HomeMy WebLinkAboutAlaska Division of Geological & Geophyscial Surveys 1981Alaska Energy Authority
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: i Soa Ce EL eect cea eat ALASKA.DIVISION OF GEOLOGICAL & GEOPHYSICAL SURVEYS
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CONTENTS
Page
Introduction..... were cceccece ceccee weer c cece nec c reece reece ee eeesseseenes 1
Background......ee+eee eee cere cree esccecces eee ce recccccesecee cer rccccees 1
Application. ....cceeeeeeeecseeees cee ec cc ccccecces seceee cece cc cccens eocee 3
Fumarole fields....... eee eeeereees sete eee eee teen eee eee e eee eeeees 3
Geologic setting..... oo ccc cece ccc ccc r ec ccnscessseccccrcce wor rccccccccccs 5
Hydrothermal resource potential... ..cseceeeececececeeccseveceessevece eee ll
Acknowledgments......... ccc ccc ccc rc ccc cee vc ceeseseecees ccc ccc creccccees 14
References Cited......eeseeeeeecees cece eccecees woe cece cc creecccceseces 16
ILLUSTRATIONS
Figure 1. Simplified geologic reconnaissance map of the northern part
of Unalaska Island...... occ cc ceccccccee ccc cccccccccee eoee 2
2. The main part of fumarole field 1, looking northeast
(July 24, 1980). ..cceeecccecccecececees perce cc crc cccccece 5
3a. Makushin Volcano, looking west-northwest (Feb. 27, 1982).... 6
3b. The main part of fumarole field 2, looking northwest
(Aug. 8, 1980).....eeeeeeee cc ccccecceees a eoveee 7
4. Part of fumarole field 3, looking northwest (Aug. 11, 1980). 8
5. Part of fumarole field 4, looking east (Aug. 8, 1980)....... 9
6. Fumarole field 5, looking west (Aug. 12, 1980).........-. eee 10
7. Fumarole field 6, looking northwest (Aug. 11, 1980)....... = 1l
8. Fumarole field 7, looking west (Aug. 1968)...... wccceee ss 12
Qa. Fumarole field 8 and Sugarloaf Cone, looking east-
northeast (Aug. 12, 1981).....eeeeeeeeeee wocccccees vecece 13
9b. Old cairn that marks location of fumarole field 8
(July 23, 1980)......... oe vecccce vee e4050ess oeteteecs a= 14
10. Diagram showing the ratios of major ions of hot-spring
waters from Unalaska Island and of hot waters from two
exploration wells near Summer Bay on Unalaska Island..... 15
TABLE
Table 1. A summary of some of the characteristics of the fumaroles
and hot springs of Unalaska Island..... ccc ccccccens coccee 4
iii
HYDROTHERMAL RESOURCES OF THE NORTHERN PART OF UNALASKA ISLAND, ALASKA
By J.W. Reeder
INTRODUCTION
During DGGS geologic investigations of Unalaska Island in the summer of
1980, previously unreported active fumaroles and hot springs were located in
the Makushin Volcano region. To date, eight fumarole fields are known to exist
there. Large vapor-dominated hydrothermal reservoirs are suspected to exist in
the area of the fumarole fields located on the southeast flank of Makushin
Volcano.
BACKGROUND
The Makushin Volcano of Unalaska Island is one of at least 36 volcanoes on
the Aleutian Islands arc that have been reported active since 1760 (Coats,
1950). Volcanic regions such as this, with shallow magma bodies and deep
tectonic fracture systems, represent a setting favorable for the existence of
large hydrothermal reservoirs.
Active hydrothermal surface manifestations have been known to exist on
Unalaska Island for some time. Dall (1897, p. 472) stated, "In Unalaska, near
Captain's Harbor, a thermal spring exists, with a temperature of 94°
Fahrenheit, containing sulphur in solution." This is believed to be the warm
spring located near Summer Bay (Reeder, 198la), 5 km east of the community of
Unalaska (fig. 1).
On rare clear days, a plume from an impressive fumarole field can be seen
near the top of Makushin Volcano. This field has received attention in the
past because of its known sulfur deposits (Maddren, 1919). Early
investigations of sulfur deposits throughout this region resulted in the
discovery of other hot springs and fumarole fields on the lower flanks of
Makushin Volcano. Some of these discoveries are still known (Henry Swanson,
R.G. Schaff, and W.E. Long, pers. commun., 1980), even though no written
documentation of these early observations have been found. Exploration pits
and cairns (Fig. 74) can still be seen at some of the fumarole fields.
Drewes and others (1961) observed fumaroles and hot springs on the
southern flank of Makushin Volcano. Later, Miller and Smith (1977) suggested
that a high-level magma chamber exists under the 3-km-dia summit caldera of the
volcano.
Warm springs were also reported to exist in the northeastern part of
Makushin Valley near Broad Bay (Swanson, pers. commun., 1980). Several large
ponds in this region were checked during 1980, but no anomalously warm waters
were found. Air reconnaissance in February 1982 showed ponds and swamps devoid
of ice and snow in the northeastern part and along the southern edge of
Makushin Valley. These unfrozen areas might be due to ground-water seeps at
normal ground-water temperatures.
166°30'
Unalasha
Bay Cone Bay
Makushin Bay
Portage Bay
Map symbols
—— Fault: dashed where approximate EE) Unaltered volcanic rocks
Fumarole field
Warm or hot spring Plutonic rocks
Recent volcanic vent
e
oO
*
ey Caldera [| Unalaska Formation
Figure 1. Simplified geologic reconnaissance map of the northern part of Unalaska Island.
The DGGS field party (Reeder, 1981b) discovered more active fumarole
fields on the flanks of Makushin Volcano, bringing the total fields to eight
(numbered clockwise in fig. 1).
APPLICATION
The Unalaska community serves the largest American fishing fleet for the
Bering and North Pacific region and it could play a major role in the
development of a bottomfish industry. In addition, there is a nearby potential
for Outer Continental Shelf oil, gas, and mineral production. Present peak
electric utility demands for Unalaska is 15 MW, including both the publicly
owned diesel generators and those operated by the private fish processors.
Projections for future energy demands are very uncertain, but peak demands by
the year 2000 could reach 50 MW. Such energy demands and the previously
mentioned observations of fumaroles and hot springs prompted the state (Markle,
1979; Reeder and others, 1980a,b) to develop a geothermal exploration plan for
Unalaska Island.
FUMAROLE FIELDS
The Makushin fumarole fields vary in character and size (table 1).
Fumarole fields 1-3 consist of fumarolic (boiling-point) activity, of warm
ground, and of outcrops of highly hydrothermally altered plutonic and
metavolcanic rocks (figs. 2-4). Field 4 consists of fumarolic activity in
lateral moraine deposits along a stream (fig. 5). Pressurized fumarolic
activity and warm ground occur in unaltered agglomerates at field 5 (fig. 6).
A fairly large steam vent and corresponding fumarole field occur near the
summit of Makushin Volcano. Part of this field occurs on a small volcanic dome
of unknown composition and within the remains of a cinder cone partly covered
with sulfur deposits (fig. 7). This field (no. 6) is located near the center
of the 3-km-dia summit caldera of Makushin Volcano. Field 7 consists of
fumarolic activity located in andesites covered by pyroclastics and tills (fig.
8). Field 8 consists of minor fumarolic activity, and of hot rock positioned
on top of a small knob (fig. 9a); the field is located just west of Sugarloaf
Cone in a region of unaltered basaltic andesites as based on the classification
scheme of Jakes and White (1972).
Some warm and hot springs were found near the fumarole fields at lower
elevations (fig. 1, table 1). Initial water analyses of some of these hot and
warm springs (Motyka and others, 1981) indicated near-neutral
sodium-bicarbonate-sulfate waters similar to hydrothermal waters described by
Mahon and others (1980), which consisted predominantly of meteoric waters that
had been heated by vapor-dominated hydrothermal systems generated from greater
than 150°C alkali-chloride waters at greater depth. The term 'vapor-dominated
hydrothermal systems' was originally coined by White and others (1971) for
those systems in which the reservoir fluids are mainly vapor, not liquid; i.e.,
wet, dry-saturated, or superheated steam. The hot springs in the Makushin
Volcano region probably derive most of their water from near surface-water or
shallow ground-water sources, and most of their heat from the vapor-dominated
hydrothermal systems that are the source of the fumaroles throughout the
region.
Table 1.
Some of the characteristics of the fumaroles and hot springs of Unalaska Island.
Fumarole | Elevation Types of Approx. area of Max. recorded | Hot Spring Max. recorded temp.
Field (meters above | exposed fumarole activity | surface temp. locat ions and corresponding ph
sea level) rock incl. warm ground | within the for hot springs
& assoc. altered | fumarole field immediately outside
bedrock of fumarole field
No. 1 350 - 370 Plutonics and some 3,000 m2 98°C W/in fumarole field, immed 68°C/5+
metavolcanics downslope from field along
stream, & upstream from
field up to 0.6 km
No. 2 650 - 910 Plutonics and meta- 0.30 km2 97°C W/in fumarole field & 90°C/5.5+
volcanics (solifluct- immed. east of field in
ion & landsliding is canyon at 600 m elevation
occurring in field)
No. 3 520 - 580 Plutonics and meta- 0.20 km2 98°C W/in fumarole field & at 96°C/6+
volcanic several locations down-
stream up to a distance of
1.0 km
No. 4 560 - 590 Lateral moraine, vol- 4,000 m2 97°C W/in fumarole field (can- -
canics & metavolcanics yon to south was not ex-
plored & might contain hot springs and/or fumaroles)
No. 5 800 - 820 Volcanic breccias 4,000 m2 97°C W/in fumarole field & to 71°C/Unknown
(agglomerate) southwest by about 0.2 km
No. 6 1650 - 1710 Volcanic dome of un- 0.1 kn’ (plus 94°C none found -
known composition, 0.2 km“ region
pyroclastics & sulphur | shows some signs of icefield thaw)
No. 7 820 and at Andesites covered by 1000 m2 96°C On the eastern margin of 67°C
860 pyroclastics and tills the lower fumarole field
No. 8 520 Basaltic andesite 1000 m2 86°C None found - (at 0.25 m
depth)
Summer 3 Metavolcanics and Main warp spring Two closely located -
Bay Warm alluvium about 2m, 2nd 2 springs occur at edge of
Springs spring @ 0.25 m - a marsh just SE of Summer
(No fum- & warm ground to Bay lake. Max. temp. of
aroles) 40 m of springs springs 35°C at pH of 7.0
Figure 2. The main part of fumarole field 1, looking northeast (July 24, 1980).
Geochemistry indicates that the fumaroles in the Makushin Volcano region
are from more than one large vapor-dominated hydrothermal system. In 1980, two
shallow exploration wells drilled into unconsolidated deposits near the Summer
Ray warm springs encountered an artesian aquifer with a temperature of 50°C and
a natural flow rate of 190 lpm (Reeder, 198la). The waters from these wells
and the main Summer Bay warm spring have similar ratios of major ions (fig.
10), which indicates a common thermal fluid ('parent'). By contrast, the
waters from the hot springs of the Makushin Volcano region all have different
ratios of major ions (fig. 10), which indicates the existence of chemically
different vapor-dominated systems. It is also possible (but less likely) that
rising vapors are mixing with chemically different shallow ground waters or
surface waters to cause different ratios of major ions at the different hot
springs.
GEOLOGIC SETTING
The rocks of Unalaska Island include an older group of altered sedimentary
and volcanic rocks designated the Unalaska Formation by Drewes and others
(1961), a group of intermediate-age plutonic rocks, and a younger group of
unaltered volcanic rocks. The three groups can be correlated with those found
throughout the eastern and central Aleutian Islands, namely, an early series of
a marine volcanic and sedimentary sequence that has been metamorphosed to a
greenschist grade, a middle series of plutonic rocks, and a late series of an
55
Figure 3a. Makushin Volcano, looking west-northwest (Feb. 27, 1982). Fumarole
field 2 dominates the foreground, as outlined by thaw region (photograph
taken 3 days after a fresh snow). Steam cloud near the summit of volcano
marks the location of fumarole field 6.
unaltered sequence of Tertiary subaerial volcanic and sedimentary rocks (Marlow
and others 1973). The early series is believed by Marlow and others (1973),
Scholl and others (1975), and DeLong and others (1978) to be Focene to middle
Miocene, 53 to 15 m.y. old. The sedimentary and volcanic rocks of the Unalaska
Formation found in the Makushin Volcano region have been altered by
albitization, chloritization, epidotization, silicification, and zeolitization.
The middle series or middle unit consists of plutons mainly of granodiorite
that have intruded the early series. These rocks have radiometric dates of 10
to 15 m.y. before present (Marlow and others, 1973; DeLong and others, 1978).
Perfit and Lawrence (1979) argued that the rocks of the Unalaska Formation were
altered mainly during the emplacement of these plutonic bodies. The late
series, which consists of basaltic and andesitic rocks that unconformably
overlie the early and middle series, is up to at least 3 m.y. in age, based on
radiometric ages from andesitic magmas (Cameron and Stone, 1970).
The region southeast of Makushin Volcano consists mainly of rock exposures
belonging to the Unalaska Formation, whereas unaltered volcanics make up the
Makushin Volcano and most of the rock exposures to the northwest of a line
extending from Pakushin Cone to Table Top Mountain (fig. 1). Except for
Pakushin Cone, the cones contained in the area have been subjected to intense
-6-
Figure 3b. The main part of fumarole field 2, looking northwest (Aug. 8, 1980)
glacial erosion. Both the Pakushin and Wide Bay Cones, which lack intense
glacial erosion, are suspected to have formed since the last glacial maximum
which ended about .11,000 yr ago (Black, 1976). The line of cones trending
toward Point Kadin (fig. 1) and the corresponding extruded lavas are believed
by Drewes and others (1961) to have formed within the last several thousand
years; they based their claim on the lack of glacial erosion on the cones and
flows, and on the degree of development of a submarine bench at Point Kadin.
On the basis of the large lichens on the surfaces of some of the scoriaceous
andesites exposed on the largest of these explosions craters, these craters are
at least several hundred years in age.
A fairly thick sequence of pyroclastic deposits, which are similar to the
flow type described by Sheridan (1979), occur in three vallevs located in the
region roughly outlined by fumarole fields 2 and 7, Bishop Point, Driftwood
Bay, and Sugarloaf Cone. These tephras collectively represent a volume of
about 0.21 km’, about half of which occupies the upper reaches of Makushin
Valley just below fumarole field 2 and near fumarole field 1. In the second
largest deposit, 0.08 km of material ocgupies the valley leading from field 7
to Bishop Point. The remainder, 0.03 km’, occurs in a valley located between
fields 7 and 8, and Driftwood Bay. There are other pyroclastic flow deposits
in the Makushin Volcano region, such as a deposit between fields 3 and 4, but
none approach a volume of more than 0.001 km.
Figure 4. Part of fumarole field 3, looking northwest (Aug. 11, 1980).
(Photograph courtesy K.E. Swanson).
A 60-m-thick section of these pyroclastics is exposed across the creek
from fumarole field 1. At the base of this section is a pyroclastic surge
deposit at least 6 m thick with antidune bed forms. Glacial tills are exposed
between pyroclastics and bedrock just upstream from this exposure, and tills
probably underlie this unit at shallow depths. Atop the unit is a 1-m-thick
ungraded mixture of ash, lapilli, blocks, rounded boulders, and assorted
debris. This lahar contains a few plutonic rock fragments. The next unit is <
welded tuffaceous agglomerate of up to 3 m thick. The dark-gray color and
hardness of this unit distinguish it from the other units; andesitic glass
blocks are plentiful here. Atop this unit are about six normally sorted
ash-lapilli flows, each up to 12 m thick. Similar ash-lapilli flow units were
recognized at the other two large pyroclastic deposits.
Figure 5. Part of fumarole field 4, looking east (Aug. 8, 1980).
of The surfaces ending, the large pyroclastic deposits, which are thought to
be related to a large eruption of Makushin Volcano that occurred since the last
glacial maximum ending about 11,000 yr ago, slope away from the volcano. The
volume of pyroclastic material involved indicates that the eruption was related
to the formation of the 3-km-dia Makushin summit caldera. On the basis of the
sequence of pyroclastic deposits at fumarole field 1, the eruption probably
began as a large vertical eruption cloud which collapsed to form large
pyroclastic surges. Following this, enough time elapsed for debris to flow
along some the drainages of the volcano. Explosions then destroyed the summit
by ejecting large quantities of material into the atmosphere, the caldera
collapsed, and large amounts of magma and debris flowed north and east. The
lack of soil horizons within this sequence indicates that the deposits were
probably formed over a fairly short period of time, certainly no longer than
several years.
The Unalaska Formation in the region of fumarole fields 1-3 has been
extensively intruded by bodies of intermediate plutonic rocks and some gabbro.
The bodies and the surrounding Unalaska Formation are extensively fractured,
especially along contact boundaries. A common joint system that strikes
between N. 30°-35° E. with an 80°-90° S. dip in this contact region aligns
roughly with fumarole fields 1, 2, 3, and 8 and with the orientation of many of
the valleys. The fractures probably serve as conduits for hydrothermal
convection. The hydrothermal surface manifestations of fumarole fields 1 and 2
=O
Figure 6. Fumarole field 5, looking west (Aug. 12, 1980).
(fig. 1) are oriented east to northeast along respective northern and southern
boundaries of an intervening plutonic body. One prominent near-vertical
fracture on the northern boundary of this pluton strikes east-west directly
through fumarole field l.
Other near-vertical fractures, striking N. 40°-70° W. and appearing to be
normal faults, were found near the fumarole fields (fig. 1). One fault, which
strikes N. 60° W. at field 2, extends nearly the entire length of the northern
part of Unalaska Island, a distance of over 36 km.
The Aleutian arc is part of a ridge-trench system associated with active
volcanism and seismicity. The Aleutian Trench is located about 180 km south of
Unalaska Island. Global tectonics has the floor of the Pacific Ocean (the
Pacific Plate) approaching the Aleutian arc (the North American Plate) in a
northwesternly direction at a rate of about 7 cm/yr (Minster and others, 1974).
On the basis of seismic models, the Pacific Plate dips about 30° under the
Aleutian arc until it reaches a depth of about 40 km, where its dip increases
abruptly to about 70° (Jacob and Hamada, 1972); thus, the Pacific Plate at the
Aleutian Trench is being thrusted under the North American Plate.
This underthrusting causes compressional stresses in the direction of
plate convergence in the arc region (Nakamura, 1977). Therefore, because the
Pacific and North American Plates converge at about N. 45° W. at Unalaska
-10-
Figure 7. Fumarole field 6, looking northwest (Aug. 11, 1980).
Island, these near-vertical northwest-striking fractures are suspected to have
been caused by compressional tectonic stresses. The fractures, even though
they correlate with most of the fumarole fields, do not appear to influence the
actual surface configuration of the hydrothermal manifestations. Moreover,
they do not appear to serve as conduits for hydrothermal convection, at least
not near the ground surface.
For Makushin Volcano, Nakamura (1977) determined, on the basis of
orientation of flank eruptions, a maximum stress orientation of N. 60° W. where
the expected azimuth should be about N. 45° W. If the expected azimuth is the
actual one, the recognized fractures ‘striking about N. 60° W. should contain a
strike-slip component. As shown in figure 1, one N. 60° W. fault near the
community of Unalaska has such a strike-slip component (based on observed
slickensides).
HYDROTHERMAL RESOURCE POTENTIAL
Large vapor-dominated hydrothermal systems probably exist in the
northeast-oriented zone roughly marked by fields 1-4. Lava flows that still
retain details of their constructional forms can be found to the a) northwest
(flows from the upper reaches of Makushin Volcano and from the prominent rift
zone near point Kadin), b) northeast (flows surrounding the Sugarloaf Cone),
and c) southwest (the volcanic rocks of the Pakushin Cone). Yet, no such
-ll-
Figure 8. Fumarole field 7, looking west (Aug. 1968). (Photograph courtesy
W.E. Long)
deposits have been found within the northeast-oriented zone or in the region to
the southeast. In fact, no unaltered volcanic rocks have been recognized as
being extruded from this zone, which indicates that no magma extrusions have
occurred in this region for the last 3 m.y.; this is corroborated by the known
range of radiometric age dates for unaltered volcanics for the Aleutian arc
(Cameron and Stone, 1970). A few basaltic and andesitic dikes of unknown age
are exposed in the region, but again no corresponding extruded deposits have
been found. There have been few, if any, extrusions because the magma probably
has not existed at depth. However, there is also the possibility that the
magma has been and may still be at depths of several kilometers where it could
be more viscous than magmas to the northeast or southwest. If such deep magma
bodies exist in the region, they might have a dikelike configuration oriented
in a direction corresponding to the N. 60° W. fractures shown in figure 1;
these bodies and any magma bodies located near the volcanic centers to the
west, northwest, and north could be the heat sources for any large
vapor-dominated hydrothermal systems.
In contrast, any hydrothermal convective systems linked to fumarole fields
5-8 are suspected to be limited to shallow zones, where any heat sources would
also be at shallow depths. Such sources might be due to either recent surface
-12-
Sugarloaf Cone
Fumarole Field No. 8 ves
Figure 9a. Fumarole field 8 and Sugarloaf Cone, looking east-northeast
(Aug. 12, 1981). (Photograph courtesy M.J. Larsen.)
volcanic flows that still contain heat, as suspected at fumarole field 8, or to
the cooling of shallow magma bodies, as reflected by the dome in field 6.
Most of the recent extrusive rocks in the northern part of Unalaska Island
are porous. Any heat in such rocks have been mostly removed, except for small
isolated areas such as the one found at field 8. Hydrothermal convective
systems might exist in the fractured Unalaska Formation and corresponding
plutonic bodies that are suspected to underlie most of the unaltered volcanic
rocks of the northern part of Unalaska Island. However, no real evidence has
been found for the existence of such systems. In fact, except for the Summer
Bay area near the community of Unalaska (Reeder, 198la), no hydrothermal
systems are known to exist in the Unalaska Formation beyond the immediate
fumarole field regions of Makushin Volcano.
A northeast-oriented zone roughly marked by fields 1-4 has been identified
as possibly containing large vapor-dominated hydrothermal reservoirs. There
are only a few places in the world, such as The Geysers, California, and
Larderello, Italy, where hydrothermal systems consist mainly of vapor (White
and others, 1971). These systems have been developed where they now represent
the major source of electrical geothermal power. Further exploration may
better define the nature of reservoirs in the Makushin Volcano region, but deep
exploratory drilling and well testing such as described by Economides and
others (1982) is required to determine their potential.
-13-
Figure 9b. Old cairn that marks location of fumarole field 8 (July 23, 1980).
ACKNOWLEDGMENTS
I thank field assistants Kirk E. Swanson (1980) and Mark J. Larsen (1981)
Roman Motyka, Mary Moorman, Shirley Liss, and Malcomb Robb helped with water -
and gas sampling of the hot springs and furiaroles. I also thank Unalaska
residents Abi Dickson, Kathy Grimmes, and the Currier family for their
extensive help and advice.
The project was funded by the Alaska Power Authority and the U.S.
Department of Energy, where these funds were administered by the Alaska
Division of Power and Energy Development.
-14-
-ST-
1 Summer Bay Warm Spring.
2 Well No.1 at Summer Bay. 60 60
3 Well No.2 at Summer Bay.
~
4 Hot Spring near Fumarole Field No.1. £ ».
5 Hot Spring near Fumarole Fleld No. 2. &
26 Hot Spring near Fumarole No. 3.
7 Hot Spring near Fumarole No.3
(Different spring site)
cl
Figure 10. Diagram showing the ratios of major ions of hot-spring waters from Unalaska Tsland and of
hot waters from two exploration wells near Summer Bay on Unalaska Island. (Data from Motyka and
others, 1981, and from author.)
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=16=
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» 1981b, Vapor-dominated hydrothermal manifestations on Unalaska
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