parts per million), 

but after higher grade energy ores are eKhausted, large volumes of rock with 10, 20, 

or 50 ppm of uranium may be utilized, as 

determined by future needs and economics. 

Similarly, if man ever has a critical need for 
4 GEOTHERMAL ENERGY 

I 

geothermal energy, he can s~arch for and find ' I " large areas of more than "Jl!ormal heat content. f 

! 

If the recovery of heat f:tl-om underground 

nuclear explosions is a possibility, the recovery of heat from magf!la chambers or 

from hot rocks is also lik~ly. Some of the 

problems of recovery are' considered in a 

later section. 

HOT SPRINGS A~EAS I 

i 

Certain "hot spots" of the earth, generally 

near areas of Recent or Pl~istocene volcanism, are discharging heat i at rates of 10 to 

more than 1, 000 times that i of areas of "normal" heat flow of comparable size. These 

are hot spring areas chara¢terized by physical transport of most of t~e total heat flow 

in water or steam. In suph areas, much 

higher temperatures existr at and very near 

the earth's surface than i!s possible where 

heat is transferred by roc~ conduction alone. 

One of the most useful parameters of a hot 

spring system is its natural rate of heat flow 

before commercial development. This parameter is a first approximation of the minimum rate at which heat can be withdrawn in 

water or steamin a geoth~rmal development 

(Benseman, 1959a; Bodvarsson, (1964a, b); it 

is also the most significant reference base 

for evaluating effects of. accelerated withdrawal from a developed geothermal field. 

In low -temperature spring systems from 

which all circulating water is discharged at 

the surface and none esqapes unseen below 

the surface, the approximate total heat flow 

is determined easily from discharge and temperature measurements~ Fukutomi (1962) 

found that conducted heat flow from such lowtemperature areas accounts for only 10 percent of the total and that discharging water 

accounts for about 90 percent. The total heat 

flow is much more difficult to determine 

where water escapes unseen below the surface {Benseman, '1959b), or where a large 

part of the total escapes in steam or by rock 

conduction {Benseman, 1959a; Dawson, 1964). 

Table 1 contains almost all estimates and 

measurements of total heat flow published 

through 1962 for hot spring areas of the world. 

Some data are only approximate, and others, 

such as the later figures for Wairakei, New 

Zealand, are based on extensive study; even 

here, agreement among authors is not notably 

close. Some of the Wairakei data suggest 

that large withdrawals of hot fluids from wells 

result in at least a temporary increase in 

total "natural" heat flow from fumaroles and 

springs and by rock conduction. This situation is the reverse of what might be expected, 

but the author has observed an increase in 

"natural" heat flow after geothermal development at Steamboat Springs, Beowawe, and 

Brady Springs, Nev. This phenomenon can 

occur in a hot water system whose temperatures at depth are close to the boiling points 

for existing pressures. If discharge of fluids 

from wells results in a decrease in water 

level or hydrostatic pressure, the new tern~ 

perature of boiling, at any given depth. iE' 

lower than the former temperature; excesS' 

heat previously stored in solid phases withir 

the zone of change is now available to vap.orize water. A temporary increase in floVI· 

of heat in activated fumaroles is eventually 

followed by a decrease if a steady-state relationship is again approached. The ne¥· 

steady-state ''natural" heat flow (from all 

sources other than production wells) shoulrl 

be less than the natural heat flow prior to 

development of the area. 

The large quantity of heat that has been 

stored in the heat reservoir of each hot spring 

system over a long period of time accounts 

for the fact that heat can be withdrawn fron1 

wells at a rate much greater than the natural heat flow. The heat content of the fluids 

does not decrease immediately if permeability and supply of fluid are adequate. 

Only a very few estimates have been made 

of the magnitude of the heat reservoirs for 

individual hot spring systems. In some systems of very low discharge that have s,? 

little convective transfer of heat that heat 

conduction is the major mode of transfer, 

temperatures will rise continuously wit~ 

depth {somewhat similar to fig. 3 of Donaldson, 1962). In most near-boiling hot sprin't 

systems, however, temperatures rise rapidly near the surface and then tend to leve:1, 

off as depth increases a few hundred fee:t 

(Banwell, 1963; Banwell and others, 1951; 

Bodvarsson and Palmason, 1964; White, 

1964); little further increase in temperatur?. 

is found within explored depths. The moE'~ 

reasonable explanation for the latter relations involves a large convection system cf 

water that circulates below explored depths 

and attains a characteristic base temperature dependent on rates of flow of water an-i 

heat in the system (similar to the models of 

Donaldson, 1962, figs. 2, 6). 
HOT SPRING AREAS 5 

Table 1.-Natural heat flows of some hot spring areas of the ~orld 

[Fstimates preceded by"'.are the author's and are based on published maps or other available data; othtlJ' estimates are by the original authors. 

Hydrothennal areas of composite type in parentheses are not included in totals] 

Area 

Qualibou, St. Lucia ___________ 

St. Vincent------------------

I>ominica ---------------- ___ Montserrat __________________ 

Total of country ______________ 

Northern belt, total--------

Southern belt, total_ ________ 

Ahuachapan group ________ 

El Playon de Ahuachapan _ 

Agua Shuca ------------

Savusavu -------------------1 

Steam fields, total heat flow __ _ 

Hengill. total _____________ _ 

Do--------------------

Hengill. southern part 

only. 

Torfajokull_ ___ -------- ___ _ Reykjanes ________________ _ 

Trolladyngja _____________ _ 

Krysuvik ----------------- Kerlingafjoll _____ -------- _ Vonarskard _________ ... ____ _ 

Grimsvotn ---------------- Kverkfjoll ________________ _ 

Askja NAmafjall ___ ~----------------- ________________ _ Krafla ___________________ _ 

Theistareykir ____________ _ 

Low temperature areas; about 

250 areas.4 

Six lines of thermal springs, 

each. , Reykjavik ________________ _ 

Reykir ___________________ _ 

Deilartunga line, total _____ _ 

Deilartunga spring-------

See footnotes at end of table. 790-001 0-65-2 

Maximum 

Approximate recorded Total 

sizel temper- heat flow3 

(km2) ature2 (106 cal/sec} 

(oC) 

British West Indies 

"'-J0.1 (S)185 

"'-J1 {S)>27 

,....,1 (S) 90 

"'-J.1 (S) 97 

El Salvador 

----------- --------- ----------- --------- ----------- --------- 80 (D)174 

"'-J.25 (S)boiling 

_,,25 (S)boiling 

Fiji Islands 

"'-J1 

50 

100 

1 

5 

10 

5 

? 

12 

10 

25 

2.5 

.5 

2.5 

-----------

1 (S)100 

Iceland 

(D)230 

{D)230 

(S)boiling 

? 

? 

(D)230 

{S)boiling 

? 

? 

? 

? 

? 

{S)boiling 

(S)boiling 

(D)146 

(S)100 

(D)146 

(D) 98 

(S) 100 

(S) 100 

8.6 

18 

17 

1.6 

200 

50 

>150 

80 

2 

{ 

630 

1,000 

5&-80 

2&-125 

28 

500 

&-25 

&-25 

&-25 

2&-125 

&-125 

12&-750 

2&-125 

&-25 

2&-125 

&-25 

&-25 

100 

&-25 

. 46 

.32 

1.7 

11 

2&-125 

24 

Source of data 

Robson and Willmore{1955). 

Do. 

I>o. 

I>o. 

Durr (1964). 

Do. 

Do. 

I>o. 

McBirney {1956) . 

I>o. 

IHealy (1960). 

Einarsson (1964). 

Bodvarsson {1964a). 

Einarsson (1964). 

Bodvarsson (1964a). 

Bodvarsson and Zoega 

(1964). 

Bodvarsson (1964a). 

I>o. 

Do. 

Do. 

I>o. 

I>o. 

Do. 

Do. 

Do. 

I>o. 

I>o. 

Do. 

Do. 

I>o. 

Bodvarsson and Zoega 

(1964). 

Do. 

Bodvarsson (1964a). 

Do. 
6 GEOTHERMAL ENERGY 

Table 1.-Natural heat nows of some hot spring areas of the world-Continued 

Area 

(Larderello) ___ ----- __ ---- ____ 

Ischia and Flegreian Fields----

(Monta Amiata) ---------------

Vulcano ______________________ 

Otaki, Ky\ishii ----------------

Atami, Shizuoka-ken ----------

Do ---------------------- Ito, Shizuoka-ken ------------· 

Obama, Nagasaki-ken---------

Beppu, Oita-ken -------------· Kawayu, Hokkaido _____________ 

Yunokawa, Hokkaido ___________ 

Yachigashira, Hokkaido ________ 

Shikabe, Hokkaido _____________ 

Toyako, Hokkaido ------------- Noboribetsu, Hokkaido: 

Hot Lake area, total_ ________ 

Jigokudani Valley (variable>-. 

Matsukawa, N. Honshu _________ 

Onikobe, N. Honshu ----------· 

Narugo, N. Honshu-----------· 

Pathe, Hidalgo ________________ 

Ixtlan, Michoacan -------------

Wairakei, 1951, 1952----------

1954 ---------------

1956? -------------- 1958, 1959----------

Waiotapu ____________________ 1958 ---------------• 

Orakei Korako _______________ _ 

Tikitere ---------------------

See footnotes at end of table. 

Maximum 

LA pproximate recorded 

si.zel temper-

(km2) ature2 

(oC) 

Italyfi 

("'50) ((D)240) 

I"J10 (D)296 

(I"J3) ((D)165) 

I"J1 (D)194 

Japan 

----------- (D)185 

5 (D)180 

----------- --------- ----------- --------- 1.5 (D)180 

,...,1Q (D)150 

.7 (S) 65 

I"J1 (S) 66 

? (S) 69 

"". 5 (D)113 

I"J3 (S) 55 

I"V•2 (D)112 

I"J.3 (D)160 

----------- (D)189 

""'80 (D)185 

----------- (D)175 

Mexico 

I"J2 (D)155 

----------- (D)150 

New Zealand 

7 (D)266 

7 (D)266 

7 (D)266 

7 (D)266 

7 (D)266 

"'15 (D)295 

""'5 (S)boiling 

5 (S)boiling 

Total 

heat fl.ow3 

(1Q6cal/sec) 

(5) 

? 

(?) 

? 

? 

16 

22 

44 

57 

19 

8 

4.0 

.5 

1.2 

2.2 

14 

_,6-11.2 

? 

? 

? 

? 

? 

133 

682 

143 

163 

101 

272 

130 

40 

Source of data 

(Composite type, see text; 

Burgassi, Battini, and 

Mouton., 1964; Boldizsar, 

1963). 

Penta and Bartolucci (1962). 

(Composite type, see text; 

Penta and Bartolucci, 

1962). 

Penta and Bartolucci (1962). 

Saito (1964). 

Watanabe (1958). 

Murozumi (1960). 

Do. 

Watanabe (1958). 

Do. 

Do. 

Fukutomi (1962). 

Do. 

Do. 

Do. 

Do. 

Do. 

Saito (1964). 

Do. 

Do. 

Penta and Bartolucci (1962). 

Anda, Isita Septien, and 

Ruiz Elizondo (1964). 

Banwell (1955, p. 50). 

Ellis and Wilson (1955). 

Benseman (1959a). 

Thompson, Banwell, Dawson, 

and Dickinson (1964). 

Fisher (1964). 

Benseman (1959a). 

Do. 

Ruiz Elizondo (1964; original data not accessible). 
HOT SPRING AREAS 7 

Table 1.-Natural heat flows of some hot spring areas of the world-Continued 

Area 

Tokopia·---------------------

Waikiti Ngatamariki ---------------------- __________________ Rotokaua _____________________ 

Ohaki ________________________ 

Taupo Spa ____________________ 

Kawerau (Onepu) 1959? --------

1962 ---------

Rotorua·---------------------

Seven scalding springs4 -------

Sulphur The Geysers Bank -----------------_________________ 

Wilbur Springs area ___________ 

Casa Diablo-Hot Creek ________ Alkali Lakes area ___________ 

(Salton Sea) ----------------

Steamboat Springs ------------

Bradys Springs---------------

Beowawe --------------------

Yellowstone Park, Wyoming----

Total, discharging water ____ _ Total, calculated9 __________ _ 

Norris Geyser Basin _____ _ 

Upper Geyser Basin ______ _ 

Mammot~Hot River------

See footnotes at end of table. 

Maximum 

Approximate recorded Total 

sizel temper- heat flow3 Source of data 

(km2) ature2 (106calfsec) 

(oc) 

New Zealand--<:ontinued 

----------- --------- 30 Ruiz Elizondo (1964; original data not accessible). 

----------- (S) 91 20 Do. 

""1 (S)hot 12.6 Gregg (1958) .. 

""5 (S)boUing 52 Do. 

~1 (S)boiling 12.8 Do. 

""3 (S)boiling? 36 Do. 

? (D)277 25 Healy (1964). 

? (D)285 18 C. J. Banwell (written 

commun., 1962). 

? (D)>160 

----------· 

Healy (1964). 

Union of South Africa 

? 1. 7 Kent (1950). 

UNITED STATES 

California 

"'1 (D)208 70.4 McNitt (1963). 

""2 (D)136 6.2 White and Roberson (1962, 

and unpub. data). 

""5 (S) 69 .4 White (1957b, p. 1675). 

>25(?) (D)180 670 (8) 

----------- --------- ----------· (""50) 

5 

"'2 

""3 

9,000 

""70 

t:.'l70 

""3 

~10 

""8 

(D,>270) (4) (Composite type, see text; 

White and others, 1963). 

Nevada 

(D)187 67 White (1957a and unpub. 

data). 

(D)168 ? Magma Power Co.,(written 

commun., 1963). 

(D)207 ? Magma and Vulcan Thermal 

Power Companies 

(written commun., 1961). 

Wyoming 

--------- (S) 138 ~---------- 207 !Allen and Day (1935, p. 136). 

(D)205 500 !White (this report). 

(D)205 8 !White (1957a, p. 1642). 

(D)180 90 Do. 

(S) 73 34 Do. 
8 GEOTHERMAL ENERGY 

Table 1.-Natural heat nows of some hot spring areas of the world-Continued 

Maximum 

Approximate recorded Total 

Area sizel temper- heat flow3 Source of data 

(km2) ature2 (106cal/sec) 

(oc) 

U.S.S.R. 

Pauzhetsk, Kamchatka _________ l"'oJ1 (D)195 1018 Averiev, Ivanov, and Piip 

(1960, p. 262); Piip, 

Ivanov,. and A veryev 

(1964). 

Total ~----------------------------------------- ~2,700 1The limits of a hydrothermal area are very difficult to define and meaningful criteria are 

difficult to apply. Depending upon the definition, the •umits• of an area can vary by at least an 

order of magnitude. The author's definition for use in this report is: 11The rather broad boundaries containing specific areas with some surface evidence for abnormally high temperatures at 

depth. The evidence can consist of one or more of the following: hot springs, fumaroles, active 

hydrothermal alteration, and abnormally high near-surface geothermal gradient. Closely spaced 

'hot spots' not separated by areas of approximately 'normal' gradient for the region are included in a single thermal area." 

2(S) indicates temperatures measured at the ~urface; (D) temperatures from drill holes. 

3From data of original author, commonly converted from other units. Most heat flows are 

relative to mean annual surface temperatures but a few are relative to 0° or 4°C; such differences are small compared to the uncertainties and have not been modified. 1 x 106 cal/cm 2fsec 

approximates the •normal" heat flow from 60 to 70 km 2. (Lee and MacDonald, 1963). 

4Heat transported in discharging water only. Other heat losses believed relatively small. 

5Heat flows of many below-boiling springs recently summarized by Uyhara (1963). 

6Heat flow computed from rate of discharge of chloride (or boron), on the assumption that all 

water has cooled from a reservoir temperature equal to maximum temperature measured (Ellis 

and Wilson, 1955; White 1957a). 

7Does not include heat losses by evaporation from ground-water body or by conduction from 

surface, which in total are considerably greater than discharging spring water. 

8Heat flow computed from rate of discharge of boron by method similar to that of Ellis and 

Wilson (1955) for chloride. Close observation of boron contents in Los Angeles Aqueduct supply 

over many years indicates a discharge of about 200 short tons of boron per year from the Casa 

Diablo-Hot Creek-Hot Lakes area, in water averaging 11 ppm of boron (Goudey, 1936; L. V. 

Wilcox, written commun., 1957; G. I. Smith, oral commun., 1962). A discharge of 520 1/sec, or 

8, 300 gpm, is indicated for the total system; temperatures to as much as 180°C have been 

measured in wells at Casa Diablo (Magma and Endogenous Power Companies, written and oral 

commun., 1960, 1961), but a conservative reservoir temperature of 150°C is used for present 

heat-flow calculations. During and since the late Pleistocene, most of the boron from this system was incorporated in saline deposits of Searles Lake. Because of this, detailed studies of 

Searles Lake by G. I. Smith (oral commun., 1962) furnished a basis for concluding that the hot 

springs have been in existence for at least 50,000 years. 

9Discharge of 2,900 1/sec of water according to Allen and Day (1935); all discharged waters 

are assumed to have cooled from 180°C at depth. This temperature is almost certainly incorrect 

for the low-temperature Mammoth-Hot River waters (700 1/ sec) but is probably more than offset by large areas of steaming ground that have little or no discharge. 

lOCalculated from discharge of 100 1/sec and heat content at depth, 17Q-190 cal/g. 

11Approximate total heat flow from those areas for which estimates have been made, excluding 

composite type in parentheses. An average has been assumed for those areas that have more 

than one estimate. 
DOMESTIC AND WORLD RESOURCES OF GEOTHERMAL ENERGY 9 

Most hard competent rocks are more highly 

fractured near the surface than at depth; therefore, mass permeabilities tend to increase 

upward, and active circulation in a hydrothermal system occurs up to the water table, 

which may be at or below the ground surface. 

The water rises by convection within the core 

of each system because of the lower density 

of the hottest fluid. Little change in temperature occurs in the rising water except where 

boiling occurs near the surface as hydrostatic 

pressure decreases. This picture is no doubt 

oversimplified for many systems but is supported by much of the evidence obtained from 

geothermal exploration to date; the assumed 

system provides a crude means of estimating 

the total heat content of a reservoir if the 

area underlain by the reservoir can be estimated and if a characteristic ''leveling-off," 

or base, temperature is ascertained by drilling. 

Rocks differ greatly in specific heats, depending on their composition, porosity, and 

degree of water saturation. This variance 

has made calculation of heat content of geothermal reservoirs by the usual methods impractical. Arthur H. Lachenbruch, U.S. Geological Survey, pointed out the important 

concept (oral commun., 1962) that specific 

heats tend to vary inversely with specific 

gravity and that most natural substances have 

volumetric specific heats that are similar 

enough to be useful for present calculations. 

Volumetric specific heats differ somewhat 

with temperature; in this report, 0. 6 cal/cm 3 

is assumed for temperatures near 100°C, 0. 7 

cal/ em 3 near 300°C, and 0. 8 cal/ cm3 near 

500°C. 

By use of these concepts of volumetric 

specific heat and a reservoir temperature 

indicated by the ''leveling-off" temperature, 

White (1964) estimated a minimum of 1.6 x 10 18 

cal of heat stored in the Steamboat Springs 

reservoir, Nevada, beneath an area of 5 sq 

km (square kilometers) and to a depth of 3 

km. This storaged heat is equivalent to present natural heat flow for 7,000 years. Similar calculations for the Upper Geyser Basin 

of Yellowstone National Park, assuming the 

area and temperature listed in table 1, indicate a heat reservoir of 3.6 x 1018. Heat 

content of a geothermal reservoir calculated 

by this method is likely to be a minimum 

content because the area of surface expression is commonly smaller than subsurface 

dimensions of hot ground and because hotter 

convection cells may underlie the explored 

cell. 

By use of different assumptions and methods of calculation, C. J. Banwell (written 

commun., 1962) found a tota1 heat content of 

2.4 x 1018 cal to a depth of 2.3 km under the 

most active Wairakei area of 6.28 km2. If 

this reservoir is assumed to extend to a 

depth of 3 km, a common base of other calculations of the present report, its heat content is about 3. 2 x 1018 cal. 

Bodvarsson ( 1964a) and Bodvarsson and 

Palmason (1964) have published estimates of 

recoverable heat content and efficiencies of 

recovery of heat from Iceland's geothermal 

reservoirs that lead to an estimate of about 

1.1 x 1019 cal for total heat content. Specific 

dimensions assumed in these calculations are 

not stated. 

The hot spring areas given in table 1 that 

show both surface area and heat flow are estimated to have a total area of about 500 km2. 

and a total heat flow of 2 x 10 9 cal/ sec or 

6 x 10 16 cal/yr (composite type not included 

here). Heat stored to a depth of 3 km in these 

hot spring systems-assumed to have an average reservoir temperature of 180°C above 

surface temperature-is about 2 x 10 2o cal. 

Much additional heat is no doubt stored at 

depths greater than 3 km and at higher temperatures but is not included in these estimates. Vertical extension of the systems with 

depth is assumed here but many, and perhaps 

most, hydrothermal systems have a horizontal 

component of up-flow controlled by waterconducting structures. 

The total stored heat of these crudely 

"measured" hot spring systems (2 x 1020 cal) 

divided by their present natural heat flow of 

6 x 10 16 cal/ yr indicates an average age of at 

least 3,000 years. If heat stored at greater 

depths and heat discharged at the surface in 

the past are also considered, the average 

spring system must be at least 10,000 years 

old. 

HYDROTHERMAL SYSTEMS OF COMPOSITE TYPE 

The Salton Sea geothermal area and some 

of the Italian areas previously mentioned are 

characterized by low natural rates of discharge of water or steam at the surface. 

Drilling in these areas has demonstrated that 

temperatures near the surface are only 
10 GEOTHERMAL ENERGY 

slightly above atmospheric temperature but 

that they increase rather steadily with depth 

at rates perhaps 5-10 times "normal." In 

many of the Italian areas and also in the Salton Sea geothermal field~ near-surface rocks 

are shale and other fine-grained sediments 

of low mass permeability; these rocks inhibit 

circulation of fluids. Temperatures of 250° 

to more than 300°C are characteristic of some 

of these areas at depths of several thousand 

feet. If these thermal systems were controlled entirely by rock conduction, temperatures close to the initial melting point of 

granite (600°C or higher, depending on water 

content) could be expected at about 10,000 ft. 

Deep drilling in Tuscany,. Italy~ has demonstrated that rocks below the capping shale 

are permeable and that temperatures show 

little additional increase with depth. The only 

reasonable explanation for such data is that 

heat is transferred through the intermediate 

permeable zone by circulating fluids. 

In the Salton Sea area~ details of temperature distribution have not been released by 

the companies involved, but the very high 

geothermal gradients of the upper 3,000 feet 

are unlikely to persist to much greater depths 

(White and others, 1963). The demonstrated 

fluid-producing capacity of recently drilled 

geothermal wells is indisputable evidence for 

at least local high mass permeability. Convective transfer of heat by circulation of the 

saline brine of the area is likely to be very 

important if permeable channels are interconnected in a three-dimensional system. 

The heat content of the Salton Sea geothermal reservoir is, by rough calculation, at 

least 2 x 1019 cal. The volume here assumed 

is equivalent to a cylinder 7 km in diameter 

and 3 km in thickness, having an average 

temperature of 250°C, and an average volumetric specific heat of 0. 7 cal/cm3. This heat 

reservoir is about 6 times larger than that of 

Wairakei, one of the largest of the explored 

hot spring systems, primarily because of the 

larger surface area assumed for the Salton 

Sea reservoir. 

Heat flows m·easured across the Larderello 

geothermal area in Italy (Boldizsar~ 1963) 

range from 6 to 14 heat-flow units~ 4-10 times 

the global average (Lee and MacDonald~ 1963). 

If an average of 7 times "normal" or 10 heat 

flow units is assumed for the conducted heat 

flow from the exploited 50 km 2 of the Larderello field (Burgassi and others, 1964), the 

total conducted heat flow of the field is 5 x 10 6 

cal/ sec. The proven size of the Salton Sea 

geothermal field is somewhat less; if a surface area of 40 km2 and a heat flow of 10 

units is assumed, the total conducted heat 

flow of this field is 4 x 1 Q6 cal/ sec. 

The heat flow from each of these composite 

hydrothermal areas is less than that from 

many hot spring systems having permeable 

upper parts (table 1). Near-surface rocks 

of low· mass permeability are evidently very 

effective in insulating a geothermal reservoir 

and in fostering high tempe1·ature and large 

size. 

Some geothermal systems of composite 

type in very tight rocks may have little or 

no upward leakage of water or gases. The 

principal evidence useful in discovering such 

a system will be abnormally high geothermal 

gradients and heat flow, distributed over a 

large area. The optimum environment for 

a composite geothermal system is a large 

heat source, such as a magma chamber at 

depth; permeable rocks, preferably at depths 

of more than 2, 000 feet, to serve as the heat 

and fluid reservoir; and a cap of impermeable 

rocks to provide an insulating cover. 

GENERAL PROBLEMS OF UTILIZATION 

At the present time, the geothermal areas 

of the earth that can most hopefully compete 

economically with other sources of energy 

are the "hot spots" that discharge steam or 

hot water as the heat-transporting medium 

or that have a permeable reservoir of hot 

fluids beneath near-surface rocks of low 

mass permeability. The essential requirements are relatively high temperature and 

permeable structures that can yield heattransporting fluids to a well. Hot volcanic 

rocks or magma chambers at or near the 

surface but lacking heat-transporting fluid 

are also possible commercial sources of 

energy; however, effective recovery of heat 

from such reservoirs has additional problems not yet solved commercially. 

The very low thermal conductivity of rocks 

imposes a restriction on effective recovery, 

especially if mass permeability is also low. 

The heat immediately adjacent to a well 

drilled in impermeable rocks can be withdrawn easily by circulating a cold fluid, but 

the amount of heat recovered is small and 

steadily decreases as rocks adjacent to the 

well are cooled. This low yield of heat is 

comparable to the low flow of an oil-test well 
GENERAL PROBLEMS OF UTll.IZATION 11 

in impermeable shale of high fluid hydrocarbon content. In contrast, large amounts of oil 

are obtainable from a single well in an oil 

pool in very permeable rocks. The possibilities for finding or creating high thermal 

•permeability• in reservoir rocks of low 

mass permeability seem limited to the following: 

1. Each well is drilled deep enough to intersect one or more highly permeable channels or aquifers that# in total# will yield commercial quantities of the hot water already 

present in the rocks; however# in most crystalline rocks and many sedimentary rocks, 

permeable channels are narrow or widely 

spaced and are likely to be less numerous at 

great depth than near the surface. 

2. The rocks adjacent to a well are artificially fractured to such an extent that the 

heat of a large volume of rock can be tapped. 

Natural permeable channels in the surrounding area must be made accessible to the 

newly broken ground to permit adequate discharge of existing deep water# or a transporting fluid must be introduced from the 

surface to recover the heat. Fracturing by 

buried nuclear explosion has also been proposed (Carlson# 1959), the nuclear energy 

providing additional heat. 

.Most deep waters are saline and are likely 

to contain high but uneconomic concentrations of elements such as boron# arsenic, 

sodium# and chloride; these elements constitute disposal problems. Some elements~ 

including boron~ potassium~ lithium~ cesium~ 

and iodine have been considered for recovery to pay for part of the costs1 but actual 

quantity and value of these elements are 

generally much below costs of recovery. 

The Salton Sea area of California is an outstandingly promising exception (White and 

others~ 1963)1 and other similar areas may 

be found. 

In some developed geothermal fields~ heat 

in being withdrawn in considerable excess 

qver the natural heat flow. Computed rates 

of withdrawal are four times the natural flow 

for Wairakei1 New Zealand (Banwell1 1964a)1 

three times for Reykir# four times for Hengill# and nine times for Reykjavik# Iceland 

(Bodvarsson and Zeoga, 1964)1 and at least 10 

times for Larderello (calculations based on 

data from Boldizsar, 1963). McNitt (1963) 

estimated an excess withdrawal of 170 times 

for The Geysers# Calif. This excess over the 

natural heat flow is far greater than the indicated overdrafts of other geothermal areas; 

thus~ additional studies should be made. 

The quantity of stored heat that can be withdrawn from a reservoir without serious decline in quantity or quality of the heat-transporting fluids will be very different for each 

area, depending on the magnitude of the whole 

reservoir, the overdraft relative to natural 

heat flow, and the physical characteristics of 

the reservoir rocks. Yield from a large volume of rocks of high porosity and mass permeability, as in much of the Wairakei and 

Larderello fields, should decline less quickly 

than that from a heat reservoir in crystalline 

or sedimentary rocks of low permeability. 

In crystalline and sedimentary rocks, circulation of fluids is localized in faults, fractures, or the more permeable sedimentary 

layers; the total surface area of rocks in direct contact with migrating Uuids thus is relatively small, and recoverable stored heat of 

the reservoir must be transferred to the circulating fluids by conduction through relatively large distances. 

Bodvarsson and Palmason (1964) suggested 

that recovery of only 10 percent of the stored 

heat is a reasonable assumption for Iceland's 

heat reservoirs, which consist largely of 

plateau basalts having few permeable channels. Banwell (1964a), however, concluded 

that recoveries of 7Q-90 percent can be 

achieved over drawoff times of 20-100 years~ 

provided that channel spacing is less than 

about 200 feet. Banwell's theoretical discussion of this subject is very useful, but several 

of his important assumptions need comment: 

1. Efficiency and percent of heat recovery 

refer onlyto heat stored above 1000C (rather 

than to that stored above mean surface ternperature, which is commonly used by others). 

Banwell's calculated efficiencies are therefore correspondingly high. 

2. Banwell's idealized models assume all 

faults and fractures to be so permeable that 

permissible rates of fluid withdrawal or 

circulation have no restrictive upper limit; 

his model also assumes a uniform rate of 

flow across all surfaces. 

Geologists familiar with permeability variations along veins, faults# and fractures in ore 

deposits will recognize the danger of accepting the latter assumption without modification. Flow rates are not stated for Banwell' s 
12 GEOTHERMAL ENERGY 

models, but if 80 percent of stored heat is 

to be recovered in 20-100 years from a 

reservoir of heat content equal to at least 

1,000 years of natural heat flow, fluid must 

be withdrawn at rates at least 10-50 times 

that of the natural discharge of the system. 

Any increase in circulation rate will be localized along interconnected channels of highest 

permeability and will result in excessive 

withdrawal of stored heat adjacent to these 

few channels and eventually in degradation 

in heat content of the recovered fluids. 

A hot spring system prior to development 

probably approximates a steady-state equilibrum relative to its recharging water supply, the pressure drive from recharge to discharge parts of the system, and frictional 

resistance along its channels of circulation. 

A highly permeable spring system that has 

unlimited recharge potential will have achieved a steady state characterized by very 

high discharge and relatively low temperature; the latter is a consequence of the low 

thermal conductivities of rocks. A high -ternperature geothermal system, on the other 

hand, can persist for thousands of years only 

if the recharge potential or the permeability 

of the most restrictive parts of the system 

is low. If the most impermeable part of a 

geothermal system is near its discharge end, 

as seems probable for the Larderello and 

the Salton Sea fields, wells can be drilled 

through the caprocks into the underlying 

more permeable rocks and the rate of 

through-flow of fluids can be increased significantly. In a thermal spring system in 

competent crystalline rocks, however, high 

permeability is likely to occur near the surface, and channels of lower permeability are 

characteristic of deep parts of the system. 

Compare, for example, the open textures of 

most shallow epithermal ore deposits with 

the tight structures of most mesothermal and 

hypothermal ore deposits. The tightest restrictions or bottlenecks, along channels of 

flow are likely to be deep in a hot spring system, below the shallow reservoir. Production 

at rates that exceed the natural discharge of 

fluids may prove to be short lived. Water 

levels, temperatures, and pressures are then 

likely to decline, and the accelerated withdrawal may be balanced only in small part by 

increased circulation through the system. 

The hydrodynamics of hot spring systems 

are very complicated and not yet well understood. Each system differs in at least some 

respects from all others. Until testing proves 

otherwise, it is hazardous to assume that 

the natural discharge of many hot spring 

systems can be as much as doubled for more 

than short periods of time. In the testing of 

a new field, nonproducing observation wells 

are essential to monitor and evaluate changes 

in water level, temperature, pressure, and 

water composition. The adequacy of fluid 

supply is likely to be more critical than 

adequacy of the heat reservoirs in limiting 

future utilization of geothermal energy. 

Other engineering problems related to 

utilization of geothermal energy are discussed 

in excellent reviews by Smith (1964; Bodvarsson 1964b). Total cost of geothermal power 

is presently considered to be about 8 mills 

per net kwhr in Iceland, and about 4.5 mills 

in New Zealand. G. Facca and A. Ten Dam 

(written commun., 1964), computed total generating costs for Italy as about 2.4-3.0 mills 

per net kwhr. At The Geysers, Calif., the 

cost of steam delivered at the powerplant is 

2.5 mills per net kwhr of electric energydelivered to the transmission line (Bruce, 1964), 

but the overall cost, including power generat:i.on, has not been published. 

Corrosion of equipment and deposition of 

mineral matter constitute problems in utilizing the heat of many thermal areas (Smith, 

1964). Corrosionislikely tobe more serious 

than deposition in the dry steam areas, but 

this problem evidently has been solved or 

adequately controlled in Italy and at The Geysers, Calif. Deposition of calcium carbonate 

or silica is characteristic of many, if not 

most, of the hot water areas. The chemistry 

of deposition has been discussed by Ellis 

(19G4), White (1964), and others. 

DOMESTIC AND WORLD RESOURCES OF 

GEOTHERMAL ENERGY 

The total amount of heat above surface ternperatures stored in the outer 100 km of the 

earth, as mentioned previously, is on the 

order of 2 x 1028 cal, which is equivalent to 

the heat content of 3 x 10 18 short tons of 

coal. Heat stored under the United States to 

a depth of 10 km is about 6 x 10 24 cal or equivalent to the heat content of 9 x 1 0 14 short 

tons of coal. Most of this heat is in areas of 

"normal" geothermal gradient and is contained in rocks that are extremely low-grade 

sources of energy. 
DOMESTIC AND WORLD RESOURCES OF GEOTHERMAL ENERGY 13 

Areas of abnormally high geothermal gradient can be classified into three types, at 

least in part overlapping: (1) areas in which 

geothermal gradient is significantly higher 

than "normal" but where notable hydrothermal 

activity is absent; in some of these areas, 

molten magma may exist within accessible 

depths; (2) hot spring areas, in which natural 

thermal fluids are discharged at the surface; 

and ( 3) hydrothermal areas of composite type 

that have little surface expression but with 

high-temperature fluids that existing beneath 

capping rocks of low permeability. 

In the first type of geothermal area, heat 

contents and thermal conductivities are so 

low, and drilling costs for very deep wells 

so high, that heat is probably not recoverable 

economically even at 10times present values. 

In the future, with research and experience, 

some energy probably can be recovered from 

heated rocks or magma if the value of the 

energy is sufficiently high, but no attempt is 

made here to compute resources of this type. 

The hydrothermal areas mentioned including both types 2 and 3 offer the gr:atest 

immediate possibilities for economic development because high temperatures occur 

relatively near the surface and hot natural 

fluids are present as energy-transporting 

media. Ge..1thermal heat is now competing 

successfully with other sources of energy in 

Italy, New Zealand, Iceland, and The Geysers 

in the United States. Experience at some 

localities indicates that heat can be withdrawn at rates of four to more than 10 times 

the natural rate of heat flow for at least 10 

years without serious effect. The heat withdrawn in excess of the natural flow is derived 

from reservoirs of previously heated rocks. 

As mentioned previously, however, many 

other geothermal areas, especially of the 

hot-water type in rocks that are at least in 

part of low mass permeability, have not been 

tested adequately; serious degradational effects may become evident in shorter times 

and at lower withdrawal rates. 

The total heat flow from hot spring areas 

for which estimates of natural heat flow have 

been made(table l)is about2.7 x 109 cal/sec 

(or 1 x 1017 cal/yr). Table 1 is probably reasonably complete for Iceland, New Zealand, 

and El Salvador but in most other countries 

only a few of the known areas are included. 

Almost none of the thousands of hot spring 

areas of low to moderate surface temperature (Waring, 196 5) are considered, and hydrothermal systems of the composite type 

that are much more difficult to recognize 

are very inadequately represented. The 

world's total natural heat flow from all hydrothermal areas is probably at least 10 

times that of the estimated areas, or about 

3 x 1010 cal/ sec (or 1 x 1018 cal/yr). 

The total stored heat of the crudely "measured" hot spring systems of table 1 (excluding the composite type) is estimated to be 

about 2 x 1o20 cal to a depth of 3 km. 

Total heat stored in other hot spring systems is difficult to estimate. Some systems 

of high discharge and low temperature may 

have less stored heat than comparable volumes of "normal" earth. Others may prove 

to be hydrothermal systems of composit~ 

type, having meager surface discharge of 

fluid but high geothermal gradients in nearsurface caprocks of low permeability. 

Heat reservoirs of hydrothermal systems 

of the composite type may be relatively 

large because their heat has been largely 

preserved by good natural insulation rather 

than dissipated by convective discharge at 

the surface. The heat content of the Salton 

Sea geothermal area, as previously stated, is 

crudely estimated to be 2x 10l9cal to a depth 

of 3 km (or 10,000 ft), and the Larderello 

system may be similar in magnitude. No 

sound basis exists for predicting how many 

large undiscovered systems of the composite type exist in the world, but 40 of the magnitude estimated for the Salton Sea area is 

not unreasonable. In addition, some hot 

spring systems of shallow circulation may 

be underlain at intermediate depth by rocks 

of low mass permeability, and these rocks 

in turn may be underlain by one or more 

circulation systems of composite type. 

A very crude estimate of 2 x 1021 cal is 

suggested as the total stored heat of all hydrothermal systems to depths of 3 km, and 

1 x 1022 cal to 10 km. 

Geothermal energy has been utilized for 

such a short period of time that estimates of 

the proportion of heat commercially recoverable from hydrothermal reservoirs are of 

doubtful value, but l percent is viewed as a 
14 

conservative estimate. On this basis~ estimated recoverable reserves are: 

Potential reserves to depths 

of 3 km~ recoverable at or near present costs ________ _ 

Additional resources to depths 

of 10 km, recoverable at 

much more than present costs ____________________ _ 

Heat content 

(calories) 

2 X 1019 

1 X 1022 

Existing worldwide utilization equivalent 

to about 1 million kw~ or 7. 5 x 1015 cal/yr, 

probably can be increased at least 10 times 

under present economic conditions and maintained for at least 50 years. The author may 

be too conservative in these estimates, but 

many technical and scientific problems are 

not yet solved, and great optimism is not 

warranted until these problems clearly can 

be solved. Although geothermal heat is now 

only of minor importance as an energy 

source, the quantities available in hydrothermal systems are large and with additional research and development, recovery 

is likely to incre~se greatly. 

The western half of the conterminous 

United States, Hawaii, and parts of Alaska 

are particularly favorable for local concentrations of geothermal energy. Of the total 

global resources of geothermal energy underlying land areas, probably 5-10 percent 

is in the United States. 

ASSUMPTIONS, STATISTICS, AND CONVERSION 

FACTORS 

Surface area of earth, 5. 1 x 1018 cm2; surface area of United States, 9.3 x 1(}16 cm2. 

Assumed temperature at 100 km depth, 

1, 100°C; average temperature of the outer 

100 km of earth, 550°C above surface temperature; average volumetric specific heat, 

0.8 cal/cm3; latent heats of any phase 

transitions are disregarded. 

Temperature gradient of outer 10 km, 20°C/ 

km (or 1 °C/ 160 ft); average temperature 

in the outer 10 km, 100°C above average 

surface temperature; average volumetric 

specific heat assumed 0.6 cal/cm3. The 

assumed gradient is probably too low for 

orogenic areas and is almost certainly too 

high for stable continental areas having 

low heat flow and higher than average thermal conductivities. 

Globalaverage heat flow about 1.5 x 10-6 cal 

/cm2 /sec or 2.5 x 102° cal/yr from the 

earth's surface. For geothermal considerations, heat flow in the range of 0.8-

2.0 x 10-6 cal/ cm2 sec may be considered 

within the "normal" range. Thermal conductivities of most rocks range from about 

3 to 10 x 10-3 cal/ sec/ em oc. Within these 

extreme limits, temperatures could increase with depth at rates ranging from 

50 to 410 feet per 1°C; the average is about 

160 feet per degree. 

1 cal (mean)=O.OOl kcal (kilocalories)=0.00116 

watt hr = 1.16 x 10-6 kwhr = 0.00397 BTU 

(British thermal unit) (mean)= 4.186 joules. 

1 year= 365 days= 8, 760 hrs = 5.26 x lOS min 

= 3.5 x 107 sec. 

Coal is assumed to have a heat content of 

13,000 BTU/lb = 6.5 x 109 cal/short ton= 7.2 

x 103 cal/g. 

Average rock of outer crust is assumed to 

contain 4. ppm uranium and 12 ppm thorium; 

uranium and thorium in 1 ton of such rock 

is equivalent in heat content to 50 tons of 

coal (Brown and Silver, 1956, p. 95). 

Solar energy striking earth, 17 x 1014 kw or 

1.3 x 10 24 cal/yr (Weinberg, 1959); United 

States assumed to be average. 

Heat available from granite magma, assumed 

liquid at 900°C, then crystallizing and cooling ~0 5000C, is about 17 5 cal/ g or 4. 7 x 1ot'T 

cal/km3 • If cooled to mean earth-surface 

temperature, nearly 300 cal/ gm or 7 x 1017 

cal/km 3 is available. Heat content of molten basalt at 1.1 OOOC is· about 37 5 cal/g, 

relative to surface temperatures. 

REFERENCES CITED 

Allen, E. T., and Day, A. L., 1935, The hot 

springs of the Yellowstone National Park: 

Carnegie Inst. Washington Pub. 466, 525 p. 

Anda, L. F. de, Isita Septien, J., and Ruiz 

Elizondo, J., 1964, Geothermal energy in 

Mexico in Geothermal energy, I: United 

Nations Conf. New Sources Energy, Rome 

1961, Proc., v. 2, p. 149-165. 

Averiev, V. V., Ivanov, V. V., and Piip [Piyp], 

B. I., 1960, Problems of using volcanic 

thermae of the Kurile- Kamchatka· Island 

arc for power: Bull. Volcano!, ser. 2, v. 

23, p. 257-263. 

Banwell, C. J., 1955, Physical investigations, 

Chap. 6 of Grange, L. I., compiler, Geothermal steam for power in New Zealand: 
REFERENCES CITFD 15 

New Zealand Dept. Sci. and Indus. Research 

Bull. 1176 p. 45-74. 

Banwell6 C. J. 6 19636 Thermal energy from 

the earth's crust. Introduction and Pt. 1: 

New Zealand Jour. Geology and Geophysics6 v. 66 p. 52-69. 

---1964a6 Thermal energy from the earth's 

crust. Pt. 2: The efficient extraction of 

energy from heated rock: New Zealand 

Jour. Geology and Geophysics# v. 76 p. 585-

593. 

---1964b6 Geothermal drillholes; Physical 

investigations1 in Geothermal energy6 I: 

United Nations Conf. New Sources Energy6 

Rome 19616 Proc.6 v. 26 p. 6D-72. 

Banwell6 C. J. 6 Cooper6 E. R.6 Thompson6 G. 

E. K. 6 and McCree6 K. J. 6 19576 Physics of 

the New Zealand thermal area: New Zealand Dept. Sci. Indus. Research Bull. 1236 

109 p. 

Benseman6 R. F. 6 1959a6 Estimating the total 

heat output of natural thermal regions: 

Jour. Geophys. Research6 v. 646 no. 86 p. 

1057-1062. 

---1959b6 Subsurface discharge from thermal springs: Jour. Geophys. Research6 v. 

646 no. 86 p. 1063-1065. 

Birch6 Francis6 19556 Physics of the crust6 in 

Poldervaart, A., ed.6 Crust of the earth--a 

symposium: Geol. Soc. America Spec. Paper 62, p. 101-117. 

Bodvarsson6Gunnar6 19576 Geothermal effects 

of the Pleistocene glaciation in Iceland: 

Reykjavik6 Iceland6 Jokull6 v. 76 p. 1-20. 

---1964a6 Physical characteristics of natural heat resources in Iceland, in Geothermal energy6 I: United Nations Conf. New 

Sources Energy6 Rome 1961, Proc.6 v. 26 

p. 82-90. 

---1964b, Utilization of geothermal energy 

for heating purposes and combined schemes 

involving power generation, heating, and/ 

or by-products6 in Geothermal energy, II: 

United Nations Conf. New Sources Energy, 

Rome 19616 Proc.6 v. 3, p. 429-436. 

Bodvarsson6 Gunnar, and Palmason, G., 1964, 

Exploration of subsurface temperature in 

Iceland, in Geothermal energy6 I: United 

Nations Conf. New Sources Energy, Rome 

1961, Proc., v. 2, p. 91-98. 

Bodvarsson6 Gunnar .• and Zoega1 Johannes, 

1964, Production and distribution of natural heat for domestic and industrial heating in Iceland6 in Geothermal energy, II: 

United Nations Conf. New Sources Energy, 

Rome 1961, Proc., v. 3, p. 449-455. 

Boldizsar, Tibor6 1963, Terrestrial heat flow 

in the natural steam field at Larderello 

[Italy]: Geofisica Pura eAppl.6 v. 566 p. 11~ 

122. 

Boldizsar, Tibor6 1964, Heat flow in the Hungarian basin: Nature, v. 202, no. 4939, 

p. 1278-1280. 

Brown, Harrison6 and Silver, L. T., 1956, The 

possibilities of obtaining long-range supplies of uranium6 thorium, and other substances from igneous rocks, in Page6 L. R., 

and others, Contributions to the geology of 

uranium and thorium by the United States 

Geological Survey and Atomic Energy 

Commission for the United Nations International Conference on Peaceful Uses of 

Atomic Energy, Geneva6 Switzerland, 1955: 

U.S. Geol. Survey Prof. Paper 300, p. 91-

95. 

Bruce, A. W •6 19646 Experience generating 

geothermal power at The Geysers power 

plant6 Sonoma County, California6 in Geothermal energy# II: United Nations Conf. 

New Sources Energy6 Rome 19616 Proc., 

v. 3, p. 284-298. 

Burgassi6 R.6 Battini6 F. 6 and Mouton,. J., 19646 

Prospecion geothermique pour la recherche des forces endogenes, in Geothermal energy# II: United Nations Conf. New 

Sources Energy6 Rome 19616 Proc.6 v. 26 

p. 134-140. 

Carlson6 R. H.6 19596 Utilizing nuclear explosive in the construction of geothermal 

power plants: U.S. Atomic Energy Commission6 2d Plowshare symposium# San 

Francisco 19596 Proc., pt. 36 p. 78-87. 

Clark6 S. P. 6 Jr.6 19616 Geothermal studies: 

Carnegie Inst.6 Washington# Year Book 606 

196Q-1961, p. 185-190. 

Dawson6 G. B. 6 19646 The nature and assessment of heat flow from hydrothermal 

areas: New Zealand Jour. Geology and 

Geophysics# v. 76 p. 155-171. 

Donaldson6 I. G. 6 1962, Temperature gradients in the upper layers of the earth's 

crust due to convective water flows: Jour. 

Geophys. Research, v. 67, no. 9, p. 3449-

3460. 

Durr6 Fritz, 1964, Review of geothermal activity in El Salvador, irt Geothermal energy, I: United Nations Conf. New Sources 

Energy, Rome 1961, Proc.6 v. 26 p. 201-203. 

Einarsson, S. S., 1964, Proposed 15-megawatt geothermal power station at Hveragerdi, Iceland6 in Geothermal energy# II: 

United Nations Conf. New Sources Energy, 

Rome 1961, Proc.6 v. 36 p. 354-364. 

Ellis6 A. J., 19646 Geothermal drillholes; 

Chemical investigations, in Geothermal energy, I: United Nations Conf. New Sources 

Energy# Rome 1961, Proc.6 v. 26 p. 208-

218. 
16 GEOTHERMAL ENERGY 

Ellis, A. J., and Wilson, S. H., 1955, The heat 

from the Wairakei-Taupo thermal region 

calculated from the chloride output: New 

Zealand Jour. Sci. and Technology, sec. B, 

v. 36, no. 6, p. 622-631. 

Fisher, R. G., 1964, Geothermal heat flow at 

Wairakei during 1958: New Zealand Jour. 

Geology and Geophysics, v. 7, p. 172-184. 

Fukutomi, Takaharo, 1962, Yunokawa, Yachigashira, Shikabe, Toya, and Noboribetsu 

hot springs in Hokkaido: Sapporo, Japan, 

Hokkaido Univ. Dept. Geophysics, Reference Data Field Researches Geophysics, 

no. 1, p. 1-10. 

Garland, G. D., and Lennox, D. H., 1962, Heat 

flow in ~estern Canada: Royal Astron. Soc. 

Geophys. Jour., v. 6, no. 2, p. 245-26 2. 

Goudey, R. F., 1936, Solving boron problems 

in Los Angeles water supply: Western 

Construction News, v. 11, September, p. 

295-297. 

Gregg, D. R., 1958, Natural heat flow from 

the thermal areas of Taupo Sheet District 

(N 94): New Zealand Jour. Geology and 

Geophysics, v. 1, p. 65-75. 

Uutenberg, Beno, ed., 1951, The cooling of the 

earth and the temperature in its interior, 

Chap. 7 of Internal constitution of the 

earth: New York, Dover Pubs., p. 15Q-166. 

Healy, James, 1960, The hot springs and geothermal resources of Fiji: New Zealand 

Dept. Sci. Indus. Research Bull. 136, 77 p. 

---1964, Geology and geothermal energy in 

the Taupo Volcanic Zone, New Zealand, in 

Geothermal energy, I: United Nations Conf. 

New Sources Energy, Rome 1961, Proc., 

v. 2, p. 25Q-258. 

Ht>rai, Ki-iti, and Uyeda, Seiya, 1963, Terrestrial heat flow in Japan: Nature, v. 199, 

no. 4891, p. 364-36 5. 

Howard, L. E., and Sass, J. H., 1964, Terrestrial heat flow inAustralia: Jour. Geophys. 

Research, v. 69, no. 8, p. 1617-1626. 

Jacobs, J. A., 1956, The interior of the earth, 

in Landsberg, H. E., ed., V. 3 of Advances 

in geophysics: New York, Academic Press, 

p. 183-239. 

Kent, L. E., 1950, The thermal water of the 

Union of South Africa and South West Africa: Geol. Soc. South Africa Trans., v. 52, 

p. 231-264. 

Khitarov, N. I., 1956, First aU-union conference on geothermal investigations in the 

U.S.S.R.: Geochemistry (Geokhimiya), no. 

2, p. 222-225. 

----eed., 1959, Problems of geothermy and 

practical utilization of Earth's heat, v. 1: 

Akad. Nauk SSSR, 254 p. [In Russian.] 

Khitarov, N. I., ed., 1961, Problems of geothermy and practical utilization of Earth's 

heat, v. 2: Akad. Nauk SSSR, 304 p. [In 

Russian.] 

Kraskovskiy, S. A., 1961, On the thermal field 

of Shields: Akad. Nauk SSSR Izv. Ser. 

Geofiz., no. 3, p. 387-392 [In Russian.] 

Lee, W. H. K., and MacDonald, G. J. F., 1963, 

The global variation of terrestrial heat 

flow: Jour. Geophys. Research, v. 68, no. 24, 

p. 6481-6492. 

McBirney, A. R., 1956, An appraisal of the 

fumarolic activity near Ahuachapan, El 

Salvador: El Salvador Serv. Geol. Nac. 

Anales Bol. 2, p. 19-32. 

McNitt, J. R., 1963, Exploration and development of geothermal power in California: 

California Div. Mines and Geology Spec. 

Rept. 75, 45 p. 

Murozumi, Masayo, 1960, Geochemical significances of similarity of hydrothermal 

activity between Atami and Ito, Part 3 of 

Geochemical investigations of hot springs 

in the Izu-Hakone district: Jour. Chern. 

Soc. Japan (Nippon Kagaku Zasshi), v. 81, 

p. 903-906 [In Japanese.] 

Penta, Francesco, and Bartolucci, G., 1962, 

Sullo stato delle "ricerche" e dell'utilizzazione industriale (termoelecttrica) del 

vapore acqueo sotterraneo nei vari paesi 

del mondo: Accad. Naz. Lincei Atti, Cl. 

Sci. Fis. Mat. e Nat. Rend. s~r. 8, v. 32, 

p. 1-16. 

Piip, B. I., Ivanov, V. V., and Averiev, V. V., 

1964, The hyperthermal waters of Pauzhetsk, Kamchatka, as a source of geothermal energy, in Geothermal energy, I: United 

Nations Conf. New Sources Energy, Rome 

1961, Proc., v. 2, p. 339-346. 

Ringwood, A. E., 1962, A model for the upper 

mantle: Jour. Geophys. Research, v. 67, 

no~ 2, p. 857-867. 

Robson, G. R., and Willmore, P. L., 1955, 

Some heat measurements in West Indian 

soufrieres: Bull. volcanol., ser. 2, v. 17, 

p. 13-39. 

Ruiz Elizondo, J., 1964, Prospection of geothermal fields and investigations necessary to evaluate their capacity, in Geothermal energy, I: United Nations Conf. New 

Sources Energy, Rome 1961, Proc., v. 2, 

p. 3--47. 

Saito, Masatsugu, 1964, Known geothermal 

fields in Japan, in Geothermal energy, I: 

United Nations Conf. New Sources Energy, 

Rome 1961, Proc., v. 2, p. 367-373. 
REFERENCES CITED 17 

Smith, J. H., 1964, Harnessing of geothermal 

energy and geothermal electricity production, in Geothermal energy, II: United Nations Conf. New Sources Energy, Rome 

1961, Proc., v. 3, p. 3-57. 

Thompson, G. E. K., Banwell, C. J., Dawson, 

G. B., and Dickinson, D. J., 1964, Prospecting of hydrothermal areas by surface thermal surveys, in Geothermal energy, I: 

United Nations Conf. New Sources Energy, 

Rome 1961, Proc., v. 2, p. 386-401. 

Verhoogen, John, 1960, Temperatures within 

the earth: Am. Scientist, v. 48, no. 2, p. 134-

159. 

VonHerzen, R.P., 1963, Geothermal heat flow 

in the Gulfs of California and Aden: Science, v. 140, no. 3572, p. 1207-1208. 

Waring, G. A., 1965, Thermal springs of the 

United states and other countries of the 

world: U.S. Geol. Survey Prof. Paper 492, 

383 p. 

Watanabe,Kazue, 1958, Thermodynamic analysis on the heat source of Obama hot 

springs in the vicinity of Unzen volcano 

district: Chigaku Zasshi, [Jour. Geography], v. 67, p. 127-152 [In Japanese.] 

Neinberg, A. M., 1959, Energy as an ultimate 

raw material: Physics Today, v. 12, p. 18-

25. 

Nhite, D. E., 1957a, Thermal waters of volcanic origin: Geol. Soc. America Bull., 

v. 68, no. 12, pt. 1, p. 1637-1658. 

White, D. E., 1957b, Magmatic, connate, and 

metamorphic waters: Geol. Soc. America 

Bull., v. 68, no. 12, pt. 1, p. 16 59-1682. 

---1964, Preliminary evaluation of geothermal areas by geochemistry, geology, 

and shallow drilling, in Geothermal energy, 

I: United Nations Conf. New Sources Energy, Rome 1961, Proc., v. 2, p. 402-408. 

White, D. E., Anderson, E. T., and Grubbs, 

D. K., 1963, Geothermal brine well-miledeep drill hole may tap ore-bearing magmatic water and rocks undergoing metamorphism: Science, v. 139, no. 3558, p. 919-

922. 

White, D. E., and Roberson, C. E., 1962, Sulphur Bank, California, a major hot-spring 

quicksilver deposit, in Engel, A. E. J., 

James, H. L., and Leonard, B. F., eds., Petrologic studies (Buddington volume): Geol. 

Soc. America, p. 397-428. 

Wyllie, P. J., and Tuttle, 0. F., 1960, Melting 

in the earth's crust: Internat. Geol. Cong., 

21st, Copenhagen 1960, Rept., pt. 18, p. 227-

235. 

Yuhara, Kozo, 1963, Some considerations on 

flow, heat, and chemical composition of 

Italian hot springs: Annali Geofisica, v. 16, 

no. 1, p. 139-156. 

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