III. Conclusions Regarding Significance of the
Hydrothermal Features of Crater Lake
B. Hydrothermal Research in Crater Lake
by U. S. Geological Survey
Review of studies concerning the presence of
thermal water inflows into Crater Lake
U.S. Geological Survey
The purpose of this section is to review
available research that is relevant to the question of existence and
significance of inflow of thermal water into Crater Lake. The pertinent research
* the formation of Crater Lake
* hydrologic and chemical balances for the lake as a well-mixed body of water
* thermal and chemical characteristics of springs on the flanks of Mount Mazama
* distributions of dissolved constituents as a function of depth in Crater Lake
* submersible observations in the deep part of the lake
The chemical balance of the lake shows that the inflow of warm, slightly saline
fluid is important to explaining the chemical balance of Crater Lake. The data
for springs on the flanks of Mount Mazama show what the temperatures and
chemistry of non-thermal springs are in order to provide a basis for
understanding the anomalies found in the lake. The distributions of dissolved
constituents with depth show how the inflow of warm, slightly saline water
affects the characteristics of water in the bottom part of the lake. The section
on submersible observations describes features such as pools and bacterial mats
on the bottom that result from the inflow of warm, slightly saline water and the
samples that were obtained from these features.
This review is a summary of a large body of research performed by investigators
primarily from Oregon State University, the National Park Service, the U.S.
Geological Survey, and other institutions. Details of the various investigations
can be found in the original references. Figures have been reproduced from the
original studies with minor modifications to labels.
FORMATION OF CRATER LAKE
Mount Mazarna is the name for the volcanic
mountain in which Crater Lake caldera formed (Bacon and Lanphere, 1990). The
oldest lavas of Mount Mazama are approximately 400,000 years old, and continuing
volcanic eruptions built the mountain to a summit elevation of approximately
3600 m. At 6850 years before present, a violent eruption destroyed the top of
Mount Mazama and created the Crater Lake caldera. The total volume of magma
erupted was approximately 50 km3 . After the formation of the caldera, eruptions
took place on its floor to form the central platform, Merriam Cone, and Wizard
Island (Figure 1). The lake reached nearly its current level before the end of
these eruptions. At about 4,000 years before present, a small dome was extruded
on the east flank of Wizard Island (Figure 1). Whether these post-caldera
eruptions are from the magma chamber related to the climactic eruption or
represent a new influx of magma is uncertain. In addition to the lava flows
mentioned above, the floor of Crater Lake is comprised of debris from the
caldera walls and relatively flat-lying sediments in the deep basins (Barber and
HYDROLOGIC AND CHEMICAL BALANCES OF CRATER LAKE
Crater Lake is 53 km2 in area, 589 m deep at
its maximum depth, and has an average depth of 325 m. The average elevation of
the lake is 1882 m, and steep caldera walls surrounding it range in elevation
from 2050 to 2480 m. Most of the water supply is as direct precipitation because
the lake area is 78 % of the total watershed area of 68 km2. The lake has no
surface outlet, and water is lost by evaporation and leakage. Phillips (1968)
and Redmond (1990) have analyzed precipitation and lake-level data to calculate
water balances for Crater Lake (Table 1). Although Redmond (1990) obtained a
greater total water supply and a correspondingly larger value for evaporation
than Phillips (1968), the two analyses are in reasonable agreement. None of the
large springs in the area adjacent to Crater Lake show clear evidence of
containing any of the leakage from Crater Lake (Thompson et al., 1990).
Table 1. Water balance for Crater Lake in units of cm/year (volume per year
divided by the lake area). Precipitation is volume per year for area of rain
The chemistry of dissolved constituents in
Crater Lake water shows that there is an input of constituents in addition to
those in precipitation and runoff from the caldera walls. For purposes of
analyzing the amounts of major ions dissolved in Crater Lake water, the lake may
be considered to be well mixed because vertical and horizontal gradients are
small. Based on the water balance, known inputs from precipitation and runoff,
and the assumption that the concentrations of dissolved constituents are
constant over time, the theoretical major-ion chemical composition of the lake
can be calculated. Because the amount of evaporation is an important parameter
in this calculation, the calculated concentrations are given in Table 2 for the
two values of the fraction of the total water supply lost to evaporation (28 %
from Phillips, 1968, and 49 % from Redmond, 1990). For each major dissolved
constituent in Table 2, the calculated concentration is substantially less than
the measured concentration. This difference is strong evidence of input of
thermal water into Crater Lake (Simpson, 1970; Nathenson, 1990b). Although the
measured amounts of dissolved constituents in Crater Lake are anomalously high,
overall the water is very low in total dissolved solids because of the direct
input of large amounts of dilute precipitation.
Table 2. Concentrations of dissolved constituents (mg/L) in Crater Lake and
calculated values based on available water supply (Nathenson, 1990b).
Analysis of historical data shows that
concentrations of dissolved constituents in Crater Lake do not appear to be
changing with time, and it is appropriate to do a steady state chemical balance
of the lake (Nathenson, 1990b). Using concentrations in the lake, precipitation,
and flow from springs on the caldera walls, along with the assumption that
inflows and leakage are steady-state, the steady-state rate of inflow of each
constituent to the lake has been calculated and summed to obtain a rate of
inflow of total dissolved solids. Nathenson (1990b) calculated an inflow of
200,000 mg/s using Phillips' (1968) value for the leakage from Crater Lake, and
Collier et al. (1991) calculated an inflow of 110,000-140,000 mg/s using
Redmond's (1990) value for the leakage. An independent calculation of the
current rate of inflow of total dissolved solids was obtained from measurements
of the vertical distribution of total dissolved solids in the bottom part of the
water column for two periods in 1989 and 1990 (McManus et al., 1991). Because
these measurements are at the limit of available precision and because a precise
measure of the area of increased dissolved solids is not possible without making
many more measurements than is practical, the rate of current inflow calculated
by McManus et al. (1991) is quite uncertain. However, the later value, which is
110,000±60,000 mg/s, demonstrates that inflow is still happening today, and that
it is approximately equal to the steady-state inflow. Because the water
residence time (lake volume divided by rate of leakage) of Crater Lake is about
220 years, a small change in the rate of inflow in the recent past would not be
easily detected from the historical data for concentrations of dissolved
constituents in Crater Lake.
SPRINGS IN THE VICINITY OF CRATER LAKE
A useful comparison for understanding thermal
measurements in Crater Lake is provided by data for springs in the vicinity (Nathenson
and Thompson, 1990; Nathenson, 1990a). To determine if a spring is thermal, the
temperatures of other springs in the area provide a basis for determining that a
spring has an elevated temperature. Figure 2 shows data for spring temperatures
in the vicinity of Crater Lake versus elevation. The line shown on the diagram
is a least-squares fit to air temperatures from twelve weather stations in
southwestern Oregon showing the decreasing air temperatures with elevation.
Except for the spring vents that are the source for the Wood River and other
springs near the Wood River south of Crater Lake National Park, spring
temperatures are generally less than air temperatures, and their variation with
elevation is similar to that for air temperatures. Also shown are temperatures
for Crater Lake taken at a time when the effects of summer heating in the upper
300 m were minimal. Temperatures in the bottom part of Crater Lake (that are
less affected by seasonal variations) are neither particularly hot nor cold
compared to spring temperatures at the surface elevation of Crater Lake.
Temperatures for the various springs that are the source of the Wood River range
from less than air temperature to more than 21C greater than air temperature.
Springs of the Wood River group also have temperatures warmer than expected for
their altitude. Temperatures for the Cedar Spring group on the other side of the
Wood River Valley are not warmer than expected.
Concentrations of dissolved constituents in springs indicate the different
processes involved in water/rock reaction. The first analysis in Table 3 is an
average of data for springs above the lake and is typical of the process of
low-temperature weathering of volcanic glass driven by dissolved carbon dioxide.
The resulting water is notably low in dissolved chloride and sulfate, because
these constituents are from precipitation, not low temperature weathering of
volcanic rock. The second analysis is for a spring on the caldera wall at Chaski
Bay slide (not included in average of springs above the lake). In addition to
constituents from low-temperature weathering, this water has excess calcium and
sulfate that are dissolved from hydrothermal minerals formed during an earlier
period of high temperature alteration in the Chaski Bay slide. The chloride
concentration in Crater Lake (the third analysis of Table 3) is noticeably
elevated compared to either of these low temperature waters. The composition of
Crater Lake is also elevated in sulfate compared to most spring water, except
for water from the Chaski Bay slide springs.
Table 3. Concentrations (mg/L) of dissolved constituents in representative
springs (Nathenson and Thompson, 1990) and Crater Lake (Nathenson, 1990b).
The last analysis of Table 3 is for one of the
more concentrated springs near the Wood River south of the park. This water is
very similar in chemistry to Crater Lake, with elevated chloride and sulfate
concentrations. The lower silica in Crater Lake is. caused by diatoms consuming
silica and then settling to the floor of the lake when they die. Based on
chemistry, one could interpret the water in springs in the vicinity of the Wood
River as leakage from Crater Lake; however, the stable isotopes of water
(deuterium and oxygen-18) show that these waters are not from Crater Lake. The
slightly elevated temperatures of the springs near the Wood River and the
similarity of their chemistry to that of Crater Lake water indicate that one
component of the mixed waters found in the Wood River Valley has probably
undergone the same reactions with rock as the water flowing into Crater Lake and
that these reactions took place at some elevated temperature.
Soda springs on Minnehaha Creek northwest of the park boundary represent another
type of anomalous water chemistry (Thompson et al., 1990; Nathenson and
Thompson, 1990; Mariner et al., 1990). Both springs have elevated chloride
concentrations (18 and 4 mg/L), but only one has elevated sulfate. Bicarbonate
concentrations are quite high compared to all other springs (2300 and 420 mg/L),
and the chemistry of these springs is probably caused by carbon dioxide
dissolving in local groundwater and reacting with rock in the near surface.
Other soda springs with high amounts of dissolved constituents and little or no
anomalous temperature occur in the Cascades, and the source of the carbon
dioxide is not well understood. The high amounts of bicarbonate relative to
chloride for the soda springs on Minnehaha Creek probably indicates that these
springs are unrelated to the process that produces the chemistry of the inflow
to Crater Lake and the Wood River springs.
CHARACTERISTICS OF THE WATER COLUMN
The distribution of temperature with depth in
Crater Lake shows that upper 200 m is well mixed and that the deep part of the
lake is minimally affected by surface heating and cooling. Figure 3 shows
profiles of temperature versus depth from the north basin for January and May
(McManus and Collier, 1990). Note that the figure has a range of temperatures
from 3.20 to 4.20C. Lake surface temperatures reach a maximum in August (about
150 to 200C). From around mid-August to the beginning of spring, the surface
temperature of the lake decreases, and the cooled water produced at the surface
sinks and mixes with warmer water in the near surface. As the surface cooling
proceeds, the mixed layer becomes progressively deeper. The January 1990 profile
shows that surface temperatures have cooled to less than 4.20C and that the
vertical mixed zone of uniform temperatures has proceeded to 200 m. Continued
cooling for the rest of the winter forces temperatures in the near surface below
3YC. Temperatures in the upper 150 m are less than temperatures shown for the
May profile and less than temperatures for the line shown for the maximum
density of water as a function of depth (pressure). At a given depth, water at
temperatures greater or less than value shown for the line is less dense than
water with a temperature on the line shown for temperature of maximum density.
In late March, surface temperatures start to increase. This warm surface-water
is actually denser than cooler water below it in the upper 150 m, because the
lake temperature is less than the temperature of maximum density, and the
surface water sinks and mixes with cooler water. The profile for May 1989 shows
uniform temperatures to a depth of 200 m, indicating that mixing has been
maintained to this depth. Salinity measurements for the upper 200 m of the lake
(not shown) confirm that the mixing process is also effective in making the
salinity uniform, except for near surface concentration due to evaporation. The
two temperature profiles are closest at about 400 m, and this is probably the
deepest effect of the seasonal heating and cooling at the surface. Continued
warming in the spring and early summer produces warmer, less dense water at the
surface of the lake that is stable.
Below 350 m, lake temperatures increase with depth, and this increase requires
an active input of thermal energy at depth. Figure 4 shows temperature for the
south and north basins of the lake and salinity for the south basin. Note that
the temperature scale in this diagram is expanded compared to Figure 3, and the
range is only 0.10C. The salinity measurements are at the limit of sensitivity
of the instrument used, and that is part of the reason for the step nature of
those values. The increase in temperature with depth below 350 m is found
throughout the lake. The difference in temperature between the south and north
basins reflects a greater input of thermal energy in the south basin. Additional
temperature data in McManus et al. (1991) show that warm water flows from the
south basin to the north basin. The increase in salinity with depth by itself
causes increased density with depth, but the increasing temperature with depth
by itself causes decreased density with depth. The stability of the deep part of
the water column is shown in the last diagram which is Sigma (theta) =
(potential density-1)x1000. The potential density is used to show water column
stability by removing the effects of pressure and adiabatic cooling on density.
Potential density is the calculated density at each depth as if the water parcel
was moved adiabatically to a common reference pressure at the surface. There is
a very slight increase in potential density with depth in Figure 4, implying
that the deep water column is stable at this time of year.
Although the increase in temperature with depth in Crater Lake is very small, it
is real and implies that there is high heat flow into the bottom part of the
lake (McManus et al., 1991). Temperature-depth profiles taken at three times
over a four-month period in 1989 (Figure Sa) show a small but significant
increase in temperature in the deep part of the lake as the year proceeds. Given
the stable depth stratification of the lake water, this increase must be due to
heat addition in the bottom part of the lake. The two profiles in Figure 5b show
that warm water in the bottom part of the January 1990 profile has mixed with
water above it by July 1990, and the bottom part of the lake has cooled by a
small but significant amount. This mixing event probably took place in February
1990 at a time of minimum stability for the water column in the upper part of
the lake. Calculation of the rate of heating shows that even though the change
in deep lake temperatures is very small, the large size of the lake yields a
heating rate of 20 megawatts of thermal energy. Dividing this total heat flow by
the 31 km2 area of the deep part of the lake as represented by the 300-m depth
contour yields an average heat flow of 650 mW/m2 (milliwatts per square meter).
Regional heat flow in the high Cascades of Oregon is 100 mW/m2 (Blackwell et
al., 1990). Thus, this estimate of the average heat flow into the bottom of
Crater Lake is approximately 6 times the regional value. Heat flow measurements
in the sediments of Crater Lake using oceanographic techniques found that 12 out
of 62 measurements were higher than 300 mW/m2 , and 7 of those 12 were greater
than 550 mW/m2 (Williams and von Herzen, 1983). Although these are conductive
heat flows because they are calculated as the product of thermal conductivity
times the measured temperature gradient, the high values reflect convection of
water either through the sediments or localized in a nearby vent. Thus the
source of the heating found from increased bottom-water temperature must be from
an inflow of warm water.
The small change in salinity with depth shown in Figure 4 indicates that major
elements measured at usual analytical sensitivities will not easily yield
diagnostic information on lake processes. Fortunately, measurements of some
other constituents are more readily diagnostic (Collier et al., 1991). For
example, helium isotopes provide a tool with much higher analytical sensitivity.
Figure 6 shows 3He versus 4He for various samples, and Figure 7 shows 3He versus
depth. In the atmosphere, 3He and 4He are found in a fixed ratio shown by the
constant slope of the broken line in Figure 6. The two samples for caldera wall
springs are close to this line, indicating that the springs are in equilibrium
with helium in air, as expected. The other samples in Figure 6 have a constant
ratio as shown by their following the solid line, and the slope of this line is
diagnostic of the input of helium derived from the mantle. Helium from the
mantle that is found near the surface is frequently associated with the
degassing of magma. The variation of 3He and 4He along the solid line in Figure
6 reflects mixing of water that was in equilibrium with air with varying amounts
of helium with a mantle ratio. Based on this model for sources of helium, the
variation with depth of 3He shown in Figure 7 is diagnostic of several lake
processes. The upper 200 m of the lake are essentially at a uniform
concentration of 3He that reflects equilibrium with the helium in air. This
confirms the interpretation of the temperature data in Figure 3 that this upper
part of the lake is very well mixed. The lower part of the profiles shows higher
concentration in the south basin where higher temperatures are found and lower
concentrations in the north basin where lower temperatures are found. The
variation with depth shows that the deep-lake mixing process is not as efficient
as the near-surface mixing process.
Inefficient deep mixing is confirmed by model calculations for the input of
anthropogenic chlorofluorocarbons that show that the time scale for complete
mixing of water in the bottom part of the lake with water in the upper part of
the lake is about 2 to 3 years (Weiss, 1991). The bottom part of Crater Lake is
well oxygenated but slightly deficient compared to the upper lake which is in
equilibrium with the atmosphere (McManus et al., 1991). The well oxygenated
character also confirms that the lake does mix to total depth, whereas the
slight deficiency confirms that this mixing is not as efficient as in the upper
lake. Additional confirmation of mixing to total depth comes from the stable
isotopes of deuterium and oxygen in water. Evaporation at the lake surface
fractionates these isotopes in a characteristic manner, and samples obtained at
various depths show that this evaporated water is found throughout water column
(Thompson et al., 1990).
That a fluid must be entering the south basin is demonstrated by the two
profiles of 222Rn versus depth in Figure 8 (Collier et al., 1991). The
radioactive element 222Rn has a half-life of 3.8 days and is added to water as
it circulates through rock. The high values at total depth in Figure 8 show that
this deepest water was recently circulating through rock.
The basic process that governs the properties of the water column in the deep
part of the lake is mixing between an inflow of a warm, slightly saline,
helium-enriched fluid with lake water that is lower in salinity and helium
concentration. The mixing of warm, slightly saline water with lake water is
shown by the plots in Figure 9 of temperature and several major dissolved
constituents versus 3He for samples of the deep water column in the south basin.
The range of variation for temperature and dissolved constituents is quite
narrow in Figure 9. Recognizing that some of the scatter is from analytical
uncertainty, the data show a linear correlation between each quantity and 3He,
which implies that there is a single source fluid that is warmer and contains
higher levels of 3He and dissolved constituents. Because of rapid dilution of
the inflow at sites of fluid venting, there could be a fluid with values of
temperature and dissolved constituents well beyond the range shown in Figure 9.
A major objective of submersible operations in
Crater Lake was to find sites of inflow and to obtain samples of the warm,
slightly saline source fluid. Deep lake observations are given in Collier et al.
(1991), which contain photographs of pools and bacterial mats found on the lake
floor. No obvious large-scale plume was visually observed; however, some
hydrothermal systems found by submersible operations in the ocean also lack
visual evidence of plumes. Features observed in the submersible operations in
Crater Lake were pools and bacterial mats in the south basin and a mat and pool
complex off Palisade Point northeast of Merriam Cone. Maximum temperatures
measured in the two major pools studied were 4.50C in Llao's Bath and 5.50C in
the Palisade Point pool. Temperatures in bacterial mats and in sediments were
significantly higher than lake temperature (Figure 10) and ranged to 18.90C in a
single-point measurement in a mat during dive CD229. The nonlinear character of
temperature versus depth in the measurements for some of the mats and sediments
(Figure 10) shows that there is movement of warm fluid through the sediments.
Fluid samples obtained from gravity cores in the sediments also have nonlinear
variations of concentrations of major dissolved constituents with depth, showing
that there is movement of more saline water through the sediments (Wheat, 1991).
Mat material was found to have iron as the most abundant material and to be
enriched in arsenic and manganese.
The pools are an ideal environment for collecting samples, but they are lower in
temperature than the bacterial mats. The mats appear to be the site of higher
temperature venting, but it is difficult to collect samples that are not well
mixed with lake water. Thus the observations of higher temperatures in the mats
cannot be connected directly with the most concentrated water samples from the
pools. Nevertheless, the correlations of temperature and solutes with 3He
(Figure 9) demonstrate that there is a source of fluid with a high value of the
ratio of temperature to solutes. That there is more than one fluid with
differing ratios of temperature to solutes is shown by data for simultaneous
measurements of temperature and salinity obtained during several dives of the
submersible (Figure 11). The data for higher salinities and temperatures define
a field that can be enclosed by projections towards two compositions of
differing ratios of temperature to salinity. Based on finding low-temperature,
more saline fluid in the pools, it seems likely that the point labeled "b" with
a low ratio of temperature to solutes is produced by fluids losing thermal
energy in the subsurface before they flow into the pools. The point labeled "a"
with a high ratio of temperature to solutes is probably that from the
higher-temperature bacterial mats. The submersible measurements define the
ratios of temperature to salinity for these two fluids but are too diluted with
lake water to define the limiting temperature or concentration. Figure 12 shows
the extrapolation of these ratios to the highest salinity fluid sampled. The
maximum temperature obtained is about 250C.
Fluid-chemistry samples from Llao's Bath and the Palisade Point pool (Table 4)
show enrichment in those elements necessary to explain the composition of Crater
Lake compared to the available water supply from precipitation and runoff from
the caldera walls. The bottom part of the table presents the major-ion chemistry
as amounts of ionic charge expressed as per cent of total cations (Ca+2 , Mg+2 ,
Na+, K+) and anions (HC03-, S0 4-2 , and C1-). The utility of looking at a water
analysis in this manner is that it allows the relative proportions of the
various dissolved constituents to be compared on a common basis. The equivalent
percentages shown in the table for magnesium and chloride, for example, indicate
that the waters from the pools are different in relative concentrations from
each other and from that in Crater Lake. This difference in relative
concentrations can be more easily assessed from the mass ratios in the last two
columns of the table. The ratio Na/Cl is 1.0 in the lake, but it is 1.8 and 2.9
in the pools. Assuming that the major determinant of lake chemistry is the
concentrated inflows, there are a couple of possibilities to explain this
difference. One interpretation is that the major fluid source feeding the lake
has not yet been found. A second is that the hydrothermal system at Crater Lake
is evolving, and that the chemistry of the water has recently changed. An
indication that this might be the case is the discovery of apparently inactive
silica spires near Skell Head in the north basin southeast of the Palisade Point
pools. The precipitation of silica indicates a higher temperature at these
locations than any yet measured on the lake floor.
The stable isotopes of water for samples from Llao's Bath and the Palisade Point
pool show that the source of the water is circulating lake water and that the
water has probably not undergone high-temperature (>200°C) reaction with rock
(Collier et al., 1991). The samples from Llao's Bath and the Palisade Point pool
have essentially the same concentrations as lake water of the stable isotopes of
deuterium and oxygen in water.
Table 4. Concentrations, equivalents per cents, and mass ratios of dissolved
constituents in the deep part of Crater Lake and for pool samples obtained by
the submersible (Collier et al., 1991).
Deuterium values in spring samples in the
vicinity of Crater Lake are quite different from lake water, because the
deuterium and oxygen isotopes in precipitation are modified by evaporation in
the lake. If the water feeding the pools had a deuterium content similar to that
of the springs before mixing with lake water, the pool samples would have to mix
with 10 to 20 parts of lake water in order for the pool samples to end up with a
deuterium content that is indistinguishable from lake water. The carbon-14
isotope content for the sample from Llao's Bath shows that it can be no more
than 50 % lake water. The dissolved oxygen content of the sample for the
Palisade Point pool is very low, indicating very little mixture with well
oxygenated lake water. Thus, the deuterium contents of the pools are only
slightly modified from that for the fluid that feeds the pools, and the source
of the fluid in the pools is lake water that has circulated to depth to be
heated and react with rock. The oxygen isotopes of lake water are also modified
by evaporation of precipitation, but they can undergo additional change if they
react with rock at high-temperature. Collier et al. (1991) have calculated the
change in oxygen isotopes of water for various ratios of water to rock as a
function of temperature. Unless the water to rock ratio is very high, the
similarity of the oxygen isotopes of the pool samples and lake water indicates
that equilibration temperatures are less than 200'C.
Chemical geothermometer temperatures calculated from major-element
concentrations for the pool samples suggest that inflow temperatures may be
higher than measured temperatures. These chemical geothermometers are
experimental or empirical relations for temperatures determined by the
equilibrium between major elements dissolved in water and rock. The calculation
of geothermometer temperatures for the composition of lake water is not strictly
appropriate, because the lake water is clearly a mixture of a more saline water
with water from precipitation and springs on the caldera walls. The pools
provide more appropriate samples for geothermometer calculations, but their high
magnesium concentration compared to calcium and potassium concentrations
indicates caution in using geothermometer temperatures. In most thermal waters,
magnesium concentrations are very low, because magnesium preferentially stays in
the solid phase when water reacts with volcanic rock at elevated temperatures.
Accordingly, in some waters, high magnesium concentrations indicate that the
water is equilibrated at low temperature. However, in other cases, high
magnesium concentrations are caused by mixing a higher-temperature water with a
cold water and subsequent reaction at the mixed temperature. Geothermometer
temperatures (chalcedony, Mg-Li, K-Mg) for Llao's Bath range from 50' to 900C
and for Palisade Point pool from 350 to 700C. Based on the high magnesium
concentration, the Mg-corrected Na-K-Ca geothermometer relations of Fournier and
Potter (1979) would indicate that the water has equilibrated at the spring
temperature (for example the 190C temperature measured in one mat). Na-Li
geothermometer temperatures are 1651C for Llao's Bath and 1 100C for Palisade
Point pool (Collier et al., 1991), but these high temperatures are not
corroborated by other geothermometers.
An interesting comparison can be made to Swim Warm Springs on the flanks of Mt.
Hood, Oregon. The measured spring temperatures range to 260C, and the
geothermometer temperatures range from 300 to 1 10'C, also with high magnesium
contents (Wollenberg et al., 1979; Mariner et al., 1990). The chemistry of Swim
Warm Springs is interpreted to result from a higher temperature water (=1 100C)
from near the central vent of the Mount Hood volcano flowing in the subsurface,
mixing with cold water, and reequilibrating some of its constituents to the
mixing temperature. A similar model could explain the chemistry of waters
sampled in the two pools in Crater Lake. Furthermore, a maximum temperature of
130"C was measured at a depth of 1067 m in a well drilled east of the park in
the Winema National Forest to a depth of 1423 m (LaFleur, 1990). Based on the
comparison to Swim Warm Springs and the high measured temperature in the well,
it is reasonable to speculate that there are higher temperatures in the inflow
to Crater Lake than those that have been measured.
Several important conclusions about
characteristics of Crater Lake and fluid inflow into the lake have been reached
as a result of recent research studies. Submersible operations have measured
temperatures of 8, 10, 13, and 19'C in the bacterial mats at the bottom of the
lake. Ambient temperatures in the lake are 3.60C, and springs at the surface
elevation of the lake range in temperature from about 20 to 50C. Thus the
measured mat temperatures are clearly anomalous and meet definitions of thermal
springs on land (several definitions are reviewed in Nathenson, 1990a and
Mariner et al., 1990). Pools found on the floor of the lake have elevated
amounts of the dissolved constituents needed to explain the chemistry of the
lake but not quite in the right proportions to account for the present
composition of lake water. Temperatures of these pools are elevated compared to
the lake temperature, but are not as high as temperatures measured in the
bacterial mats. Geothermometer temperatures calculated from analyses of pool
samples tend to indicate that the fluid is low-temperature (300 -900C). The
presence of 3 He/4He with a mantle signature and significant amounts of
chloride, boron, and lithium permit speculation that there is an
intermediate-temperature (>900C) water that has been modified by subsequent
reaction. The source of the fluid in the pools is lake water that has circulated
to depth to be heated and react with rock. Although no obvious venting was
visually observed, the nonlinear gradients of temperature and dissolved
constituents found in the sediments and bacterial mats clearly indicate flow
into the lake, as does the presence of pools with anomalous fluid compositions.
The warm, slightly saline water that flows into the lake is found to collect in
the bottom part of the lake for much of the year. At the time during the year of
minimum water-column stability, the warmer bottom water mixes imperfectly with
surface water. Although the greater salinity of the added water tends to make
the bottom water stable, the added temperature tends to make it unstable.
Without the input of warm water at depth, the mixing properties of the deep lake
likely would change, and Crater Lake might no longer mix to total depth. Lake
Tahoe depends on winter storms to mix and mixes to total depth only in some
years (Goldman and Jassby, 1990). Because of the much smaller average radius of
Crater Lake (4.1 km) compared to Lake Tahoe (12.6 kIn) but similar total depths
(589 m compared to 501 m), winter storms might not be effective in mixing Crater
Lake to total depth.
In summary, the research program at Crater Lake has demonstrated an inflow of
thermal water that is important to lake dynamics. The characteristics of this
thermal water and its impact on lake dynamics remain imperfectly understood.
Bacon, C. R., and Lanphere, M. A., 1990, The
geologic setting of Crater Lake, Oregon, in Drake, E. T., Larson, G. L., Dymond,
J., and Collier, R., eds., Crater Lake, An Ecosystem Study: Pacific Division,
American Association for the Advancement of Science, San Francisco, p. 19-27.
Barber, J. H., Jr., and Nelson, C. H., 1990,
Sedimentary history of Crater Lake caldera, Oregon, in Drake, E. T., Larson, G.
L., Dymond, J., and Collier, R., eds., Crater Lake, An Ecosystem Study: Pacific
Division, American Association for the Advancement of Science, San Francisco, p.
Blackwell, D. D., Steele, J. L., Frohme, M. K.,
Murphey, C. F., Priest, G. R., and
Black, G. L., 1990, Heat flow in the Oregon
Cascade Range and its correlation with regional gravity, Curie point depths, and
geology: Journal of Geophysical Research, v. 95, p. 19,475-19,493.
Collier, R. W., Dymond, Jack, and McManus,
James, 1991, Studies of Hydrothermal Processes in Crater Lake, OR: College of
Oceanography Report #90-7, Oregon State University, 317p.
Fournier, R. O., and Potter, R. W., II, 1979,
Magnesium correction to the Na-K-Ca chemical geothermometer, Geochimica et
Cosmochimnica Acta, v. 43, p. 1543-1550.
Goldman, C. R., and Jassby, Alan, 1990, Spring
mixing depth as a determinant of annual primary production in lakes, in Tilzer,
M. M., and Serruya, Colette, eds., Large Lakes, Ecological Structure and
Function. Springer-Verlag, Berlin, p. 125-132.
LaFleur, Joe, 1990, Letter to Jim Larson,
National Park Service: California Energy Co., Santa Rosa, California, 5 p.
Mariner, R. H., Presser, T. S., Evans, W. C.,
and Pringle, M. K. W., 1990, Discharge rates of fluid and heat by thermal
springs of the Cascade Range, Washington, Oregon, and northern California:
Journal of Geophysical Research, v. 95, p. 19,517-19,531.
McManus, James, and Collier, Robert, 1990, The
physical limnology of Crater Lake, OR: Mechanisms for the redistribution of heat
and salt in the water column, in Collier, R. W., Dymond, Jack, and McManus,
James, Studies of Hydrothermal Processes in Crater Lake, OR. A Report of Field
Studies Conducted in 1989 for The National Park Service, Oregon State
University, p. A.1- A.30.
McManus, James, Collier, Robert, and Dymond,
Jack, 1991, On the physical limnology of Crater Lake, Oregon: Mechanisms for the
redistribution of heat and salt in the water column, in Collier, R. W., Dymond,
Jack, and McManus, James, 1991, Studies of Hydrothermal Processes in Crater
Lake, OR: College of Oceanography Report #90-7, Oregon State University, p. A.1-
Nathenson, Manuel, 1990a, Temperatures of
springs in the vicinity of Crater Lake, Oregon, in relation to air and ground
temperatures: U.S. Geological Survey Open-File Report 90-671, 19 p.
Nathenson, Manuel, 1990b, Chemical balance for
major elements in water in Crater Lake, Oregon, in Drake, E. T., Larson, G. L.,
Dymond, J., and Collier, R., eds., Crater Lake, An Ecosystem Study: Pacific
Division, American Association for the Advancement of Science, San Francisco, p.
Nathenson, Manuel, and Thompson, J. M., 1990,
Chemistry of Crater Lake, Oregon, and nearby springs in relation to weathering,
in Drake, E. T., Larson, G. L., Dymond, J., and Collier, R., eds., Crater Lake,
An Ecosystem Study: Pacific Division, American Association for the Advancement
of Science, San Francisco, p. 115-126.
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East, and Davis Lakes, Oregon: U.S. Geological Survey Water-Supply Paper 1859-E,
Redmond, K. T., 1990, Crater Lake climate and
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San Francisco, p. 127-141.
Simpson, H. J., Jr., 1970, Closed basin lakes
as a tool in geochemistry: Ph.D. Thesis, Columbia University, New York, 325 p.
Thompson, J. M., Nathenson, Manuel, and White,
L. D., 1990, Chemical and isotopic compositions of waters from Crater Lake,
Oregon, and nearby vicinity, in Drake, E. T., Larson, G. L., Dymond, J., and
Collier, R., eds., Crater Lake, An Ecosystem Study: Pacific Division, American
Association for the Advancement of Science, San Francisco, p. 91-102.
Weiss, R. F., 1991, Deep water renewal rates in
Crater Lake deduced from the distribution of anthropogenic chlorofluoromethanes
(freons), in Collier, R. W., Dymond, Jack, and McManus, James, 1991, Studies of
Hydrothermal Processes in Crater Lake, OR:College of Oceanography Report #90-7,
Oregon State University, p. G.1- G.2.
Wheat, C. G., 1991, Fluid circulation and
diagenesis in the basement of Crater Lake, Oregon: Pore water constraints, in
Collier, R. W., Dymond, Jack, and McManus, James, 1991, Studies of Hydrothermal
Processes in Crater Lake, OR: College of Oceanography Report #90-7, Oregon State
University, p. F.1- F.31.
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On the terrestrial heat flow and physical limnology of Crater Lake, Oregon:
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and Strisower, Beverly, 1979, Geochemical studies of rocks, water, and gases at
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Figure 1. Bathymetric map of Crater Lake along with topography of immediate
vicinity (Collier et al., 1991). Bathymetry in meters and topography in feet.
Detailed study area is where most of the sampling and submersible operations
Figure 2. Elevation versus temperature for
springs in the vicinity of Crater Lake (Nathenson, 1990a). Cedar Springs group,
source of Wood River, and Wood River group are located south of the park
boundary. Solid line is least squares fit to air temperatures from weather
stations in southwestern Oregon. Broken line shows water temperature versus
elevation for Crater Lake for 19 June 1971 when effects of near-surface heating
or cooling are minimal.
Figure 3. Depth versus temperature for Crater Lake for May 1989 and January 1990
(McManus and Collier, 1990). The line marked Tmd shows the temperature at which
water is at its maximum density versus depth (pressure). The adiabatic
temperature increase shows what temperatures would occur in the deep part of the
lake if the lake were well mixed from 310 m to total depth.
Figure 4. Plots at expanded scales for data in
the bottom 100 meters of the south basin and for temperature only of the north
basin of Crater Lake (Collier et al., 1991). (a) Plot of temperature versus
depth for south basin (diamonds) and north basin (points). The adiabatic
temperature increase shows what temperatures would occur in the deep part of the
lake if the lake were well mixed from 350 m to total depth. (b) Plot of salinity
versus depth for south basin. (c) Plot of Sigma (theta) = (potential density-l)xlOOO
Figure 5. Temperature versus depth in the north basin of Crater Lake (McManus et
al., 1991). Note that depth range is different in the two diagrams. (a)
Temperatures in 1989 showing addition of thermal energy over a four-month
period. (b) Temperatures in 1990 showing decrease in bottom-water temperature
from January to July 1990.
Figure 6. Concentration of 3He versus 4He for Crater lake and spring samples
collected in 1989 (Collier et al., 1991). Broken line shows variation for
samples in equilibrium with air of ratio Ra = (3He/4He)a = 1.4xl106 Solid line
shows relation for Crater Lake samples with a slope R = 7.1 Ra. SB is south
Figure 7. 3He concentration (cubic centimeters
at standard temperature and pressure per gram of water) versus depth for north
basin (NB) and south basin (SB) of Crater Lake (Coffier et al., 199 1).
Figure 8. 222Rn contents in disintegrations per
minute per 100 liters (dpm/100 L) versus depth for two profiles in the south
basin (Collier et at., 1991). Vertical broken line shows the amount of m2Rn from
the decay of 226Ra dissolved in the lake water.
Figure 9. Concentration of 3He versus temperature and dissolved constituents for
samples from 1987 and 1988 (Collier et al., 1991). Dissolved constituents are
reported as millimoles (mM) and micromoles per liter. Molar concentrations are
obtained from weight concentrations by dividing by molecular weight of
Figure 10. Temperature versus depth for mats
(top) and sediments (bottom) obtained by using probe from submersible (Collier
et at., 1991). Key is for locations named in Collier et al. (199 1). Generalized
bottom water temperature shown.
Figure 11. Temperature versus salinity data taken during listed submersible
dives in the south basin and adjacent area (Collier et al., 1991). Lines on
figure are isopycnals or lines of constant density calculated at a pressure of
43.5 bars. Circled letters show correlations of fluid properties towards a high
temperature/salinity ratio fluid (a), a low temperature/salinity ratio fluid
(b), and an intermediate fluid (c).
Figure 12. Temperature versus salinity showing
extrapolation to highest salinity found in pool samples (McManus et al., 1991).
Lines on figure are isopycnals or lines of constant density calculated at a
pressure of 46.5 bars. Circled letters show extrapolated values from
correlations of fluid properties from Figure 11 towards a high
temperature/salinity ratio fluid (a), a low temperature/salinity ratio fluid
(b), and an intermediate fluid (c). Area of Figure 11 is shown by small box in
lower left comer of figure.