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Evidence from Heat-Flow Measurements for Laterally

Extensive Geothermal Fluid Systems in the Yellowstone

Caldera, Yellowstone National Park, Wyoming, USA

PAUL MORGAN

Department of Physics/Earth Sciences, New Mexico State University, Box 3D, Las Cruces, New Mexico 88003,

' USA

DAVID D. BLACKWELL

ROBERT E. SPAFFORD

Institute for the Study of Earth and Man, Department of Geological Sciences, Southern Methodist University,

ABSTRACT

Dallas, Texas 75275, USA

ROBERT B. SMITH

Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA

Yellowstone Lake straddles the boundary of the Quater-

nary Yellowstone caldera and marine-type heat-flow deter-

minations have been made in the lake to study the thermal

regime in the caldera and its transition across the caldera

boundary. Twenty-two temperature gradient sites were

occupied in the lake, and gradients ranging from 120 to

20 420°C / km were measured. Results from thermal con-

ductivity determinations on the lake sediments indicate a

uniform conductivity and the heat flow values are in the

range 2.2 to 370 tical cm-2 sec-1. There is a sharp transition

from lower to higher heat flow between the southern and

the northern parts of the lake. This is interpreted as the

result of a laterally extensive thermal fluid system beneath

the northern portion of the lake. A similar, but shallower

system is thought to cause even more heat flow in West

Thumb, but the very high values determined are probably

the result of localized, possibly structurally controlled,

hydrothermal convection systems. The average heat flow

in the caldera is on the order of 15 Fcal cm-2 sec-' and

convective heat loss represents shallow redistribution of

the conductive flow.

INTRODUCTION

Yellowstone National Park is famous for its remarkable

array of geysers, hot sbrings, and other thermal phenomena

which in total number and variety are unsurpassed through-

out the world. The main area of the park is in northwestern

Wyoming, overlapping on the north and west boundaries

into Montana and Idaho (Fig. 1 ). It lies on the east boundary

of the broad region of high heat flow in the western United

States which includes the Basin and Range, Columbia

Plateau, and Northern Rocky Mountain provinces, an exten-

sive tectonic zone characterized by Cenozoic volcanism and

5151

tectonic and seismic activity (Roy, Blackwell, and Decker,

1972; Smith and Sbar, 1974). The Yellowstone area is also

at the east end of the Snake River plains, an east-west

trending volcanic zone marked by a spatial migration from

west to east of the onset of silicic volcanism, starting in

western Idaho with the eruption of the Columbia River basalt

around 14 to 18 m.y. and progressing easterly toward

Yellowstone (Armstrong, Leeman, and Malde, 1975). The

oldest cycle of volcanic activity in Yellowstone area is

approximately 1.9 m.y. in the Island Park caldera and

includes the western part of the Yellowstone caldera. This

easterly progression appears to be continuing with a second •.

volcanic cycle confined to the Island Park area around 1.2

m.y. and activity initiating in the Yellowstone caldera shortly

after that event. Around 600 000 years ago a catastrophic

eruption of rhyolite, pumice, and ash totalling more than

900 km3 resulted in the collapse along ring fracture zones

to form the Yellowstone caldera (Eaton et al., 1975), 70

km long and 45 km wide with its long axis trending in

a northeasterly direction (Fig. 1 ). Rhyolite flows were

erupted from vents in a fracture system encircling the central

part of Yellowstone Park, forming a large rhyolite plateau.

The flows are as young as 70 000 years (Christiansen, 1974,

personal commun.; Eaton et al., 1975); and although there

are no historical records of volcanic eruptions in Yellowstone

Park, there is no evidence that the volcanic activity has

ceased because of the long period of intermittent activity

in the caldera area.

Geophysical studies have provided evidence for the exis-

tence of a magma chamber beneath the caldera (Smith et

al., 1974; Eaton et al., 1975). A large gravity low is found

over the rhyolite plateau with a steep gradient inside the

mapped caldera boundary. Extensive seismic activity has

been recorded around the caldera, but earthquakes within

the caldera are much less abundant. The earthquakes that

1155

1156

Figure 1. Location map for Yellowstone National Park, theYellowstone caldera, and Yellowstone Lake. (Caldera boundary

from Keefer, 1971 ).

have been located within the caldera boundary originateat shallow depths of less than 5 km as compared to maximumdepths of 15 km outside the caldera (Smith et al., 1974).Studies of seisinic waves crossing the caldera indicateattenuation and local delays of P-waves and shadowing ofS-waves. These data are consistent with the existence ofa shallow magma chamber or magma plexis beneath thecaldera. The extensive loss of heat by convection fromthe hydrothermal areas (Fournier, 1974), the extensivehistory of silicic volcanism, and the high conductive heat-flow values energetically require the presence of a largevolume of still partially molten or recently solidified silicicrock. Hydrothermal activity in the park is structurallycontrolled by the caldera, and heat-flow studies in boreholeshave been interpreted as indicating hydrothermal convectioncontrolled by fracture systems (White et al., 1975). In spiteof extensive thermal studies for the whole park and drillingand subsidiary studies in the hydrothermal systems (White,Muffler, and Truesdell, 1971; White et al., 1975), no conduc-tive heat flow studies have been reported previous to thisstudy.

Yellowstone National Park undoubtedly contains the larg-est localized reservoir of geothermal energy in the UnitedStates. Although designation of the area as a national parkprevents exploitation of this resource, scientific studies inthe park can provide useful base data and definition ofearth models for interpretation of less extensively studiedgeothermal areas and for the interpretation of caldera sys-tems in different stages of development. This paper presentsnew marine-type heat-flow data from Yellowstone Lake.The data were collected during an extensive geophysicalstudy of Yellowstone Lake including geologic, seismic,magnetic, and thermal studies (Smith et al., 1974; AmericanGeophysical Union, 1974; Otis, Smith, and Wold, 1974).

Yellowstone Lake straddles the southeastern boundaryof the Yellowstone caldera (Fig. 1 ), and heat-flow data from22 sites in this lake provide information on the transitionof the heat flow from outside the caldera to within thecaldera boundary. The new data provide evidence on thegeothermal conditions in the portion of the caldera overlainby the lake. Yellowstone Lake is approximately 32 km in

length with a maximumwidth of 23 km (Fig. 2). The maximumdepth of the lake is 94 m, and the bottom water temperatureranges from 3.2 to 4.0°C during the year. The location ofthe caldera boundary relative to the lake is shown in Figure1. The ring fracture zone and the steep gravity gradientoccur 5 to 10 km inside the caldera boundary. Mary Bayand West Thumb (Fig. 2) are thought to be relatively recentsubcalderas on the margins of the lake (Christiansen andBlank, 1972). A brief description of the data collectiontechniques will be given before the presentation of the data.The data will then be discussed with regard to the geothermalimplications.

HEAT-FLOW DETERMINATIONSSediment temperature gradient measurements in the lake

were made with a 4.5-m probe with four thermistor tempera-ture sensors at 1 -m intervals. Thermistor resistances weremeasured as a function of time for a period of up to 40min after penetration of the probe into the sediment toallow extrapolation to undisturbed temperatures. The ex-trapolated temperatures are generally considered to be withind:0.002°C of the in place temperatures. Sensors in the probeindicated the angle of penetration, which was within 10degrees of vertical for all stations reported here, and thedepth of penetration of the probe was indicated by a slideon the probe. Twenty-two temperature gradient measure-ment sites were occupied as shown in Figure 2. Thermalconductivity determinations were made by the needle probemethod (Von Herzen and Maxwell, 1959) on sediment corescollected by a piston corer at eight sites throughout thelake. Corrections to the temperature data have been calcu-lated for the effects of bottom-water temperature changes,topography, thermal refraction, and sedimentation; but thesedo not significantly alter the general pattern of the resultsnor the implications of the data and will be discussed inanother publication.

The temperature data are shown in Table 1 and thegradients are plotted in Figure 2. The lowest temperature

Figure 2. Temperature gradient measurement sites in Yel-lowstone Lake. Meantemperature gradients (28Julyto 3 August

1974) are given at each site in °C/km.

MORGAN, BLACKWELL, SPAFFORD, AND SMITH

45° 00'

44'30'

44 ° 30'--

44°20' -

111° 00' 110° 00'/-h, ,12 ./ h. 'L.-1 .» r1

-., MONTANA . 1WYOMING 4 31

<CALDERA 1 r-PARK,---••Z BOUNDARY V BOUNDARY

.k·)-.4

i erW2 \\e ,\/--1):: tr

1YELLOWSTONE•LAKE

0 I0LOCATION MAP FOR YELLOWSTONE LAKE 1,1•661SCALE, km

110 ° 30 110020 110 ° 10'

Mory Bcy20,420 x648)< x440 2,165XS,Wit:..8 X6,660

x 860X 640x885x860 X505X910 /1,100.XI,835 160 xx255< -\•West Thumbx 2,110 \1 *160 x 130

x330Southeost 120XArm SouthArmX x130245

0 5/=L•L=1=1-1km

GEOTHERMAL FLUID SYSTEMS IN THE YELLOWSTONE CALDERA 1157

23456789

10111213141516171819202122

Station LocationNorth LakeNorth LakeMary BayMary BayMid LakeMid LakeSoutheast Arm S.Southeast Arm M.Southeast Arm N.W. Thumb ChannelW. Thumb N.W. Thumb S.South Arm N.South Arm S.South LakeSouth LakeMid LakeMid LakeW. Thumb NEW. Thumb ChannelSouth LakeMid Lake

44030'

44020'

59.457.927.421.389.989.976.282.382.353.392.483.553.361.080.879.283.873.236.645.782.382.3

Table 1. Yellowstone Lake heat-flow data.Waterdepth

(m)

110030'

3.873.873.873.864.394.544.424.424.934.754.754.854.984.554.785.034.785.165.083.874.554.78

Penetration(m)

4.8474.26842.918.2305.4595.0313.8533.8083.8965.4248.6809.5134.3423.8443.9944.0014.839

20.1656.4264.9724.1175.359

Thermistor temperatures fC)Next to Next to

Top top bottom Bottom

110020

5.4634.67366.339.9256.3405.6923.9543.9294.0486.313

10.55511.5214.7064.0974.1784.1865.371

26.9957.6205.8034.3806.255

6.0505.167

87.0212.387

7.2426.3364.1104.0674.1837.186

12.40713.7095.0454.3714.3404.3445.877

33.7388.8026.8384.6587.145

6.7335.590

104.1714.7238.0376.9434.2374.1734.2848.003

14.18415.8475.3394.5844.4794.4726.358

40.1489.7297.7034.8888.013

110 ° 10'

Averagegradient(°C/km)

630440

20 4202 165

860640130120130860

1 8352110

330245160160505

66601100910255885

Figure 3. Sediment temperature gradient contours for Yellowstone Lake. Contour interval is 100°C/km below 1000°C/km,and 500°C/km above.

l 1

\%11 2000\\--k - -» -1000

\1 /}•6660•J•21

'0* --/Se,.. %4•--ii- -S**00 00 --%-41-((«

300 -

0

200-

0 5llllll

km

1158 MORGAN, BLACKWELL, SPAFFORD, AND SMITH

gradients, ranging from 130 to 330°C/km, were measured

in the southern half of the lake. Intermediate temperature

gradients, in the range 440 to 910°C / km, were measured

in the main body of the lake. The highest gradients charac-

teristic of a large area, about 2000°C/km, were measured

in West Thumb. Extremely high gradients, 20 420°C /km

measured in Mary Bay and 6600°C/km near Stevenson

Island, are in the immediate vicinity of bottom hydrothermal

features. A contour map of these gradient data is shown

in Figure 3.

Thermal conductivity determinations all resulted in similar

values: 1.8 + 0.2 meal cm-1 °C-1 sec-1. No significant

variation of thermal conductivity with depth was detected

nor was any correlation between thermal conductivity and

gradient noted. The temperature gradient pattern is therefore

taken to be indicative of the heat-flow pattern. The heat

flow in the southern portion of the lake is within the range

2.2 to 5.9 hfu (Fcal cm-2 sec-1 ), and in the main body

of the lake ranges from 7.9 to 16 hfu. The heat flow in

West Thumb is approximately 35 hfu, and the maximum

value in Mary Bay is 370 hfu. As noted above, the various

corrections to be applied to the data will systematically

change the values but will not alter the relative magnitudes.

IMPLICATIONS OF HEAT-FLOW DATA

the gradients within the bay (the northern site was relocated

three times on two different occasions, and resulting gra-

dients only differed by a few percent) is not characteristic

of heat-flow regimes in known local hydrothermal areas

in other parts of Yellowstone Park (White et al., 1975).

If a temperature gradient of 2000°C/km is extrapolated

downward it meets the steam point approximately 100 m

below the sediment surface. Seismic reflection profiles in

West Thumb indicate a high acoustic impedance and loss

of signal penetration at about 100 m. This reflector may

be the steam-water interface (Otis, Smith, and Wold, 1974)

and would be due to the large impedance change between

the gas- and liquid-saturated porous rock, and thus would

be analogous to the bright-spot reflectors found over gas

reservoirs in oil exploration. The large temperature gradients

in West Thumb are thereby interpreted as indicating a broad

area of hot fluids below West Thumb reaching the steam

point at about 100 m. Beneath this depth temperatures might

remain constant or possibly increase along the boiling point

under hydrostatic pressure (Muffler, White, and Truesdell,

1971 ).

The very rapid transition in temperature gradient between

the southern and northern parts of the lake indicates a

structural control on the cause of the high temperature

gradients to the north. This transition is 5 to 10 km north

of the mapped caldera boundary and appears to truncate

There are three immediately striking features to the the earthquake activity extending north from the South Arm

heat-flow data from Yellowstone Lake: (1) the large magni- (Smith et al., 1974). It approximately coincides with a sharp

tude of some of the temperature gradients; (2) the extremely gravity gradient and is about 5 km north of the mapped

sharp transition between the normal regional heat-flow ring fractures. Thus in the area of Yellowstone Lake, the

values (Blackwell, 1969; Roy, Blackwell, and Decker, 1972) most significant geothermal boundary is about 5 km inside

in the southern portion of the lake and the higher values the mapped caldera boundary. The geothermal boundary

to the north; and (3) the remarkable uniformity of the high is possibly linked to the mapped boundary by a series of

gradients in the northern part of the lake. All of the faulted blocks increasing the downward displacement of

temperature gradients, except perhaps in Mary Bay and the caldera roof up to the heat-flow boundary.

near Stevenson Island, indicate a conductive heat-transfer Possibly the most significant feature of the Yellowstone

regime in the upper sediments of the lake. With such shallow Lake heat-flow data is the relative uniformity of the high

data, however, other geological and geophysical parameters gradients in the main body of the lake. Even allowing for

must be carefully considered before the temperatures can a conductivity increase by a factor of 2 or 3 with depth,

be extrapolated downwards. the temperature gradients cannot be extrapolated to more

The extremely high temperature gradient in Mary Bay than 3 km before the rock melting point is exceeded. This

(20 420°C/km) is in an area of known hydrothermal activity. is an unacceptably shallow depth for the magma chamber

Steam continuously erupts from vents in rocks at the margins beneath Yellowstone. From a similar reasoning to that used

of the bay and in several places in Mary Bay bubbles of with the West Thumb data, the high gradients cannot be

hot gases can be seen rising in the lake. Piston coring in explained by hydrothermal convection systems of the di-

"1973 retrieved cores from Mary Bay that were too "hot mensions of geyser basins with much lower heat flow

to touch when brought on board the vessel. This high heat in-between. The data are therefore taken to indicate the

flow value is therefore attributed to a local hydrothermal presence of a large, extensive, relatively stable thermal fluid i

convection system similar to the other hot-spring systems system beneath the lake. The uniformity of the data suggests

in the park as investigated by White et al. ( 1975). that the water is contained in the intergranular porosity

The high gradient in the main body of the lake (6660°C /km) of quasi-stratigraphic units rather than in discrete fracture

is in an area where continuous seismic reflection profiles systems. Based on the drilling and geochemical evidence,

indicate a continuity of sediments; but it is within 2 km the temperature of this reservoir would be at least. 200 to •

of an area south of Mary Bay that shows a discontinuity 250°C.

of reflections, thought to be related to a large impedance

contrast and lack of signal penetration that would result -CONCLUSION

from a steam-filled sediment layer (Otis, Smith, and Wold,

1974). No surface thermal manifestations are visible at this The implications of the measurements bounding and

location, but the high heat flow is attributed to a local outside the caldera, and a correlation of the thermal data J

structurally controlled convection system. with the seismic structure of the caldera will be described

The high gradients in West Thumb (around 2000°C /km) in a separate publication. The geothermal implications of i

are not thought to be the result of a small local convection the data are emphasized here. The results of the heat-flow •

system. There are extensive surface thermal manifestations measurements in Yellowstone Lake suggest that an extensive 4

in the rocks surrounding the bay, but the uniformity of hydrothermal reservoir underlies the northern part. of the *

lake and is associated with the downfaulted caldera fill.

The average heat flow in the caldera, excluding the data

from West Thumb and Mary Bay is on the order of 15

pcal cm-2 sec -1. This value is of the same order as the

average convective heat loss' from the caldera area based

on geochemical estimates (Fournier, 1974). Thus the con-

ductive heat loss m an area where the surface is covered

by a layer of .impermeable sediments and the average

convective heat lossforthe caldera are similar. We speculate

that the thermal regime beneath Yellowstone. Lake is similar

to that beneath the remainder of the caldera except for

the presence of a cap of recent sediments. The convective

heat loss represents shallow redistribution of :the average

conductive heat loss for the caldera into the hot-springs

areas by shallow hydrologic conditions. Rainfall on the high

elevations around the hot-spring basins i.; discharged in the

basins and carries with it the heat thatwould be lost through

the highlands in the absence of the water flow. Thus 'the

local heat loss is redistributed, but the total magnitude of

the heat loss for the whole area is maintained. Underlying

the whole caldera at a depth of 300 to 500 m is an extensive

reservoir with temperatures of 200 to 250°C or more.

We suggest that the hydrothermal regime in the caldera

may be characterized as one or more laterally extensive

quasi-stratigraphic reservoirs interconnected by fracture

systems raiher·than by vertically extensive circulation along

major fracture zones. Possible reservoir rocks include the

rhyolite flows, caldera infall representatives of. the tuffs,

and pre-caldera units sach as the Paleozoic carbonates that

are the source of the thermal fluids of Mammoth hot springs.

We see no evidence for broad regions of deep recharge

or upflow in the vicinity of the lake and infer that the

surface manifestations in the caldera reflect shallow ground-

water interaction having a vertical extent of a few hundred

meters, with the uppermost extensive thermal fluid reservoir

indicated from the lake measurements. This interaction may

redistribute, but not increase significantly, the conductive

heat loss from the shallow fluid reservoir.

Marine-type heat-flow measurementshave proved an

economical and rapid way of obtaining heat-flow data in

the. Yellowstone caldera. The presence of an impermeable

layer of recent sediments blanketing the lake allowed inter-

pretation of the deeper thermal conditions from the shallow

measurements. Based on the results described here, the

use of marine-type heat-flow measurements in lakes in

geothermal areas as a research and exploration tool is

emphasized.

ACKNOWLEDGMENTS

This work was sponsored by the U.S. National Science

Foundation with grant GA 43424 to the UnNersity of Utah.

We are grateful to personnel from the Universities of Utah

and Wisconsin-Milwaukee for assistance with the field work.

REFERENCES CITED

American Geophysical Union,1974, Fall Annual Meeting 1974,Yellowstone sessions I and II (abstract): EOS (Am.

Geophys. Union Trans.), v. 56, p. 1189.Armstrong, R. L.,:Leeman, W. P., and Malde, H. E., 1975,

K-Ar dating, Quaternary and Neogene volcanic rocksof the Snake River plain, Idaho: Am. Jour. Sci., v.

275, p. 225.

Blackwell, D. D., 1969, Heat flow determinations in thenorthwestern United States: Jour. Geophys. Research,

v. 74, p. 992.

Christiansen, R. L., and Blank, H. R., Jr., 1972, Volcanic

stratigraphy of the Quaternary rhyolite plateau'in Yel-lowstone National Park: U.S. Geol. Survey Prof. Paper729-B, 18 p.

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

M., Mabey, D. R., Blank, H. R., Jr., Zietz, I., and

Gettings, M. E., 1975, Magma beneath YellowstoneNational Park: Science, v. 188, p. 787.

Fournier, R. 0., 1974, Convective heat flow: Oral presenta-

tion, Am. Geophys. Union Fall Annual Meeting, San

Francisco,Keefer, W. R., 1971, The geologic story of Yellowstone

National Park: U.S. Geol. Survey Bull. 1347,92 p.

Muffler, L. J. P., White, D. E., and Truesdell, A. H., 1971,Hydrothermal explosion enters in Yellowstone Nation-

al Park: Geol. Soc. America Bull., v. 82, p. 723.Otis, R. M., Smith, R. B., and Wold, R. J., 1974, A seismic

reflection survey of Yellowstone Lake: EOS (Am.

Geophys. Union Trans.), v. 56, p. 1190.Roy, R. F., Blackwell, D. D., and Decker, E. R., 1972,

Continental heat flow, in Robertson, E. C., ed., Nature

of the solid earth: New York, McGraw-Hill, p. 506-543.

Smith, R. B., and Sbar, M. L.,1974, Contemporary tectonicsand seismicity of the western United States with em-phasis on the Intermountain seismic belt: Geol. Soc.

America Bull., v. 85, p. 1208.

Smith, R. B., Shuey, R. T., Freidline, R. 0., Otis, R. M.,and Alley, L. B., 1974, Yellowstone hot spot: Newmagnetic and seismic evidence: Geology, v. 2, p. 451.

Von Herzen, R. P., and Maxwell, A. E., 1959, The measure-ment of thermal conductivity of deep-sea sediments

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v. 64, p. 1557.White, D. E., Fournier, R. 0., Muffler, L. J. P., and Truesdell,

A. H., 1975, Physical results of research drilling in

thermal areas of Yellowstone National Park, Wyoming:U.S. Geol. Survey Prof. Paper 892,70 p.

White, D. E., Muffler, L. J. P., and Truesdell, A. H., 1971,Vapor-dominated hydrothermal systems compared with

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GEOTHERMAL FLUID SYSTEMS IN THE YELLOWSTONECALDERA 1159