GEOLOGICAL SURVEY CIRCULAR 866 

Geothermal Resources of 

Southern Idaho 

Geothermal Resources of 

Southern Idaho 

By Don R. Mabey 

GEOLOGICAL SURVEY CIRCULAR 866 

1983 
United States Department of the Interior 

JAMES G. WATT, Secretary 

Geological Survey 

Dallas L. Peck, Director 

Free on application to Branch of Distribution, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 
xtension .. 

opography . 

Geot ermal systems 

ydrothermal convection systems . 

ot dry rock . 

agma ..... 

esource grade 

Geot ermal areas . 

CONTENTS 

Page 

1 Geothermal areas-Continued 

1 Weiser area . . . . . . . 

2 Idaho batholith . . . . . 

2 Western Snake River Plain 

6 Camas Prairie . . . . . . . . 

7 Owyhee area . . . . . . . . 

8 Eastern Snake River Plain 

8 Island Park area . 

9 Lost River area . 

9 Southeastern Idaho 

9 Conclusions . . . 

9 References cited . . . . 

ILLUSTRATIONS 

FIG RE 1. Graph showing thermal energy within 3 km of the surface in hydrothermal systems with reservoir temperaPage 

9 

12 

12 

14 

14 

15 

18 

20 

20 

22 

22 

Page 

tures ;;::::oooc. . . . . . . . . . . . . . . . • . . . . . • . . . • . . . . . . . . . . . . . . . . . . . . • . . . 2 

2. Map of southern Idaho showing areas dicussed in text, and location of geothermal and volcanic systems. . 3 

3. Map of the western United States showing Idaho, the Cordilleran Thermal Tectonic Anomaly (CTTA), areas 

of higher than 2.5 heat-flow units, and the Humbolt zone. . . . . . . . . . . . . . 4 

4. Map of southern Idaho showing contours on the base of the magnetized crust. . . . . . . . . . . . . . . 5 

5. Histogram of number of hydrothermal systems with reservoir temperatures ;;::::90°C. . . . . . . . . . . . 10 

6. Profile across the eastern Snake River Plain through Arco and Blackfoot showing Bouguer Gravity anomaly 

and inferred density model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

TABLE 

Page 

TAB E 1. Geothermal systems in Idaho with calculated reservoir temperatures ;;::::90°C. . . . . . . . . . . . . . . . . 11 

III 

Geothermal Resources of Southern Idaho 

By Don R. Mabey 

ABSTRACT 

The geothermal resource of southern Idaho as assessed by 

the U.S. Geological Survey in 1978 is large. Most of the known 

hydrothermal systems in southern Idaho have calculated reservoir temperatures of less than 150°C. Water from many of 

these systems is valuable for direct heat applications, but is 

lower than the temperature of interest for commercial generation of electricity at the present time. Most of the known and 

inferred geothermal resources of southern Idaho underlie the 

Snake River Plain. However, major uncertainties exist concerning the geology and temperatures beneath the plain. By 

far the largest hydrothermal system in Idaho is in the 

Bruneau-Grand View area of the western Snake River Plain 

with a calculated reservoir temperature of 107°C and an energy 

of 4. 5 x 1020 joules. No evidence of higher temperature water 

associated with this system has been found. Although the geology of the eastern Snake River Plain suggests that a large 

thermal anomaly may underlie this area of the plain, direct 

evidence of high temperatures has not been found. Large volumes of water at temperatures between 90° and 150°C probably 

exist along the margins of the Snake River Plain and in local 

areas north and south of the plain. Areas that appear particularly promising for the occurrence of large high-temperature 

hydrothermal systems are: the area north of the Snake River 

Plain and west of the Idaho batholith, the Island Park area, 

segments of the margins of the eastern Snake River Plain, 

and the Blackfoot lava field. 

INTRODUCTION 

The assessment of the geothermal resources of 

the United States completed by the U.S. Geological Survey in 1978 (Muffler, 1979) indicates that 

the geothermal resource in southern Idaho (south 

of 46° N. lat) is large. The evaluated igneous systems in southern Idaho contain 40 percent of the 

energy in such systems outside of National Parks. 

This percentage is more than that of the second 

and third states, California and Alaska, combined. 

The 36 known hydrothermal convection systems 

in southern Idaho with reservoir temperatures 

2::90°C contain 32 percent of the total energy in 

such systems in the United States outside of N a1 

tional Parks (fig. 1). This percentage is second 

only to that of California. The energy estimated 

for undiscovered hydrothermal convection systems 

in Idaho is 34 percent of the United States totalthe largest of any state. The total estimated energy in discovered and undiscovered hydrothermal 

convection systems in Idaho 2::90°C is greater than 

that of any other state. The Idaho resource in systems ;::::: 150°C is less impressive. Most of the 

known resources in Idaho are lower than 150°C 

and much of it is in one large system, BruneauGrand View, with a calculated reservoir temperature of 107°C. In addition to the resource 2::90°C, 

water in the temperature range from 30° to 90°C 

underlies large areas of southern Idaho. 

Although the geothermal resource in southern 

Idaho is large, the only commercial development 

has been for space heating, fish farming, and bathing. Only one of the systems, Raft River, has 

been investigated in detail. Considerable uncertainty exists concerning the distribution and size 

of the resource. In 1977, the U.S. Geological Survey, the U.S. Department of Energy, The Idaho 

Department of Water Resources, and the U.S. 

Forest Service began a coordinated study of the 

geothermal resources of southern Idaho designed 

to develop a better understanding of the resource. 

This program has involved geological, geophysical, 

hydrological, and geochemical studies supported 

by drilling. Numerous investigators from the four 

agencies and from several universities and commercial organizations have been involved in the 

work. Some of the studies have been completed 

and others are continuing. This report does not 

attempt to synthesize the results of all the studies 

but rather present conclusions as to what the 

studies indicate about the geothermal resources of 

southern Idaho at temperatures 2::90°C. The Idaho 

Department of Water Resources has the primary 
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EXPLANATION 

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FIGURE 1.-Thennal energy (in 1018 joules) within 3 km of 

the surface in hydrothennal systems with reservoir temperatures of :2:90•c. Data from Brooks and others (1979). 

responsibility for the investigation of the low-temperature resources (lower than 90°C) and a report 

on their work has been published (Mitchell and 

others, 1980). 

All the known geothermal resources in Idaho 

at temperatures ~9ooc are south of 46° N. lat and 

most are in an arc 100 km wide that extends across the state from Yellowstone National Park to 

the Weiser area and that includes the Snake River 

Plain (fig. 2). The Snake River Plain, which is a 

topographic depression completely covered with 

Cenozoic sedimentary and volcanic rocks, contains 

most of the energy in identified geothermal sys2 

terns and most of the postulated undiscovered resource in the state. Although significant geothermal systems occur outside the Snake River Plain, 

their energy content is small compared with the 

resource in or immediately adjacent to the plain. 

REGIONAL FEATURES 

All of southern Idaho lies within the Cordillerian 

Thermotectonic Anomaly (CTT A) (fig. 3) as defined by Eaton and others (1976). The CTTA, 

which contains 70 percent of the identified hydrothermal convection systems with estimated reservoir temperatures ~90°C in the conterminous 

United States, is centered near the southwest 

corner of Idaho. The CTT A was defined on the 

basis of several characteristics. Primarily they 

were: (1) High heat flux indicated by heat-flow 

measurements, thermal springs and volcanism; (2) 

rapid crustal spreading indicated by extensional 

faults and earthquakes with fault-plane solutions 

indicating extension; and (3) by the generally high 

surface elevation. These three criteria, supplemented by additional geological and geophysical 

considerations, can be used to examine the regional geological and geophysical features of 

southern Idaho as indicators of the geothermal resources in the region. 

HEAT FLUX 

Hydrologic disturbances over much of southern 

Idaho limit the usefulness of heat-flow measurements obtained in shallow holes, and the regional 

heat-flow patterns are not well defined. Brott and 

others (1976, 1978) and Brott and others (1981) 

have made numerous heat-flow measurements in 

southern Idaho, and Blackwell (1978) used the earlier data to conclude that a large heat-flow high 

underlies much of southern Idaho but that the 

heat flow in the Idaho batholith is lower and near 

the normal for the area of the CTT A. Brott and 

others (1981) suggest a model in which a large 

heat source is emplaced under the Snake River 

Plain beginning about 20 million years ago in the 

west and moving eastward in time toward Y ellowstone National Park. High heat-flow values 

near the margins of the western Snake River Plain 

and lower values on the plain are attributed by 

them to a combination of thermal refraction caused 

by Cenozoic rocks with low thermal conductivity 

underlying the plain and low heat generation in 
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IDAHO BATHOLITH 

EXPLANATION 

ESTIMATED THERMAL ENERGY IN UNITS OF 1018JOULES 

> 100 

10-100 

1-10 

< 1 

Hydrothermal 

systems 

> 10,000 

1,000-10,000 

100-1,000 

Volcanic 

systems 

FIGURE 2,-Map of southern Idaho showing named areas (bounded by dashed lines) discussed in text, location of geothermal systems with number designations referenced to table 1, and volcanic systems with letter designations referenced 

to text. 

the crust under the plain. The low heat flow under 

the eastern Snake River Plain is attributed to the 

movement of ground water in volcanic aquifers. 

Some physical properties of rocks are temperature-dependent, and measurements of these physical properties by geophysical techniques provide 

an indication of temperature at depth. By far the 

most important mineral in determining the magnetism of rocks is magnetite. The Curie temperature 

of magnetite is about 580°C; therefore, large magnetic anomalies are not likely to be produced by 

rocks much higher than this temperature. The 

magnetic anomalies over southern Idaho have 

been analyzed to determine the elevation of the 

3 

base of the magnetized crust, which is assumed 

to coincide with the Curie isotherm surface and 

indicate temperature distributions deep in the 

crust (Bhattacharyya and Mabey, 1980). Extremely high electrical conductivity also may indicate high temperature. Magnetotelluric soundings 

have been used in southern Idaho to map a highly 

conductive layer in the crust, and variations in the 

depth to this layer may provide an indication of 

major thermal anomalies (Stanley and others, 

1977). Deeper conductivities have been determined in the area of the eastern Snake River Plain 

using measurements of geomagnetic induction. The 

analysis of teleseismic P-wave delays recorded 
I 

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CTTA 

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FIGURE 3.-Map of the western United States showing Idaho, the Cordilleran Thermal Tectonic 

Anomaly (C'ITA; from Eaton and others, 1976), areas (stippled) of higher than 2.5 heat-flow 

units (HFU; from Blackwell, 1978), and the Humboldt zone (from Mabey and others, 1978). 

WSRP, western Snake River Plain; ESRP, eastern Snake River Plain. 

in southeastern Idaho define a low-velocity zone 

that may indicate a large, deep thermal anomaly. 

Information from all these techniques has been 

used in an attempt to define regional thermal 

anomalies in southern Idaho. 

A map of the elevation of the base of the magnetized crust is shown in figure 4. This map is based 

on the work of the late B. K. Bhattacharyya and 

is modified from figure 8 of Bhattacharyya and 

Mabey (1980). It differs from the early version in 

that some determinations considered less reliable 

(B. K. Bhattacharyya, written commun., 1979) 

have been rejected. Little correlation is apparent 

between the computed elevation of the base of the 

magnetized crust and the Snake River Plain. This 

4 

lack of correlation suggests that the Snake River 

Plain is not related to a simple thermal event. The 

thickness of the magnetized crust thins northeastward toward Yellowstone National Park where 

previous analysis indicated a thickness of 5-6 km 

(Bhattacharyya and Leu, 1975). A high in the base 

of the magnetized crust coincides with The Craters of the Moon, an area of young basalt flows, 

and with a series of young rhyolite domes on the 

eastern Snake River Plain (CM, fig. 4). Another 

high is coincident with the Albion Mountains (AM, 

fig. 4), where Precambrian metamorphic rocks 

crop out. The Raft River geothermal system is 

in the area of this high. The contouring suggests 

a connection between the high in the Albion Moun-
117' 

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I 

FIGURE 4.-Map of southern Idaho showing contours on the base of the magnetized crust (modified from Bhattacharyya 

and Mabey, 1980). Contours are kilometers below sea level based on elevations determined at points marked by plus 

signs. CM, Craters of the Moon; AM, Albion Mountains. 

tains area and a high north of the plain over the 

eastern edge of the Idaho batholith and the area 

to the east. Intermediate elevations were determined over most of the western Snake River 

Plain. 

Magnetotelluric soundings m southeastern 

Idaho have been interpreted to indicate a highly 

conductive zone in the crust at elevations ranging 

from 9 to 25 km below sea level (Stanley and 

others, 1977). The conductive layer is nearest the 

surface in the area of the Albion Mountains and 

deepest along the axis of the eastern Snake River 

Plain. W. D. Stanley (oral commun., 1979) reported that the conductive layer generally appears 

to be nearer the surface along the margins of the 

eastern Snake River Plain than along the axis. 

Stanley and others (1977) believed that the conductive zone is related to high temperatures. Good 

agreement exists between the elevation of the 

magnetized crust as determined by Bhattacharyya 

and the elevation of the conductive layer in the 

crust determined by magnetotelluric sounding in 

southeastern Idaho (Bhattacharyya and Mabey, 

1980, fig. 9). Preliminary results from the 

geomagnetic induction surveys over the eastern 

5 

Snake River Plain do not indicate a deep conductivity anomaly coincident with the plain (Fitterman, 1979; and Wier, 1979). 

The intermountain seismic belt extends into 

eastern Idaho from the north and south but does 

not extend onto the eastern Snake River Plain, 

where few earthquakes occur (Smith, 1978). The 

low number of earthquakes on the eastern Snake 

River Plain may indicate that the temperature in 

the crust under this part of the plain is so high 

that the strain is accommodated without fracturing. Evans and Iyer (1979) and J. R. Evans (written commun., 1981) have interpreted the delay in 

teleseismic P-waves in the eastern Snake River 

Plain as indicating a low velocity body 100 km 

wide and extending from 100 to 250 km below the 

surface. They concluded that this body increases 

in size but decreases in velocity contrast to the 

southwest. The seismic data appear to suggest a 

thermal anomaly under the northeastern side of 

the Snake River Plain that is not apparent in the 

resistivity data. 

Volcanic rocks of Neogene and Quaternary age 

occur over all the Snake River Plain, much of the 

area southwest of the plain, west of the Idaho 
batholith, and locally southeast of the Snake River 

Plain. In the east, these rocks overlap the north 

edge of the plain but are not known to extend 

far north of the plain. The volcanic rocks are predominantly basalt and rhyolite with basalt being 

the most common youngest and oldest rock and 

rhyolite the most abundant intermediate-age rock. 

In general, the rhyolite is progressively younger 

toward the east (Armstrong and others, 1975; 

Christiansen and McKee, 1978) but young basalt 

flows are widespread over the plain. The beginning of the Neogene volcanic activity appears to 

coincide with the start of large-scale extension of 

the northern Basin and Range province. Although 

the fundamental structure of the Snake River 

Plain and the relationship between the plain and 

the widespread Neogene and Quaternary volcanism in the region are still debated, the origin 

of the plain and the volcanism probably are both 

related to the development of the CTTA and extension of the northern Basin and Range province. 

EXTENSION 

Lachenbruch and Sass (1978) have developed a 

model of an extending lithosphere and related heat 

flow for the Basin and Range province. The model 

assumes that heat is supplied to the extending 

lithosphere by the intrusion of basalt dikes or by 

the underplating of basalt on the lithosphere. The 

model indicates that an increase of 40 m W/m2 (milliwatts per square meter) in heat flow could be 

produced by 1-2 percent extension per million 

years. Thus, an understanding of the nature and 

age of the extensional tectonics of southern Idaho 

is important in the evaluation of the geothermal 

resources. 

Although generally east-west extension characterizes most of the CTTA, the orientation and effect on the upper crust of the extension varies 

with time and location. South of Idaho, a symmetry to the extension is apparent with the axis 

of symmetry near the north-south axis of the 

CTTA (Eaton and others, 1978). In southern 

Idaho, the symmetry of the extension is not nearly 

so pronounced. 

Beginning about 17 million years ago, large volumes of basalt were erupted in the Columbia 

Plateau and in eastern Oregon and western Idaho 

at about the same time that regional extension appears to have begun in the northern Basin and 

6 

Range province and perhaps throughout the entire 

area of the CTT A. The crust in the region probably was relatively competent at the beginning of 

the extensional tectonics, and rifts developed with 

basalt rising to the surface and accumulating in 

subsided areas that developed on the surface. The 

Cortez or Nevada rift in north-central Nevada was 

one of these rifts (Zoback and Thompson, 1978; 

Mabey and others, 1978), and the western Snake 

River Plain may have been the site of another. 

Christiansen and McKee (1978) suggested that 

these two features originated as a continuous rift 

that was subsequently offset by transform displacement along the northern boundary of the 

Great Basin. The Cortez rift is well exposed, but 

most of the evidence for a rift under the western 

Snake River Plain is deeply buried under younger 

sedimentary and volcanic rock. The primary data 

relating to the structure of the western Snake 

River Plain are: (1) normal faults (plain side down) 

near the margins of the plain and a regional dip 

of the Cenozoic rocks toward the axis of the plain, 

(2) the seismic refraction data indicating a thinning 

of the upper crust and a thickening of the lower 

crust under the plain, (3) a large gravity high 

cresting near the axis of the plain, ( 4) a magnetic 

anomaly consisting of a high along the southern 

edge of the plain and a low along the north edge, 

and (5) a few deep drill holes. 

These data can be interpreted in several ways. 

Mabey (1976) presented an interpretation that infers that the crest of the gravity high overlies the 

thinnest upper crust and that the entire western 

plain is a deep subsidence containing a dense magnetic mass that is probably Miocene basalt similar 

to that cropping out north of the western end of 

the plain. After this interpretation was published, 

a correlation between an appendage of the magnetic high over the western Snake River Plain and 

an outcrop of Eocene volcanic rocks southwest of 

the plain was noted. This correlation suggests that 

Eocene volcanic rocks as well as Miocene basalt 

may underlie the plain. The data from deep drill 

holes west of Boise (Bowen and Blackwell, 1975) 

appear consistent with the interpretation that a 

large mass of dense and highly magnetized volcanic rock underlies the plain, but the data from a 

deep hole near Grand View do not support this 

interpretation. The latter hole is reported to have 

bottomed in granite without having penetrated 

any great thicknesses of basalt or other dense, 
highly magnetized rocks that would produce the 

measured magnetic and gravity anomalies (Mcintyre, 1979). The source of the magnetic anomaly 

and most likely part of the gravity anomaly must 

underlie the area of the hole. Perhaps the granitic 

body is thin, being part of a sill or a large slide 

block from much higher areas of granite a few kilometers to the southwest. Gravity data indicate 

that the western Snake River Plain is underlain 

by a large positive mass anomaly and that the 

plain is in approximate isostatic equilibrium with 

the adjoining highlands. The most recent subsidence of the plain may relate primarily to isostatic 

adjustment and may not reflect major extension 

of the crust. 

As the temperature of the crust in the CTTA 

increased, the style of the extensional deformation 

changed. No more major rifts developed and the 

direction of extension rotated clockwise (Zoback 

and Thompson, 1978). In late Miocene and Pliocene time, large volumes of silicic volcanic rocks 

were erupted in southeastern Idaho, and the center of this activity migrated northeast along the 

current location of the Snake River Plain. In 

southeastern Idaho, extensive basins developed 

and filled with great thicknesses of sedimentary 

and volcanic rock of the Miocene and Pliocene Salt 

Lake Formation. These basins, which may have 

extended onto the area of the present eastern 

Snake River Plain, have not been studied in detail 

but they appear to have been considerably broader 

than the present topographic basin~ north and 

south of the eastern Snake River Plain. 

The boundary between the eastern and western 

Snake River Plain used in this report is based on 

a marked contrast of gravity and magnetic 

anomalies. The subsurface geology of the eastern 

Snake River Plain is largely speculative with only 

one deep drill hole for control and the geophysical 

and geological data subject to conflicting interpretations. Almost certainly the eastern plain is, at 

least in part, an extensional feature with a complex of calderas underlying the northeastern end 

of the plain. A major regional magnetic lineament 

(Eaton and others, 1975; Mabey, Zietz, and 

others, 1978) has been interpreted as indicating 

that the location of the eastern Snake River Plain 

is controlled by an old crustal feature (The Humboldt zone, fig. 3). Christiansen and McKee (1978) 

proposed that the eastern Snake River Plain is 

part of the northern transform boundary of the 

7 

Great Basin and that shear melting at the base 

of the crust developed into a self-sustaining melting anomaly. Brott and others (1981) suggest that 

the entire Snake River Plain represents the track 

of a moving thermal source now under Y ellowstone National Park. 

When the eastern Snake River Plain began to 

develop as a major depression has not been determined. Subsidence likely occurred in the area at 

the beginning of regional extension about 17 million years ago. Whether this took the form of a 

single elongate depression or a group of smaller 

basins is not clear. In Mabey (1978a) the long 

wave length gravity anomalies are intepreted as 

reflecting a series of the early basins, but the evidence for these is not compelling. Most of the 

large Basin and Range structures south of the 

eastern Snake River Plain do not extend to the 

edge of the plain, and the geophysical data suggest 

that similar structures north of the plain do not 

extend far onto the plain. The major Basin and 

Range faults, which have been active in late Cenozoic time, are likely to be predominantly listric 

faults that flatten at depth and merge with the 

underlying thrust faults. The extension of the 

crust in eastern Idaho in late Cenozoic time appears in part to involve some of the same fault 

surfaces that formed during the shortening of the 

crust in late Mesozoic time. We do not know if 

the thrust faults underlie the eastern Snake River 

Plain. 

TOPOGRAPHY 

Although all of southern Idaho lies within the 

regional topographic high associated with CTT A, 

the dominant topographic feature of southern 

Idaho is the Snake River Plain with low surface 

relief and an elevation that is generally about 500 

m below the average elevation of the adjoining 

highlands. The northwest end of the Snake River 

Plain, which is less than 1 km above sea level, 

is the lowest large area within the CTT A. Toward 

the east, the elevation of the plain rises as does 

the average elevation of the adjoining highlands. 

Brott and others (1981) relate the eastward increase in the elevation of the Snake River Plain 

to increasing temperature of the crust and resulting thermal expansion. Gravity studies indicate 

that the plain is in approximate isostatic equilibrium with the adjoining highlands but that most 

of eastern Idaho is undercompensated and stands 
at a higher regional elevation than can be 

explained by isostasy (Mabey, 1978a). Basin and 

Range topography is prominent in eastern Idaho 

outside the Snake River Plain. The development 

of the topography of southern Idaho was closely 

related to the development of some of the geothermal systems. 

GEOTHERMAL SYSTEMS 

HYDROTHERMAL CONVENTION SYSTEMS 

The geothermal resources in southern Idaho 

that are of most immediate economic interest are 

hydrothermal convection systems. Hot water can 

be produced from these systems and the energy 

extracted from the water. Most of the known hydrothermal convection systems in Idaho have been 

identified because water from the system is being 

discharged at the surface in a spring or was encountered in drilling a water well or oil and gas 

exploration well. Thermal water in springs and 

shallow wells is usually part of a hydrothermal 

convection system that may contain large volumes 

of water at depth with a higher temperature than 

the near-surface water. The hot water discharged 

from springs can be used in applications where 

small volumes of relatively low-temperature water 

are required such as small-scale space heating. 

But in applications that require either large volumes of water or high-temperature water, for example the generation of electricity, the deeper 

parts of the system must be tapped. 

The major elements of a hydrothermal convection system are: (1) conductive heat flow supplying 

heat to the systems; (2) cold water descending 

through a permeable zone, such as fratured rock, 

to a zone of high temperatures where the water 

is heated by direct contact with hot rock; (3) the 

hot water being less dense ascending toward the 

surface in a porous zone; ( 4) a reservoir of porous 

rocks where hot water is stored and where the 

reservoir rock is heated; and (5) some systems discharge hot water into shallow aquifers or at the 

surface in springs. The system is continually 

transporting heat toward the surface, but the primary resource is the heat that is stored in the 

water and rock_ of the reservoir. Development of 

the resource normally will be by drilling wells into 

the reservoir and extracting the hot water from 

the reservoir. Some heat will be supplied to the 

8 

reservoir by convection in the system during the 

period of production, but the rate of resupply will 

usually be small compared to the rate of production and the reservoir will be depleted. Although 

the reservoir may eventually recover, the time required normally will be too great to be of interest 

in a resource study. Therefore, the resource of 

primary interest is the recoverable energy in the 

reservoir. 

Although considerable uncertainty eXists about 

what the representative deep thermal gradients 

are for southern Idaho, some conclusions can be 

drawn from existing data. Thermal gradients of 

31° and 47°C/km were measured in holes in granite 

southwest of the Snake River Plain (Urban and 

Diment, 1975). Brott and others (1978) reported 

gradients of 40° to 100°Cikm in numerous areas 

of southern Idaho, but their highest values probably reflect near-surface rocks with low thermal 

conductivity and the presence of hydrothermal 

convection systems. Lachenbruch and Sass (1978) 

indicated that thermal gradients of from 25° to 

50°C/km are representative of the northern Basin 

and Range province where surface heat flow 

ranges from 55 to 125 m W /m2

• The normal gradients over most of southern Idaho probably fall 

in the 25° to 50°C/km range. Possible support for 

this conclusion is provided by the Curie isotherm 

depths suggested by the magnetic anomalies that 

indicate that a temperature of about 550°C occurs 

at 10 to 23 km below the surface over most of 

southern Idaho. The maximum calculated reservoir temperatures for any known system in Idaho 

is 171 oc (Brook and others, 1979), and this could 

be obtained by water circulating to depths of 

about 6 km in low-gradient areas and 3 km in highgradient areas. The depths could be substantially 

less in areas underlain by great thickness of rocks 

with low thermal conductivity. Thus, the temperatures calculated for all known geothermal systems 

in southern Idaho could be obtained by water circulating to moderate depths in the geothermal 

gradients that are estimated to be normal for the 

region. Any local heat sources that may exist will 

enhance the thermal gradients and reduce the 

depth of circulation required to obtain a given 

temperature. 

Some hydrothermal reservoirs develop without 

·important convection with water in a confined 

aquifer heated by the conductive heat flow. This 

kind of reservoir will not contain water at as high 
temperature at a given depth as will the reservoir 

in a convection system. 

HOT DRY ROCK 

Large amounts of thermal energy are contained 

in hot rock that, because of low permeability, does 

not contain significant amounts of extractable 

water. These reservoirs are often referred to as 

"hot dry rock" although the rock is probably never 

completely dry and some reservoirs may contain 

substantial amounts of water. Research directed 

toward the development of technology to extract 

the heat from these reservoirs by circulating a 

fluid through the rock is in progress. Hot impermeable rock is known to exist in several areas 

of Idaho at depths of less than 3 km, and at great 

depths the energy content of these rocks must be 

considerable. However, until the technology and 

economics of extracting energy from these hot impermeable rocks is demonstrated, they cannot be 

termed a resource. 

MAGMA 

A third type of geothermal reservoir is molten 

or partially molten rock. Great amounts of heat 

are transported toward the surface of the earth 

as magma moves upward to form plutons or to 

be extruded at the surface as lavas or ash flow 

tuffs. Bodies of molten magma, both basalt and 

rhyolite, may exist within the crust in several 

areas of southern Idaho. However, the technical 

problems of extracting heat from these reservoirs 

are formidable and commercial development is not 

likely to occur for many years, if ever. 

RESOURCE GRADE 

Although the geothermal resource of southern 

Idaho is large, most of the energy is in hydrothermal convection systems with estimated temperatures less than 150°C. Figure 5 illustrates the reservoir temperature of the southern Idaho systems 

as well as those of adjacent states and all those 

in the conterminous United States. Note the absence of any extremely high temperatures for the 

Idaho system. Most of the temperatures are based 

on geochemical thermometry of water from 

springs and shallow wells. The computed temperatures assume that the thermal waters have not 

mixed with cold water unless mixing could be 

proved and quantitatively evaluated; this was 

rarely possible with data available (Brook and 

others, 1979). The average precipitation in Idaho 

is relatively high-more than twice that of Nevada 

and western Utah. In many of the geothermal 

areas of Idaho there is a shallow ground water 

that is probably mixing with the thermal water 

leaking from the underlying geothermal reservoirs. Geochemical temperatures determined for 

these mixed waters generally will be lower than 

the actual reservoir temperatures. Additional 

work will likely result in an increase of the calculated reservoir temperatures of some of the Idaho 

systems. 

9 

GEOTHERMAL AREAS 

The geology of Idaho is varied and complex and 

the characteristics of the geothermal systems that 

occur throughout the southern part of the State 

are determined by the local and regional geology. 

Southern Idaho is here divided into nine areas on 

the basis of the regional geology and the character 

of the geothermal systems (fig. 2). The geothermal 

systems with reservoir temperatures ~90°C are 

listed in table 1 as are estimated mean reservoir 

temperature, volume, and energy. The data for 

this table are from Brook and others (1979). Additional data on most of the systems are available 

in Young and Mitchell (1973). Plate 1 of Mitchell 

and others (1980) is a 1:500,000 scale map that 

shows the location of these systems as well as data 

on the systems with reservoir temperatures 

::::;90°C. 

WEISER AREA 

Five geothermal systems with estimated reservoir temperatures higher than 130°C are in one 

area north and northwest of Boise, here called the 

Weiser area (fig. 2). This area includes the extreme northwest end of the western Snake River 

Plain, a strip along the west edge of the Cretaceous Idaho batholith, and an area north of the 

plain underlain by basalt and sedimentary rock of 

Miocene age. The Weiser area is adjacent to the 

Vale geothermal area in eastern Oregon where 

reservoir temperatures of 127°C, 157°C, and 188°C 

are estimated for three systems. Heat flow and 

thermal gradients over most of the area appear 

to be high. Although the systems in the Weiser 
UTAH 

0 

10-.--~--------~----~--~--~~--~-----.--~-----.--~--------~--------~--.--. 

0 

en 30 ~ 

w 

..... 

en 

>- en 

LL 

0 

a: 

w 

m 20 

~ 

~ 

z TOTAL ALL CONTERMINOUS 

STATES,EXCEPTIDAHO 

10 

0 

20-.--.---.--.--.---.--.--~--.--.--~--.--.--.---.--.--.---.--.--.---.--.--~--.--. 

10 

0 

100 150 200 250 300 

TEMPERATURE, IN DEGREES CELSIUS 

FIGURE 5.-Histogram of number of hydrothermal systems with reservoir temperatures :=::90°C (data from Brook and others, 

1979). 

10 
No. 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

33 

34 

35 

36 

TABLE 1.-Geotkermalsystems in Idaho with calculated reservoir temperatures ;?::90 oc 

[See figure 2 for location ofsystems. "C, degrees Celsius; krn3, cubic kilometers; J, joules; avg. temp., average temperature] 

System 

Name 

Mean 

temperature 

ec> 

Weiser area (avg. temp. 16:ZOC) 

White Licks Hot Springs ...................................... . Crane Creek-Cover Creek ..................................... . Weiser ................................................................. . Roystone Hot Springs ........................................... . Payette River area ............................................... . 

Idaho batholith (avg. temp. 110°C) 

Red River Hot Springs ........................................ . Ri~gins Hot Sprin~s ............................................. . Krigbaum Hot Spnngs .......................................... . Vulcan Hot Springs .............................................. . Car barton Hot Springs ......................................... . Boiling Springs ..................................................... . Bonneville Hot Springs ........................................ .. N einmeyer Hot Springs ........................................ . Owl Creek Hot Springs ........................................ . Big Creek Hot Springs ......................................... . Sunbeam Hot Springs ........................................... . Slate Creek Hot Springs ....................................... . Worswick Hot Spnngs .......................................... . 

Lost River area 

Sharkey Hot Springs ........................................... .. 

Camas Prairie (avg. temp. 116°C) 

Magic Reservoir area ............................................ . Wardrop Hot Springs ........................................... . Barron's Hot Springs ............................................ . 

139 

171 

130 

135 

131 

92 

102 

103 

122 

106 

99 

105 

94 

126 

162 

112 

107 

94 

114 

149 

97 

103 

Western Snake River Plain (avg. temp. 10TC) 

Latty Hot Springs ............................................... .. Radio Towers area ............................................... . Gravel Pits area ................................................... . Bruneau-Grand View area ..................................... . White Arrow Hot Springs ..................................... . Banbury .............................................................. . 

Owyhee area 

Murphy Hot Springs ............................................ .. 

124 

125 

103 

107 

103 

117 

103 

Eastern Snake River Plain (avg. temp. l00°C) 

Newdale area ....................................................... . Ashton Warm Springs .......................................... . 

Southeast area (avg. temp. 132°C) 

Raft River area ................................................... . Maple Grove Hot Springs ..................................... . Riverdale area .................................................... .. Wayland Hot S:prings ........................................... . Squaw Hot Spnngs .............................................. . 

11 

100 

92 

149 

93 

99 

113 

119 

Mean 

volume 

(km3) 

3.3 

39.0 

4.4 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

3.3 

1830.0 

5.8 

27.0 

3.3 

89.0 

3.3 

21.0 

3.3 

3.3 

3.3 

3.3 

Mean 

reservoir 

ener~rV 

(1018'J) 

1.1 

16.4 

1.4 

1.1 

1.0 

0.69 

0.78 

0.79 

0.96 

0.82 

0.76 

0.81 

0.71 

1.0 

1.3 

0.87 

0.82 

0.71 

0.89 

1.2 

0.74 

0.79 

0.98 

0.99 

0.79 

450.0 

1.4 

7.4 

0.79 

20.0 

0.69 

7.4 

0.70 

0.76 

0.88 

0.94 
area have not been studied in detail, no common 

element is apparent in the geology of the five systems. Three are in the area of Miocene sedimentary rock and basalt and two are in rocks of the 

Idaho batholith. 

The largest and hottest system in the Weiser 

Area is Crane Creek-Cove Creek (system 2, fig. 

2) (Young and Whitehead, 1975b; Mcintyre, 1976). 

This system is expressed at the surface by springs 

on Crane and Cove Creeks about 11 km apart. 

The similarity of water chemistry, the continuity 

of geophysical anomalies and extensive alteration 

leads to the conclusion that one large system underlies the area. The volume of the reservoir is 

estimated to be 39 km3 (Brook and others, 1979) 

but geophysical anomalies and alteration are consistent with the existence o( a larger reservoir. 

At 171 oc calculated reservoir temperature, this is 

the hottest known system in Idaho. One deep test 

hole has been drilled in the area but data from 

the test have not been released. 

Thermal waters occur in an area of several 

square kilometers around Weiser Hot Springs 

(system 3, fig. 2). Reservoir volume for this system is estimated to be 4.4 km3 and the reservoir 

temperature 130°C. The extent of the resistivity 

low (Hoover and Long, 1976) in this area suggests 

that the system may be much more extensive than 

assumed in the resource estimate. 

Both the Crane Creek-Cove Creek and Weiser 

systems are in an area underlain by a thick sequence of Miocene sedimentary rocks and basalt 

and the springs occur along north-northwest 

trending fault zones. Both are areas of complex 

structure with dominant north-northwest systems 

of faults and folds being intersected by northeast 

trending faults. Fractured zones in the Miocene 

and pre-Miocene rocks are the most likely reservoirs. Detailed geophysical surveys should provide 

targets for exploration drilling. 

Little is known about the remaining three 

geothermal systems in the Weiser area. Roystone 

Hot Springs (system 4, fig. 2) issue from alluvium 

near Miocene basalt, but the basalt probably is 

underlain by rocks of the Cretaceous Idaho 

batholith at depths of a few hundred meters. 

White Licks Hot Springs (system 1, fig. 2) and 

the Payette River area (system 5, fig. 2) are in 

the Idaho batholith. No geophysical data are available to indicate the extent of these three systems. 

Any large reservoir that may exist at depth is 

probably fractured granitic rock. 

12 

IDAHO BATHOLITH 

More hydrothermal convection systems are 

within the Idaho batholith or in areas thought to 

be underlain by the batholith (fig. 2) than in any 

other area of Idaho. None of them is known to 

be large, and Brook and others (1979) assumed 

each to have the arbitrary minmum reservoir volume (3.3 km3

) assigned to systems when no information was available to indicate the dimensions 

of the reservoir. A wide range of heat flow has 

been measured in the batholith. Hot springs are 

most common in the major stream valleys, many 

of which are thought to be controlled by faults 

or shear zones. A model that is consistent with 

all available data assumes fracture permeability 

permitting water to circulate to depths of 3 or 4 

km where temperatures in the 100°-150°C range 

are expected. Twelve of the 13 systems have calculated reservoir temperatures ::S126°C. The exception: Big Creek Hot Springs (system 15, fig. 

2) which issues from Precambrian rock about 7 

km from the nearest outcrop of Idaho batholith. 

The estimated reservoir temperature of this system is 162°C, which is substantially higher than 

any other system in the area of the batholith. An 

adequate evaluation of the geothermal resource of 

the Idaho batholith will require detailed studies 

of representative systems involving hydrological, 

geological, and geophysical surveys supported by 

drilling. Current information suggests a large but 

relatively low-temperature resource distributed 

through many small systems in the batholith. 

WESTERN SNAKE RIVER PLAIN 

The western Snake River Plain (fig. 2) contains 

87 percent of the discovered hydrothermal resources of Idaho ~90°C and about one-fourth of 

the nation's geothermal resource in hydrothermal 

convection systems outside of National Parks. 

Most of the energy is in the Bruneau-Grand View 

system and all is in the Mountain Home-Twin 

Falls area of the plain. Although parts of this area 

have been intensely studied and several deep holes 

have been drilled, several fundamental questions 

relating to the systems remain unanswered. 

Estimated reservoir temperatures for systems 

in the western Snake River Plain range between 

103° and 124°C with the Bruneau-Grand View system at 107°C. 1 Fifteen heat-flow measurements in 

1Young and Lewis (1980) concluded that the average reservoir temperature in 

the Bruneau-Grand View system is about 90"C. 
the area range from -20 to 140 mW/m2 and average 80 mW/m2 (Brott and others, 1976), which is 

lower than on the margin of the plain. Brott and 

others (1978) attributed the low heat flow over 

the western Snake River Plain relative to the margins to thermal refraction produced by thick sedimentary rocks underlying the plain and to higher 

heat generation in the upper crustal material adjoining the plain. They postulated that a large heat 

source underlies the plain. 

Although the model proposed by Brott and 

others (1978) produces good agreement between 

the measured and the computed heat flow values, 

the model does not appear to be completely consistent with other data. A hole drilled on the Snake 

River Plain 45 km south of Boise was reported 

bottomed in granite at about 3.4 km below the 

surface (Mcintyre, 1979). Above the granite, the 

section was 25 percent sediments, 9 percent 

basalt, and 66 percent rhyolite. Thus, at this location, the thickness of Cenozoic rock is only about 

3.5 km rather than the 6 km assumed in the model 

proposed by Brott and others (1978) and the abundance of rhyolite indicates an average thermal conductive significantly higher than the 3 TCU (thermal conductivity units) assumed in the model. 

Therefore, the amount of thermal refraction may 

be less than computed. If granitic crust underlies 

the plain at 3.5 km and 66 percent of the overlying 

rock is rhyolite, the heat production under the 

Snake River Plain may be substantial rather than 

zero as assumed in the model. If the top of the 

heat source postulated in the thermal model is 

coincident with the top of the lower crust, the 

gravity data suggest a much smaller extent than 

the model assumes and suggest that the area of 

greatest elevation of the lower crust is confined 

to the area underlying the plain. Because of these 

apparent inconsistencies, other thermal models 

should be considered in addition to that proposed 

by Brott and others (1978). 

The Snake River Plain in the Mountain HomeTwin Falls area is downwarped toward the axis 

and faulting is locally prominent along the margins. Displacement on the faults is predominantly 

plain-side down. Subsidence of this area of the 

plain may be, in part, an isostatic response to the 

large positive mass anomaly underlying, which 

may not be completely compensated. Current extension may be concentrated along the margins of 

the plain with related crustal thinning and resultant high heat flow. 

The Bruneau-Grand View geothermal system is 

unique in Idaho because of its size. Water at temperatures as high as 84. 5°C is produced from artesian aquifers in Cenozoic volcanic rocks in wells 

that are as deep as 1100 m (Young and Whitehead, 

1975a). Temperature measurements have not been 

made in the producing wells, but the temperature 

of the water produced increases with total depth 

of the well at an average rate of about 65°C per 

kilometer, which is about equal to the thermal gradient measured in heat-flow holes in the area 

(Brott and others, 1976). Thus, the wells do not 

appear to be in a freely convecting hydrothermal 

system. The increase of temperature with depth 

suggests that the water may be heated in place 

and that the temperature gradient at depths 

greater than about 500 m is not greatly perturbed 

by convection. However, the silica and sulfate 

geothermometers indicate reservoir temperatures 

of 63°-140°C with no apparent correlation with 

well depth or water temperature. 

Young and Lewis (1979) proposed a model in 

which water moves northward through semi-confined aquifers and is heated by an average heat 

flow of 92 m W/m2• They calculated that about twothirds of heat entering the bottom of the system 

is lost at the surface through conduction and 

spring discharge, and that one-third is carried 

north by the movement of ground water. 

The Bruneau-Grand View system is in an area 

of complex structure probably produced by an interaction of the forces that have controlled the formation of the western and eastern segments of 

the plain. The well-defined topographic front that 

forms the boundary between the plain and 

Owyhee Mountains terminates here, and to the 

south and southeast no clearly defined topographic 

border exists. The southwest edge gravity and 

magnetic anomalies associated with the plain bend 

to the south. On the opposite side of the plain, 

the northern edge begins a wide arc along which 

it rotates about 80°. A structure extending from 

the south end of the Owyhee Mountains eastnortheast across the plain is suggested by the 

gravity data. To the south, the gravity high along 

the axis of the plain includes a secondary high 

south of the primary high. The silicic volcanic 

rocks, which are the primary deep aquifer, are 

probably highly fractured in this area to produce 

the permeability that accounts for the productivity 

of the water wells. The depth to which this permeability extends has not been determined, and, 

13 
thus, the thickness and likely maximum temperatures are not known. More information is needed 

to define the extent of the Bruneau-Grand View 

system and the source of the hot water. Although 

additional deep drilling may discover hot zones in 

the system, there is no evidence in the existing 

data to indicate that large volumes of water much 

hotter than that indicated by the geothermometers will be found associated with the BruneauGrand View system within 3 km of the surface. 

Five smaller systems have been identified in the 

Mountain Home-Twin Falls area to the east of the 

Bruneau-Grand View system. In these systems, 

water is produced from springs or shallow wells 

with calculated reservoir temperatures ranging 

from 103° to 125°C. However, the geothermometers for some of these waters may be unreliable 

(Brook and others, 1979). Some or all of these systems may be continuous with the Bruneau-Grand 

View system. If they are, the resource of hot 

water is much larger than is indicated in Brook 

a~d others (1979). Some of the systems east of 

Bruneau- Grand View appear to be related to the 

northeast margin of the plain and the projection 

of this margin onto the plain, and they may be 

distinct from the Bruneau-Grand View system. 

Fractured volcanic rock is the most likely aquifer, 

and the possiblity exists that significantly higher 

water temperatures occur at depth than are indicated by the existing geothermometer data. Although warm water occurs widely over most of 

the western Snake River Plain and may be of considerable value in low-temperature applications, 

there is no evidence that a high-temperature resource exists outside the Mountain Home-Twin 

Falls area. 

CAMAS PRAIRIE 

Camas Prairie (fig. 2) is an east-trending basin 

lying north of and parallel to the central part of 

the Snake River Plain (Mitchell, 1976a). Although 

separated from the Snake River Plain by a highland, the Mount Bennett Hills, the structure of 

Camas Prairie and the Mount Bennett Hills is 

closely related to the Snake River Plain. Two 

thermal gradient measurements in Camas Prairie 

yielded gradients of about 90°C/km and heat-flow 

values of 125 mW/m2• 

Three geothermal systems are known in the 

area of the prairie that have estimated reservoir 

temperatures of 2::90°C. Wardrop Hot Spring (systern 21, fig. 2), with an estimated reservoir temperature of 97°C, is on the northside of the western part of the prairie where Quaternary alluvium 

is probably underlain at depths of a few tens of 

meters by the Cretaceous Idaho batholith. Barron's Hot Springs (system 22, fig. 2), estimated 

reservoir temperature 103°C, is on the south side 

of the western part of the prairie where Quaternary alluvium is underlain by an unknown thickness of Cenozoic volcanic rocks. The water chemistry of Wardrop Hot Springs is typical of the Idaho 

batholith systems. The water from Barron's Hot 

Springs has slightly higher concentrations of dissolved solids but is within the range of Idaho 

batholith systems. Although considerable volumes 

of water at about 100°C may occur in the central 

and western part of Camas Prairie, there is no 

evidence that indicates the presence of a high-temperature reservoir there. 

The Magic Reservoir system (system 20, fig. 2), 

at the east end of Camas Prairie, has a calculated 

reservoir temperature of 149°C, the third highest 

in Idaho. The Magic Reservoir thermal water has 

high concentrations of dissolved solids (1210 mg/L 

(milligrams per liter)) with high chloride (85 mg/L) 

and high bicarbonate (735 mg/L) (Mitchell, 1976a). 

These values are more like the systems in eastern 

Idaho than those of western Idaho. No systems 

;::::90°C to the west have as much bicarbonate and 

only one, White Licks Hot Springs, has as much 

chloride. The Magic Reservoir system is located 

in an area of complex structure and high heat flow. 

A wide variety of reservoir rocks, including Paleozoic sedimentary rocks, may underlie the area. A 

significant high-temperature reservoir may exist 

there. 

OWYHEE AREA 

The Owyhee area (fig. 2) is southwest of the 

Snake River Plain. Cretaceous intrusive rocks, 

similar to those of the Idaho batholith, and 

metamorphic rock crop out in the Owyhee Mountains, but most of the area is overlain by Tertiary 

silicic volcanic rock. Murphy Hot Springs (system 

29, fig. 2), with an estimated reservoir temperature of 103°C, is the only geothermal system in 

this area that has a reservoir temperature 2::90°C. 

No deep holes have been drilled in this area and 

the deep subsurface geology is not known. Regional heat flow is probably high, and structures 

14 
favorable to the development of hydrothermal convection systems are likely to exist. Additional 

geophysical exploration and drilling is needed to 

evaluate the geothermal resource of this area. 

EASTERN SNAKE RIVER PLAIN 

The size of the geothermal resource that underlies the eastern Snake River Plain is highly uncertain. Abundant recent volcanic activity, including 

large rhyolite domes as young as 300,000 years 

old, and the proximity and possible relationship 

to the huge geothermal systems in Yellowstone 

National Park suggest that a large resource might 

underlie this segment of the plain. However, only 

two hydrothermal convection systems with estimated reservoir temperatures ~oooc have been 

found in the area of the eastern Snake River Plain. 

These systems are along the edge of the plain and 

have estimated reservoir temperatures of only 92° 

and 100°C. Measurement of the regional heat flow 

in the eastern Snake River Plain is difficult because of the effect of water flowing southwest 

through the fractured volcanic rock that underlies 

most of this part of the plain (Brott and others, 

1981). One 3-km-deep drill hole and two shallower 

drill holes on Idaho National Engineering Laboratory are believed to have been drilled deep enough 

to permit measurement of the heat flow below the 

effect of the aquifer. One hole about 900 m deep 

located 8 km from the margin of the plain had 

a gradient in the lower part of the hole of 60°Cikrn 

and indicated a heat flow of 110 mW/m2

• The second hole is 600 m deep and is 25 km from the 

margin near East Butte. Here, the gradient in the 

lower part of the hole is 40°C/km and indicated 

a heat flow of less than 70 m W /m2• The average 

thermal gradient in the deep drill hole about 7 

km from the margin of the plain was about 40°C/ 

km. Heat flow measured in the upper part of the 

hole was 109 mW/m2• Numerous high heat-flow 

values have been obtained from measurements in 

shallow wells near the margins of the eastern 

Snake River Plain. 

Brott and others (1981) have computed the average heat flow below the major aquifer in the eastern Snake River Plain to be 190 mW/m2• This computation is based on measured heat flow at the 

surface of the plain and on calculated heating of 

the water as it moves through the aquifer. They 

pointed out that this value is twice the average 

Basin and Range heat flow and 30 percent higher 

than the average for the Battle Mountain heatflow high in northerl) Nevada. 

Although much has been learned about the eastern Snake River Plain in recent years, large uncertainties still exist concerning the subsurface 

geology and the fundamental structure of the 

plain. Preliminary interpretation of data from an 

extensive seismic refraction survey of the eastern 

Snake River Plain indicate a velocity structure 

similar to that of the western Snake River Plain 

with an upper crust about 10 km thick and a thick 

lower crust (Braile and Smith, 1979). Thus, the 

entire Snake River Plain appears to be underlain 

by an upper crust that is thinner and a lower crust 

that is thicker than adjoining regions to the north 

and south. These seismic data indicate that the 

simple crustal-thinning model of the eastern Snake 

River Plain suggested by Mabey (1978a) on the 

basis of the gravity anomaly is not valid. A model 

consistent with crustal structure inferred from the 

seismic data is consistent with the gravity anomaly (fig. 6). Although the velocity structure 

under the eastern and western parts of the Snake 

River Plain are similar, the contrast in gravity 

and magnetic anomalies and topography suggest 

important differences. The thinning of the upper 

crust under the western Snake River Plain appears to be most pronounced along the axis of the 

plain, whereas under the eastern Snake River 

Plain, the thinned crust is a broader feature extending well beyond the plain. The magnetic 

anomalies indicate a much more complexly magnetized crust under the eastern plain. The topography and gravity data indicate that recent subsidence has been greatest along the axis of the western plain and along the margins of the eastern 

plain. 

The computed map of the base of the magnetized crust (fig. 4) does not indicate a high coincident with the eastern Snake River Plain. The 

magnetotelluric data suggest that the average 

depth to the conductive layer in the crust is greater under the plain than in the adjoining regions. 

Thus, these two data sets do not provide support 

for the presence of high thermal gradient under 

the eastern Snake River Plain. 

The two known hydrothermal convection systems in the area of the eastern Snake River Plain 

with calculated reservoir temperatures ~90°C are 

along the margin of the plain near its northeast 

15 
I~'( --SNAKE RIVER PLAIN-----:;.t)l en 

a: 

w 

1-

w 

-0.3 G/CM3 

:2: 

0 

...J 

~ 0 50 KILOMETERS 

FIGURE 6.-Profile across the eastern Snake River Plain through Arco and Blackfoot, Idaho, showing Bouguer gravity 

anomaly and inferred density mode. g/cm3

, grams per cubic centimeter. 

end. The Newdale system (system 30, fig. 2) is 

within the Rexburg caldera complex (B, fig. 2) 

(Mabey, 1978b; Prostka and Embree, 1978) near 

the northeast edge of the caldera complex. Thermal water with a calculated reservoir temperature 

of 100°C has been found in several wells as deep 

as 250 m, and thermal gradient measurements indicate that the anomaly extends over an area of 

several square kilometers (Brott and others, 

1976). The Newdale system probably involves convection in a zone of complex structure associated 

with the margin of the caldera, perhaps involving 

an intersection of the caldera edge with a normal 

fault or with another caldera. Several other areas 

of thermal water are known within the Rexburg 

caldera complex but the calculated reservoir temperatures are all less than 90°C. Heise Hot 

Springs is near the southern edge of the Rexburg 

caldera complex and is associated with prominent 

gravity and magnetic highs. These anomalies have 

been interpreted as reflecting an uplifted block of 

pre-Tertiary basement underlain by a large intrusive body that may be young enough to be a 

source of heat (Mabey, 1978b). The concentration 

of chloride of the waters is 2400 mg/L. Of the hydrothermal systems in Idaho ?:90°C, only the 

northern Cache Valley group have a higher 

16 

chloride content. Although the geothermometers 

indicate a relatively low reservoir temperature for 

the Heise Hot Springs system, the geological and 

geophysical data suggest that the system is unusual and merits special attention. 

Ashton Warm Springs (system 31, fig. 2) has 

an indicated reservoir temperature of 92°C. The 

subsurface geological setting is obscured by Pleistocene basalt flows. It is between the Rexburg 

caldera complex and the Island Park caldera, and 

as yet undefined caldera structures may underlie 

the basalt in this area. 

Low-temperature thermal waters occur in several other areas near the margins of the eastern 

Snake River Plain, but available evidence does not 

suggest that these areas are underlain by a hightemperature resource. 

Crustal extension and thinning of the upper 

crust may have been important in the formation 

of the eastern Snake River Plain. Crustal extension would be accompanied by high heat flow but 

when the extension stops, the heat flow eventually 

will decline and may drop below the original level 

because of the lowered heat-generating capacity 

of the thinned crust. A broad low-amplitude topographic high is approximately coincident with the 

axis of the eastern Snake River Plain. Two expla-
nations for this high exist: (1) the high is a constructional feature produced by a thicker accumulation of volcanic rock near the axis than along 

the margins, and (2) the margins are subsiding 

more than the axis. Most of the surface basalts 

were erupted from vents along rifts extending 

from the margin of the plain to near the axis. The 

distribution of these vents does not suggest that 

a greater volume of basalt was erupted near the 

axis. In addition to the basalt flows, sediment derived from the adjoining highlands has been deposited near the margins of the plain and is interlayered with the flows. The topographic low and 

the underlying sediments along the margins of the 

plain suggest that the margins of the plain have 

subsided more than the axis. A more rapid subsidence of the margins of the eastern Snake River 

Plain suggests that extension of the crust is concentrated along the margins and that heat flow 

is probably higher there. 

Extension normal to the plain would explain the 

topographic and heat-flow patterns, but there is 

little evidence in the surface geology of active regional extension normal to the plain. Volcanic rifts 

and open fractures are common on the eastern 

Snake River Plain. Most are alined approximately 

normal to the axis of the plain and parallel to the 

basin and range structures to the north and south. 

Apparently, current regional extension is accommodated north of the plain by movement along 

preexisting listric faults in the upper brittle layer 

and by ductile extension in the underlying layer. 

On the plain, this extension produces fissures in 

the upper layer. The greater subsidence along the 

margins of the plain relative to the axis might reflect a thinner ductile layer and incomplete isostatic adjustment of the margins of the plain. 

The most recent volcanic activity on the eastern 

Snake River Plain can be classified into two types: 

eruption of basalt from volcanic rifts and the development of rhyolite calderas and domes. Large 

volumes of basalt have erupted on the eastern 

Snake River Plain in Quaternary time and some 

of the flows are only a few thousand years old. 

The basalt probably originates at great depth and 

rises to the surface in narrow conduits with most 

of the heat released by cooling of the basalt at 

the surface. Only a small part of the heat in the 

molten basalt is used in heating rock adjacent to 

the conduits or stored in magma reservoirs in the 

upper crust. However, the rhyolite may come 

17 

from high-level silicic magma chambers that are 

large reservoirs of heat while they cool. These 

magma chambers could be major sources of heat 

on the eastern Snake River Plain (see Smith and 

Shaw, 1979). The geological and geophysical data 

in the area of the Rexburg caldera complex 

(Mabey, 1978b) are consistent with the existence 

of a high-level magma chamber in this area. Smith 

and Shaw (1979) have estimated the remaining 

thermal energyin this system at 8400 x 1018 jou1es, 

making this the fourth largest energy content in 

igneous systems in the United States and only 

slightly less than in Valles caldera system in New 

Mexico. Although other large caldera systems may 

underlie the eastern Snake River Plain, most are 

likely to be older and thus to contain less thermal 

energy. Big Southern Butte (C, fig. 2) and several 

smaller buttes to the east are rhyolite domes. 

Ages of the rhyolite of these buttes range from 

0.3 to 1.4 m.y., with Big Southern Butte the 

youngest (Kuntz and Dalrymple, 1979). Smith and 

Shaw (1979) estimated the remaining thermal energy in the big Southern Butte igneous systems 

at 240 x 1018 jou1es. However, the available 

geophysical data do not provide any confirmation 

that a large high-level magma chamber underlies 

Big Southern Butte or the other buttes on the 

eastern Snake River Plain. 

In 1979, the U.S. Department of Energy drilled 

a geothermal test well on the Snake River Plain 

at the Idaho National Engineering Laboratory 

about 7 km from the northwest edge of the plain. 

The hole bottomed at 3155 min rhyodacite, which 

may be either a dense welded tuff or a high-level 

intrusive (Doherty and others, 1979). 2 The bottom-hole temperature was 142°C, the permeability 

was low below 500 m, and little water cou1d be 

produced. The hole was located on a zone of 

geophysical anomalies trending parallel to the 

northwest margin of the plain. The anomalies 

were interpreted as reflecting a major boundary 

between an area along the margins of the plain 

where pre-Tertiary rock was generally within 2 

km of the surface, and an area to the southeast 

where the depth and nature of the pre-Tertiary 

rock was uncertain. Subsequent to the drilling, 

Doherty and others (1979) have proposed that the 

well was drilled in intracaldera fill of a large caldera. The caldera they proposed extends north 

from Big Southern, Cedar, Middle, and East 

2L. A. McBroome (oral commun., 1981) reports that this rock is 12.6 m.y. old. 
Buttes and has a maximum dimension of about 30 

km. Kuntz and Covington (1979) inferred that several other large calderas underlie the eastern 

Snake River Plain. Prostka (1979) also postulated 

several calderas underlying the eastern Snake 

River Plain. 

Extensive geological and geophysical surveys 

and drilling in recent years have failed to provide 

any direct evidence for the existence of a major 

high-temperature geothermal resouce underlying 

the central part of the eastern Snake River Plain. 

However, exploration for geothermal energy on 

the eastern Snake River Plain is difficult and much 

more work must be done before the resource in 

this area can be evaluated with confidence. 

The primary information to be considered in estimating the geothermal resource underlying the 

eastern Snake River Plain is: (1) abundant evidence of recent volcanic activity and a suggested 

relationship to the known huge geothermal resource in Yellowstone National Park, (2) warm 

water in springs and shallow wells near the margins of the plain and heating of the water in the 

major aquifer under the plain, (3) three boreholes 

on the plain drilled through the aquifer in which 

were modest thermal gradients and heat flow, (4) 

an indication in the P-wave delay data and the 

low seismisity that the temperatures at depth 

under the plain may be higher than adjoining 

areas, (5) the absence of any indication of anomalously high temperatures under the plain in the 

interpretation of the magnetotelluric and magnetic 

data, (6) an indication in the topography and subsurface data that the margins of this plain may 

have subsided more than the axis, and (7) 

geophysical and borehole information on the subsurface that can be used to infer where the structure would permit the development of hydrothermal convection systems. These data can be interpreted in several ways. 

Brott and others (1981) concluded from their 

thermal studies in the eastern Snake River Plain 

that the heat flow below the major aquifer is high 

and that a large geothermal potential exists below 

the cooling effect of the aquifer. They assumed 

that the aquifer is from 200 to 300 m thick. Thus, 

they suggested that a considerable resource may 

exist within 3 km of the surface. 

Four holes drilled to depths of more than 600 

m for geothermal information are only a small 

sample of the large area of the eastern Snake 

River Plain but they provide the most direct tern18 

perature and geologic data available on the subsurface. One hole about 50 km northeast of Idaho 

Falls was drilled mostly in fractured rhyolite and 

appeared to be in the regional aquifer to the total 

depth of 700 m. The thermal gradient was disturbed by water movement, but the average temperature in the aquifer was about 15°C. Three 

holes drilled on Idaho National Engineering Laboratory appear to have been drilled through the 

regional aquifer. The temperature in the aquifer 

ranged from 13° to 23°C, the gradients below the 

aquifer ranged from 42° to 56°C/km, and the heat 

flow below the aquifer ranged from 66 to 110 mW/ 

m2 with the lowest values for the hole nearest the 

axis of the plain. The deepest hole was 3157 m 

with a bottom-hole temperature of 142°C. The hydraulic conductivity below the regional aquifer 

was generally low and little water could be produced. 

Although any conclusion about the entire eastern Snake River Plain based on these four holes 

should be drawn with great care, the data from 

these holes do not provide support for a high heat 

flow under the central part of the plain. Nor is 

there evidence in the temperatures of the regional 

aquifer that major heating of the water is occurring in the central part of the plain. The inability 

of the deep hole to produce fluid indicates that, 

at least in one location, the deep structure is not 

favorable for the development of hydrothermal 

convection systems. 

Although an uncertainty exists concerning the 

likelihood of a major geothermal resource occurring under the central part of the eastern Snake 

River Plain, along the margins conditions appear 

favorable for the development of geothermal systems. Here, regional heat flow is high and complex 

structures favoring deep circulation of water are 

likely to exist at the intersection of the margins 

of the plain with calderas on the plain and with 

basin and range structures approximately normal 

to the plain. Geophysical data suggest that pre-Cenozoic sedimentary rocks that are known to be 

reservoirs south of the plain extend at least a 

short distance onto the plain. Exploration should 

progress from the margins of the plain toward the 

axis. 

ISLAND PARK AREA 

The Island Park area (fig. 2) as used in this 

report is the area lying between the northeast end 

of the Snake River Plain and Yellowstone National 
Park. It is approximately coincident with the 

Upper Henrys Fork basin. Except for the north 

edge, the entire area is underlain by Cenozoic volcanic rocks that locally are overlain by Quaternary 

alluvium. 

Several thermal springs have been identified in 

the Island Park area (Whitehead, 1978), but none 

have water temperatures ;:::25°C or geochemical 

temperatures ;:::90°C. The average annual precipitation in this area is estimated to be 90 em and 

the water table is commonly near the surface. The 

volcanic rocks are probably highly fractured, and 

the movement of nonthermal water in these fractures is likely to result in mixing between cold 

ground water moving down and laterally with ascending thermal waters. Thus, the absence of surface water with high measured or geochemical 

temperatures does not preclude that high temperatures exist at depth. 

The primary indication that a large geothermal 

resource might occur in the area is the geology 

(Christiansen, 1975). Between about 1 and 2 million years ago, large volumes of rhyolite were 

erupted from the Island Park area during three 

cycles of activity associated with the development 

of the Island Park caldera system (A, fig. 2). This 

rhyolite is presumed to have come from a large 

rhyolitic magma body underlying the caldera system. Although this body has been cooling for more 

than a million years, considerable thermal energy 

must remain in the system. Smith and Shaw (1979) 

estimated that the system contains 16,850x 1018 

joules of thermal energy. Of the igneous systems 

in the United States, this energy content is second 

only to that of the Yellowstone caldera system and 

is more than twice that of the third largest system-Valles caldera. Although anticipated temperatures are lower than for younger systems, a 

large geothermal resource might exist. 

The geophysical data from the area are inconclusive. A comparison of the geophysical anomalies 

with two other caldera systems in the region illustrates the problem of interpreting the data from 

the Island Park area. To the east, the Yellowstone 

caldera system is younger and larger. To the 

southwest, the Rexburg caldera complex is of 

similar size but is older. Large gravity lows are 

related to both the Yellowstone and the Rexburg 

features but no large gravity anomaly is coincident 

with the Island Park caldera system. The Y ellowstone gravity low has been interpreted as reflecting a molten rhyolite body (Eaton and others, 

19 

1975). The Rexburg low is thought to reflect 

primarily the rock filling the caldera, but some 

effect of a solidified magma body also may be present (Mabey, 1978b). The absence of a gravity anomaly at Island Park can be explained in either 

of two basic ways: (1) There may be no large lateral density contrasts. If there is thick caldera fill 

or a large intrusive body underlying the area, they 

are enclosed in rock of approximately equal 

density. (2) Positive anomalies (basalt) and negative mass anomalies (rhyolite and sediments) of 

approximate equal amplitude occur and the total 

effect is near zero. 

The regional free-air anomaly values in the Island Park area suggest that the area is in approximate isostatic equilibrium with the areas to the 

north, west, and south. On the east and extending 

a short distance into the Island Park area is the 

large gravity low associated with Yellowstone 

caldera system. The suggestion was made first by 

Eaton and others (1975) and later was expanded 

upon by Brott and others (1981) that the formation 

of the eastern Snake River Plain could in part relate to the cooling and contraction of a hot mass 

underlying the plain. The Island Park area, which 

is intermediate between the Yellowstone caldera 

system and the Snake River Plain in both gravity 

and topography, may be underlain by a partly 

cooled mass. Perhaps with further cooling and contraction, the area will subside and a Bouguer gravity high will develop. 

The magnetic intensity in the Island Park area 

is generally low, suggesting more of an affinity 

with the Yellowstone caldera system than with the 

eastern Snake River Plain, but areas of low intensity of almost equal area do occur on the eastern 

Snake River Plain. Computations of the elevation 

of the Curie isotherm surface for southeastern 

Idaho (Bhattacharyya and Mabey, 1980) did not 

produce a determination in the central part of the 

Island Park area, but a determination near the 

north border and outside of the magnetic low is 

11 km below sea level, or about 13 km below the 

land surface. Within the central part of the area, 

the Curie isotherm surface may be significantly 

higher but not likely as high as under the Y ellowstone caldera system where the Curie isotherm 

is computed to be 5-6 km below the land surface 

(Bhattacharyya and Leu, 1975). 

The Island Park caldera system is a major resistivity anomaly. Audio-magnetotelluric measurements (Hoover and Long, 1976) indicated that the 
resistivity of the rocks within 2 km of the surface 

are generally more than 100 ohm-m (ohm meter) 

and as high as 1000 ohm-m. Deeper probing magnetotelluric sounding (Stanley and others, 1977) 

indicates that rock with high resistivity (more 

than 500 ohm-m) extends to the depths perhaps 

as much 8 km. In contrast, resistivities in the Y ellowstone caldera system are low, with resistivities 

generally less than 50 ohm-m in the upper 5 km 

and less than 5 ohm-m at depths greater than 5 

km. 

The resistivity data from the Island Park area 

can be interpreted in two quite difference ways 

as an indication of temperatures at depth. Hoover 

and Long (1976, p. 1063) concluded that the high 

resistivities determined from audio-magnetotelluric and telluric current surveys "implies that the 

caldera has cooled, that there is little rock alteration, and that the area is not now a very promising 

exploration target." W. D. Stanley (oral commun., 

1979), on the other hand, postulated that the high 

resistivities at depth under the Island Park caldera system reflect zones from which water has 

been purged by high temperatures. 

Few large earthquakes occur in the Island Park 

area. The seismic activity is thus similar to the 

eastern Snake River Plain which is almost void 

of earthquakes, as contrasted with intense earthquake activity in some areas of the Yellowstone 

caldera system. 

Although no data are available for the Island 

Park area to define the deep thermal gradient, 

the geology and measured thermal gradients in adjoining areas suggest that the thermal gradients 

below the hydrologically disturbed zone are likely 

to be at least 50°Cikrn and perhaps considerably 

higher. Regional tectonic patterns suggest that 

the area is likely to be extending. The resulting 

fracture systems should provide conduits for deep 

circulation of water and the development of hydrothermal convection systems. The existence of 

large volumes of rock and water at temperatures 

higher than 150°C within 3 km of the surface appears likely. Deep drilling will be required to 

evaluate the geothermal resources of the Island 

Park area but the existing data suggest that it 

is one of the most promising areas in southern 

Idaho. 

LOST RIVER AREA 

The Lost River area (fig. 2), which is east of 

the Idaho batholith and north of the Snake River 

Plain, contains only one geothermal system with 

an estimated reservoir temperature ~90°C. Early 

Tertiary intrusive and extrusive rocks are common in this area but younger volcanic rocks are 

not. The elevation of this region is the highest 

in Idaho, and the movement of cold ground water 

downward and away from this high area may inhibit the development of geothermal systems in 

the near surface. The regional heat flow has not 

been defined, but the magnetic data suggest a 

high in the Curie isotherm surface over most of 

this region. An active seismic zone extends across 

the northern part of the area and evidence of 

young normal faulting occurs throughout much of 

the area. Active extension with attendant crustal 

thinning and high heat flow is suggested. Large 

gravity lows occur in Lemhi, Birch Creek, 

Pahsimeroi, and Little Lost River Valleys and 

smaller lows in the Big Lost River Valley. These 

anomalies reflect thick accumulations of Cenozoic 

rock that may act as thermal blankets and increase 

the temperature at depth under the valleys. The 

Cenozoic rocks underlying the valleys are potential reservoir rocks, as are Paleozoic and Precambrian sedimentary rocks that are several kilometers thick. Although the direct indicators of a 

geothermal resource are lacking, the area offers 

sufficient promise to justify additional exploration 

to determine the regional heat flow. 

SOUTHEASTERN IDAHO 

Southeastern Idaho (fig. 2) as used here is south 

of the eastern Snake River Plain. It is an area 

of basin and range structure with Cenozoic fill in 

the basins as much as 4 km thick. The Albion 

Mountains have a core of Precambrian metamorphic rock and the next east, the Cottrel and Jim 

Sage Mountains, is entirely Cenozoic volcanic and 

sedimentary rocks. The ranges to the east are predominantly Precambrian, Paleozoic and Mesozoic 

sedimentary rocks locally overlain by Cenozoic 

sedimentary rocks. One area of young volcanic 

rocks, Blackfoot lava fields, is of special geothermal interest. 

Five geothermal systems with calculated reservoir temperatures of ~oooc have been identified 

in this area. The Raft River system (Williams and 

others, 1976) (system 32, fig. 2) is near the western edge of the area and has measured reservoir 

temperatures of from 146° to 149°C. Water from 

this system may be used by the U.S. Department 

of Energy in a demonstration project to produce 

20 
electricity from this relatively low-temperature 

water, and it is the most intensely studied system 

in Idaho. The other four systems with calculated 

reservoir temperatures from 93° to 119°C are 

along the Bear River within 27 km of Preston 

(Mitchell, 1976a). 

Heat-flow data are sparse in southeastern 

Idaho, but the available values are generally low 

except near· the margins of the Snake River Plain 

and in the vicinity of the Raft River system where 

a background value of about 100 mW/m2 is indicated. Although other local heat-flow anomalies of 

substantial extent may exist in the area, the average heat flow is probably less than 80 m W /m2 • 

The primary reservoir of the Raft River system 

is in fractured Tertiary sedimentary rocks overlying a less disturbed pre-Tertiary basement complex (Williams and others, 1976; Mabey, Hoover, 

and others, 1978). The fracturing of the reservoir 

rock has been interpreted as resulting from movement along a low-angle detachment fault near the 

contact between the Tertiary sedimentary rocks 

and the underlying basement. Deeper circulation 

may be controlled by a northeast-trending shear 

zone. Both a magnetotelluric sounding and the 

magnetic data have been interpreted as indicating 

a high-temperature anomaly in the crust. The extent of the Raft River system has not been determined, but reservoir volume is estimated to be 

about 20 km3• Other similar systems may occur 

around the Albion Mountains. 

Gravity data (Peterson and Oriel, 1970) suggest 

that three of the four systems near Preston are 

along normal faults bounding a prism of Cenozoic 

sediments about 2 km thick underlying northern 

Cache Valley. No information exists on the volume 

of the reservoirs associated with these systems. 

Two of them-Squaw Springs and Wayland Hot 

Springs (systems 35 and 36, fig. 2; Wayland Hot 

Springs is called Battle Creek Hot Springs in 

Mitchell, 1976a)- are 1. 6 km apart, have similar 

indicated reservoir temperatures and water 

chemistry (9,000-12,000 mg/L total dissolved solids). They are along the same fault suggested by 

gravity data and may be a part of the one large 

system. Possible reservoir rocks are lower units 

in the Tertiary section or pre- Tertiary rocks. No 

evidence of a local heat source has been reported 

and the water is probably heated by deep circulation. Although the area merits more study, there 

is no suggestion in the available data of a large 

21 

reservoir of water at a temperature higher than 

120°C. 

The Riverdale system (system 34, fig. 2; Ben 

Meek Well in Mitchell, 1976a) has a lower calculated reservoir temperature and lower concentration of dissolved solids. Structural control and reservoir rocks for this system are likely to be similar 

to those of the system on the west side of the 

valley. Mitchell (1976a) described the northeasttrending Mink Creek-Bear River lineament passing through the three Cache Valley systems and 

suggests that the intersection of the lineament 

with a north-trending normal fault may control the 

hot-spring activity on the west side of the valley. 

The same situation exists on the east side of the 

valley. Other northeast-trending features parallel 

to the axis of the eastern Snake River Plain can 

be identified in southern Idaho, and their intersection with north-trending fault zones merit special 

attention in geothermal exploration. 

Maple Grove Hot Spring (system 33, fig. 2) is 

located where a deep canyon of the Bear River 

cuts into Paleozoic sedimentary rock. The calculated reservoir temperature is lower (93°C) and 

the concentration of dissolved solids is lower (2000 

mg/L) than the waters of the Cache Valley systems. Although several lower temperature hot 

springs occur about 2 km to the north, there is 

no evidence that a large high-temperature reservoir exists in this area. 

Several oil tests drilled in the overthrust belt 

of southeastern Idaho have reported high temperatures and large flows of water from pre-Tertiary 

rocks. Not enough information has been made 

public to evaluate the possible resource found in 

this drilling. 

Quaternary basalt flows occur in several valley 

areas in a north-trending zone about 35 km wide 

and about 100 km long extending from near the 

southeast margin of the Snake River Plain to a 

few kilometers south of Soda Springs. The known 

vents for. these flows are considerably less extensive than the flows but are widespread. Some of 

the flows are slightly deformed and inversely magnetized, indicating that they are more than 

700,000 years old. The age of the youngest basalt 

flows have not been determined, but most of the 

basalt is probably more than 50,000 years old. 

Near the center of the zone of basalt flows is the 

Blackfoot lava field (D, fig. 2), which appears to 

have been the source area for much of the basalt. 

Here, in addition to about 25 basalt cinder cones 
and vents are several rhyolite domes. Some of 

these domes have been dated at about 50,000 

years B.P. (before the present; Leeman and Gettings, 1977). 

The geophysical data from the Blackfoot lava 

field have been interpreted in several ways. 

Mabey and Oriel (1970) concluded that the gravity 

low and magnetic high could be produced by a volcanic collapse related to the withdrawal of magma 

at depth, but they recognized that part of each 

anomaly might be produced by an underlying intrusive body. Leeman and Gettings (1977) interpreted the same data to indicate a large hot laccolithic body at shallow depth. Audio-magnetotelluric data (D. B. Hoover, written commun., 1975) 

show that the resistivities to depths of 2 km are 

high, but magnetotelluric data (J. E. O'Donnell, 

written commun., 1980) indicate a highly conductive zone at depths of less than 10 km. 

Numerous thermal springs occur in the general 

area of the Blackfoot lava field and a high temperature has been reported in an oil test a few kilometers to the north; however, none of the waters 

from springs or shallow wells have temperatures 

higher than 42°C and calculated reservoir temperatures are generally low (Mitchell, 1976c). Large 

volumes of cold ground water are moving through 

the basalt aquifers and may be mixing with ascending hot water and masking surface evidence 

of high temperature underlying the Blackfoot lava 

field. Geological and geophysical evidence of a 

large thermal anomaly in the area suggest that 

it is the most promising area in southeastern 

Idaho for the occurrence of large high-temperature geothermal reservoirs. Geothermal gradient 

surveys in drill holes several hundred meters deep 

will be required to confrrm or refute the existence 

of a major geothermal anomaly. 

CONCLUSIONS 

Large volumes of hot water are known to occur 

in southern Idaho. The large-scale development of 

this resource for direct application to space heating and industrial processes is highly likely. Thus, 

geothermal energy appears to be an important future source of energy in southern Idaho. Reservoirs of water at the higher temperatures that are 

currently considered necessary for the commercial 

generation of electricity have not been found in 

southern Idaho. Exploration for geothermal energy in Idaho is unusually difficult because of an 

22 

extensive cover of young sedimentary and volcanic 

rocks in many areas of interest and problems related to obtaining heat-flow and thermal-gradient 

data undisturbed by ground-water movement. 

However, on the basis of the geology and geophysical anomalies, several areas have been identified 

as favorable for the occurrence of high-temperature resources and additional exploration in these 

areas has promise of locating a large high-temperature resource. 

REFERENCES CITED 

Armstrong, R. L., Leeman, W. P., and Malde, H. E., 1975, 

Quaternary and Neogene volcanic rocks of the Snake River 

Plain, Idaho: American Journal of Science, v. 275, p. 225-

251. 

Bhattacharyya, B. K., and Leu, L. K., 1975, Analysis of magnetic anomalies over Yellowstone National Park-Mapping 

of Curie-point isothermal surface for geothermal reconnaissance: Journal of Geophysical Research, v. 80, p. 4461-

4465. 

Bhattacharyya, B. K., and Mabey, D. R., 1980, Interpretation 

of magnetic anomalies over southern Idaho using 

generalized multibody models: U.S. Geological Survey 

Open-File Report 80-457, 49 p. 

Blackwell, D. D., 1978, Heat flow and energy loss in the Western United States, in Smith, R. B., and Eaton, G. P., 

eds., Cenozoic tectonics and regional geophysics of the 

Western Cordillera: Geological Society of America Memoir 

152, p. 175-208. 

Bowen, R. G., and Blackwell, D. D., 1975, The Cow Hollow 

geothermal anomaly, Malheur County, Oregon: The Ore 

Bin, v. 37, p. 109-121. 

Braile, L. W., and Smith, R. B., 1979, The structure of the 

crust in the Yellowstone-Snake River Plain area and adjacent provinces and implications for crustal evolution: EOS, 

v. 60, p. 941. 

Brook, C. A., Mariner, R. H., Mabey, D. R., Swanson, J. 

R., Guffanti, Marianne, and Muffler, L. J. P., 1979, Hydrothermal convection systems with reservoir temperatures ~90°C, in Muffler, L. J. P., ed., Assessment of 

geothermal resources of the United States-1978: U.S. 

Geological Survey Circular 790, p. 18--85. 

Brott, C. A., Blackwell, D. D., and Mitchell, J. C., 1976, 

Geothermal investigations in Idaho, part 8, Heat flow 

study of the Snake River Plain, Idaho: Idaho Department 

of Water Resources Water Information Bulletin 30, 195 

p. 

Brott, C. A., Blackwell, D. D., and Mitchell, J. C., 1978, Tectonic implications of the heat flow of the western Snake 

River plain: Geological Society of America Bulletin, v. 89, 

p. 1697-1707. 

Brott, C. A., Blackwell, D. D., and Ziagos, J. P., 1981, Thermal and tectonic implications of heat flow in the eastern 

Snake River Plain, Idaho: Journal of Geophysical Research, v. 86, no. B12, p. 11709-11734. 

Christiansen, R. L., 1975, Origin and geothermal potential of 

Island Park, eastern Idaho: Geological Society of America 

Abstracts with Programs, v. 7, p. 595-596. 
Christiansen, R. L., and McKee, E. H., 1978, Late Cenozoic 

volcanic and tectonic evolution of the Great Basin and Columbia intermountain regions, in Smith, R. B., and Eaton, 

G. P. eds., Cenozoic tectonics and regional geophysics in 

the Western Cordillera: Geological Society of America 

Memoir 152, p. 283-312. 

Doherty,· D. J., McBroome, L. A., and Kuntz, M. A., 1979, 

Preliminary geological interpretation and lithologic log of 

the exploratory geothermal Test Well (INEL-1), Idaho 

National Engineering Laboratory, eastern Snake River 

Plain, Idaho: U.S. Geological Survey Open-File Report 79-

1248, 10 p. 

Eaton, G. P., Christiansen, R. L., lyer, H. M., Pitt, A. M., 

Mabey, D. R., Blank, H. R., Zietz, Isidore, and Gettings, 

M. E., 1975, Magma beneath Yellowstone National Park: 

Science, v. 188, p. 787-796. 

Eaton, G. P., Prostka, H. J., Oriel, S. S., and Pierce, K. L., 

1976, Cordilleran thermal tectonic anomaly; I. Geophysical 

and geological evidence of coherent late Cenozoic intraplate magmatism and deformation [abs.]: Geological Society of America Abstracts with Programs, v. 8, no. 6, p. 

850. 

Eaton, G. P., Wahl, R. R., Prostka, H. J., Mabey, D. R., 

and Kleinkopf, M.D., 1978, Regional gravity and tectonic 

patterns; Their relation to Late Cenozoic epeirogeny and 

lateral spreading in the Western Cordillera: Geological Society of America Memoir 152, p. 51-92 

Evans, J. R., and lyer, H. M., 1979, Deep structure} under 

Yellowstone and the eastern Snake River Plain from teleseismic P-wave delays: EOS, v. 60, n. 46, p. 942. 

Fitterman, D. V., 1979, Geomagnetic induction study across 

the eastern Snake River Plain, Idaho: EOS, v. 60, no. 

46, p. 944. 

Hoover, D. B., and Long, C. L., 1976, Audio-magnetotelluric 

methods in reconnaissance geothermal exploration: United 

Nations symposium on the development and use of geothermal resources, 2d, San Francisco, Calif., 1975, Proceedings, v. 2. p. 1059-1064. 

Kuntz, M. A., and Covington, H. R., 1979, Do basalt structures and topographic features reflect buried calderas in 

the eastern Snake River Plain: EOS, v. 60, no. 46, p. 945. 

Kuntz, M. A., and Dalrymple, G. B., 1979, Geology, geochronology, and potential volcanic hazards in the Lava 

Ridge-Hells Half Acre area, eastern Snake River Plain, 

Idaho: U.S. Geological Survey Open-File Report 79-1657, 

72p. 

Lachenbruch, A. H., and Sass, J. H., 1978, Models of an extending lithosphere and heat flow in the Basin and Range 

province, in Smith, R. B., and Eaton, G. P., eds., Cenozoic tectonics and regional geophysics of the Western Cordillera: Geological Society of America Memoir 152, p. 209-250. 

Leeman, W. P., and Gettings, M. E., 1977, Holocene rhyolite 

in SE Idaho and geothermal potential: EOS v. 58, no. 12, 

p. 1249. 

Mabey, D. R., 1976, Interpretation of a gravity profile across 

the western Snake River Plain, Idaho: Geology, v. 4, p. 

53-55. 

--1978a, Regional gravity and magnetic anomalies in the 

eastern Snake River Plain, Idaho: U.S. Geological Survey 

Journal of Research, v. 6, no. 5, p. 553-562. 

--1978b, Gravity and aeromagnetic anomalies in the Rexburg area of eastern Idaho: U.S. Geological Survey OpenFile Report 78-382, 19 p. 

23 

Mabey, D. R., Hoover, D. B., O'Donnell, J. E., and Wilson, 

C. W., 1978, Reconnaissance geophysical studies of the 

geothermal system in southern Raft River Valley, Idaho: 

Geophysics, v. 43, no. 7, p. 1470--1484. 

Mabey, D. R., and Oriel, S. S., 1970, Gravity and magnetic 

anomalies in the Soda Springs region , southeastern Idaho: 

U.S. Geological Survey Professional Paper 646-E, 15 p. 

Mabey, D. R., Zietz, I., Eaton, G. P., and Kleinkopf, M. D., 

1978, Regional magnetic patterns in part of the Cordillera 

in the Western United States, in Smith, R. B., and Eaton, 

G. P., eds., Cenozoic tectonics and regional geophysics of 

the Western Cordillera: Geological Society of America 

Memoir 152, p. 93--106. 

Mcintyre, D. H., 1976. Photogeologic map of Cambridge quadrangle and western half of the Council quadrangle, western Idaho: U.S. Geological Survey Open-File Report 76-

857. 

--1979, Preliminary description of Anschutz Federal No. 

1 drill hole, Owyhee County, Idaho: U.S. Geological Survey Open-File Report 79-651, 14 p. 

Mitchell, J. C., 1976a, Geothermal investigations in Idaho, part 

7, Geochemistry and geologic setting of the thermal wat~rs 

of the Camas Prairie area, Blaine and Camas Counties, 

Idaho: Idaho Department of Water Resources Water Information Bulletin 30, 44 p. 

--1976b, Geothermal investigations in Idaho, part 5, 

Geochemistry and geologic setting of the thermal waters 

of the northern Cache Valley area, Franklin County, 

Idaho: Idaho Department of Water Resources Water Information Bulletin 30, 47 p. 

--1976c, Geothermal investigations in Idaho, part 6, 

Geochemistry and geologic setting of the thermal and mineral water of the Blackfoot Reservoir area, Caribou 

County, Idaho: Idaho Department of Water Resources 

Water Information Bulletin 30, 47 p. 

Mitchell, J. C., Johnson, L. L., and Anderson, J. E., 1980, 

Geothermal investigations in Idaho, part 9, Potential for 

direct heat application of geothermal resources: Idaho Department of Water Resources Water Information Bulletin 

30,396 p. 

Muffler, L. J. P., ed., 1979, Assessment of geothermal resources of the United States--1978: U.S. Geological Survey Circular 790, 163 p. 

Peterson, D. L., and Oriel, S. S., 1970, Gravity anomalies in 

Cache Valley, Cache and Box Elder Counties, Utah, and 

Bannock and Franklin Counties, Idaho, in Geological Survey Research, 1970: U.S. Geological Survey Professional 

Paper 700-C, p. C114-C118. 

Prostka, H. J., 1979, Buried Pliocene calderas of the eastern 

Snake River Plain: EOS, v. 60, no. 46, p. 945. 

Prostka, H. J., and Embree, G. F., 1978, Geology and geothermal resources of the Rexburg area, eastern Idaho: U.S. 

Geological Survey Open-File Report 78-1009, 15 p. 

Smith, R. B., 1978, Seismicity, crustal structure, and intraplate tectonics of the interior of the Western Cordillera, 

in Smith, R. B., and Eaton, G. P., eds., Cenozoic tectonics 

and regional geophysics of the Western Cordillera: Geological Society of America Memoir 152, p. 111-144. 

Smith, R. L., and Shaw, H. R., 1979, Igneous-related geothermal systems, in Muffler, L. J. P., ed., Assessment of 

geothermal resources of the United States--1978: U.S. 

Geological Survey Circular 790, p. 12-17. 
Stanley, W. D., Boehl, J. E., Bostick, F. X., and Smith, H. 

W., 1977, Geothermal significance of magnetotelluric 

soundings in the eastern Snake River Plain-Yellowstone 

region: Journal of Geophysical Research, v. 82, p. 2501-

2514. 

Urban, T. C., and Diment, W. H., 1975, Heat flow on the 

south flank of the Snake River rift [abs.]: Geological Society of America Abstracts with Programs, v. 7, p. 648. 

Whitehead, R. L., 1978, Water resources of the upper Henrys 

Fork Basin in eastern Idaho: Idaho Department of Water 

Resources Water Information Bulletin 46, 91 p. 

Wier, Stuart, 1979, Geomagnetic variation anomalies on the 

central Snake River Plain: EOS, v. 60, no. 46, p. 944. 

Williams, P. L., Mabey, D. R., Zohdy, A. A. R., Ackermann, 

Hans, Hoover, D. B., Pierce, K. L., and Oriel, S. S., 1976, 

Geology and geophysics of the southern Raft River Valley 

geothermal area, Idaho, USA: United Nations symposium 

on the development and use of geothermal resources, 2d, 

San Francisco, Calif., 1975, Proceedings, v. 2, p. 1273-

1282. 

24 

u U.S. GOVERNMENT PRINTING OFFICE: 1983- 776-041-4002 

Young, H. W., and Lewis, R. E., 1980, Hydrology and 

geochemistry of thermal groundwater in southwestern 

Idaho and north-central N evaoa: U.S. Geological Survey 

Open-File Report 80-2043, 40 p. 

Young, H. W., and Mitchell, J. C., 1973, Geothermal investigations in Idaho, part 1, Geochemical and geologic setting 

of selected thermal waters: Idaho Department of Water 

Administration Water Information Bulletin 30, 43 p., 

Young, H. W., and Whitehead, R. L., 1975a, Geothermal investigations in Idaho, part 2, An evaluation of thermal 

water iri the Bruneau-Grand View Area, southwestern 

Idaho: Idaho Department of Water Resouces Water Information, Bulletin 30, 126 p. 

--1975b, Geothermal investigations in Idaho, part 3, An 

evaluation of the thermal water in the Weiser area, Idaho: 

Idaho Department of Water Resources Water Information 

Bulletin 30, 35 p. 

Zoback, M. L., and Thompson, G.A., 1978, Basin and Range 

rifting in northern Nevada; Clues from a mid-Miocene rift 

and its subsequent offsets: Geology, v. 6, p. 111-116.