Geysers and the Earth's Plumbing Systems
GS662 12 december 1996
Geysers are essentially hot springs that become thermodynamically and hydrodynamically unstable. However, geysers are extremely rare on the surface of the earth, indicating that a complex set of parameters must be exactly right for geysers to occur. Figure 1 shows locations and Table 1 lists names of geyser fields of the world. It is worth noting that there are only approximately fifty geyser fields known to exist on earth and around two-thirds of those fifty contain five or fewer active geysers. Yellowstone National Park in Wyoming, U.S.A. has, by nearly an order of magnitude, more geysers than any other field known, and has been the site of extensive study of the properties and characteristics of geysers.
Because geysers are so rare, there have been several investigations
into the conditions that must exist for geyser activity. It has been found
that at least three essential conditions must be met, but there are many
other contributing factors that influence the type and frequency of eruptions.
The basic elements of a geyser are: 1) a water supply, 2) a heat source,
and 3) a reservoir and associated plumbing system (Figure 2).
Cross-section of a typical geyser (after White, 1967)
The vast majority of water in a geyser system is meteoric. Often geyser fields are found on the banks of rivers, which is usually a significant component of a geyser's water source. Rainfall and circulating groundwater also play a significant role. Analysis of the tridium content of geyser water indicates that groundwater expelled from a geyser system is on the order of 500 years old; this is obviously the amount of time it takes for groundwater to circulate to depth, become heated, and move back up to shallower levels.
The eruption of water or gas from a geyser is due in significant part to an interaction between hot and cool fluid sources. This necessitates a constant, steady supply of heat. Every geyser field in the world is located near some sort of volcanic, shallowly-lying heat source. Frequently, geyser fields are located near lithospheric plate boundaries, which are typically characterized by active volcanism. Other geyser fields, such as Yellowstone, are assumed to lie above hot spots or plumes. The overwhelming majority of all geyser fields lie above large bodies of rhyolite, although some fields are associated with more mafic volcanic rocks such as andesite or basalt.
The six types of reservoir systems
(after Rhinehart, 1980)
Reservoirs and associated plumbing systems
The third essential component of geyser systems is the reservoir and plumbing system. Although these systems are thought to be extremely complex and unique for every geyser, there are six generic classifications of reservoir types from which all are thought to be derived. Figure 3 shows these six basic types:
Type A: Geysers that fall into this category generally have a single standpipe that is
connected to an underground reservoir with a raised cone at the surface. Such geysers usually erupt at fairly regular intervals and their eruptions are typically long, with the geysers playing at considerable heights. Old Faithful in Yellowstone National Park is an example of a geyser that is thought to have this type of plumbing system. In 1992, a probe equipped with temperature and pressure sensors and a video camera was sent down Old Faithful's standpipe. It found, at a depth of 45 feet, a cavern the size of a (rather large) automobile filled with vigorously boiling water (Bryan, 1995).
Type B: The plumbing systems of Type B are deep, rather narrow, shafts. Geysers that are associated with this type of reservoir system are usually quite violent and their eruptions are short, such as the Round Geyser in Yellowstone National Park. The Round Geyser erupts on the order of every 8 hours, to a height of about 25 meters.
Type C: Reservoir systems such as the one depicted in Type C of Figure 3 have standpipes similar to those in a Type A system. However, cones do not form around their surface openings. Instead their openings have slightly raised rims and are in pools of water. This makes their behavior somewhat different from a purely columnar geyser such as Old Faithful and purely fountain geysers such as the Grand or Great Fountain in Yellowstone.
Types D, E, and F: These three types of reservoir systems are typical of fountain geysers, such as the Grand and Great Fountain, Narcissus Geyser in Yellowstone, and the Great Geysir of Iceland. A geyser with a system similar to that of Type D erupts as a series of explosions with intermediate periods of quiescence. This is probably due to a complex set of interconnected reservoirs, which empty in series. Types E and F are other configurations for fountain or pool geysers. Eruptions for both of these systems are long and fairly regular, but not particularly violent.
If these essential elements (water supply, heat source, and reservoir)
are not present in the correct configuration, there will still be geothermal
activity in the area, but geysers will not develop. Fumaroles, or steam
vents, form when there is very little water, but an intense heat source.
Or, if there is an abundance of hot water, but it is in a highly permeable
rock, the water will be supersaturated in silt and clay, which forms a
mudpot. Or, if there is an excess of water, but the heat source is not
hot enough for a geyser to develop, a hot pool will form. If water in the
reservoir is somehow constricted from circulating or if the standpipes
are too large, then a boiling hot spring will develop. Generally, in areas
where geysers are found, most of these other phenomena also occur.
The processes by which geysers erupt, which are generally easily understood in a qualitative fashion, can also be explained using a more analytical approach. There are two basic types of eruptions: those that arise from fountain or pool geysers and those from columnar geysers. Figure 4 shows a cross-section of a typical pool geyser. Immediately after an eruption, the basin or reservoir of the geyser will slowly fill with water. It is generally thought that most geyser systems have two separate water sources, one through which large amounts of shallow, cool water flow and a separate source that brings in small quantities of boiling water from depth. The water will mix in the reservoir with the hot, less dense water moving upward in the basin, and the cooler, more dense water moving downward. This system moves in a convective fashion until the reservoir fills and the entire system gradually increases in temperature. When the water is heated to a critical temperature, a blob of the upward-moving hot water will retain enough heat energy after mixing to stay at its boiling point. When this happens, the water turns into steam as it reaches the surface. This agitates all the water in the system, and more hot water begins to rise and there is a succession of explosions.
Columnar geysers have a slightly more violent eruptive pattern than pool geysers. In a reservoir system for a columnar geyser, hot water that is circulating at depth flows into the geyser's plumbing system and mixes with cooler water, as in a pool geyser. The reservoir fills up and continues heating the entire system until all the water is at its boiling point for a given depth. If the heat source is constant, then the system will heat enough so that the steam bubbles stop collapsing as they reach the surface of the water. Initially, the bubbles begin to rise and move through the plumbing system with no difficulties. However, as more and more steam bubbles begin to rise they get caught in a constriction in the geyser's plumbing system (in columnar geysers, constrictions in the channels are critical for eruption). The pressure builds until it lifts overlying water up and out of the channel so that the steam bubbles can escape. When the overlying water is lifted out of the system, the resulting drop in pressure lowers the boiling point of the residual water in the reservoir. This water, which was already boiling, boils even more vigorously and forms more steam bubbles. This steam rapidly expands and and the reservoir empties itself catastrophically. The eruption will continue until either the reservoir is out of water or the temperature of the system drops below boiling.
It is also useful to analyze the thermal and hydrologic regimes of geyser eruption in a more quantitative fashion. For a columnar geyser, after eruption the reservoir begins to refill with a mixture of cool water and hot water or steam. If the heat source is steam, then after condensation of the steam, the temperature of the heat source is equal to water at a temperature of:
Th=Ts + (r/c) where
r = heat of vaporization of water
Ts = temperature of steam
c = specific heat of water
As the reservoir fills, all the residual water in the reservoir is at a temperature of To. As time goes on, the amount of water (which is a mix of hot and cold waters) increases at a steady rate given by:
p(dV/dt) = q1 where
q1 = the rate of flow of hot and cold water (qh +
p = density of fluid
As the reservoir is filling, the temperature of the entire reservoir is changing, given by:
pV(dT/dt) = qh(Th - To) + qc(Tc
where qh = rate of influx of hot water
qc = rate of influx of cold water.
Let Te1 = the equilibrium temperature after mixing, so:
Te1 = (qcTc + qhTh)/q1.
Geysers will only be active when Te1 > To (that is, the temperature after mixing has to be greater than the temperature of the residual water in the reservoir).
So, the time (t1) that it takes to fill the reservoir is given by:
t1 = (VA - VO)(p/q1) where
VA = the volume of water in the full reservoir
VO = the volume of water initially present
p = the density of the water mixture
q1 = the rate of flow
The temperature (T1) at the time, t1 is:
T1 = Te1 + (VO/VA)(TO
Once the reservoir is full, water begins to move up the standpipe (or exit channel). The volume of water in the reservoir must remain constant since the reservoir is of fixed size, but the overall pressure in the reservoir increases due to the weight of the water in the standpipe. The pressure increase in the reservoir system makes the pressure differential between the water in the surrounding rock mass and the water in the reservoir smaller. This decreases the rate of overall flow into the reservoir. However, this difference is largely negligible in a competent igneous rock so that the time at which the standpipe also fills, t2, is:
t2 = t1 = Vc/q where
Vc = volume of the standpipe
q = rate of flow into the standpipe
When both the reservoir and the standpipe are full, the water in the entire system begins to heat up. Eruption will commence when the temperature of the water, T2, becomes greater than the boiling point of the water, Tb, at a given depth. When this occurs, the steam bubbles begin forcing overlying water out of the standpipe, the overall pressure drops, and violent boiling begins and the geyser erupts. The amount of time between the end of one eruption and the beginning of the next will simply be the amount of time it takes to fill the reservoir, plus the amount of time it takes to fill the standpipe, plus the time it takes to heat the entire system to its boiling point. Therefore, the total time interval between eruptions:
t = (VA - VO)/q1 + (VA/q1)ln[(Te2 - T2)/(Te2 - Tb)]
Geysers and Gas-An Intimate Coupling
Gasses are ubiquitous in geyser systems, and their presence can significantly
affect geyser behavior. Carbon dioxide is the most abundant gas (about
80-100% of all gas in any geyser) with other minor gases including oxygen,
carbon monoxide, hydrogen, methane, nitrogen, argon, and hydrogen sulfide
(Allen and Day, 1935; Barth, 1950). Most gases are volcanic in origin,
all though some gases probably have an atmospheric component as well. The
presence of a significant amount of gas can affect the hydrostatic pressure
of a system such that water that contains gas can boil at a temperature
much lower than its own boiling point. Therefore, the presence of large
amounts of gases may precipitate eruptions even if the heat source is not
intense enough to heat water to its boiling point. Gassy geysers, or geysers
that are driven by the presence of gas instead of the temperature of the
system, are commonly found in oil and gas-producing regions, such as the
eastern U.S.A. Such geysers usually eject fluids and gas that are well
below the boiling point of water at atmospheric conditions, although the
pools of water appear to be boiling vigorously. Geysers can be driven by
different gases, such as carbon dioxide or hydrocarbons. Many times, these
geysers erupt with no warning, but are fairly regular in their eruption
Behavioral Changes In Geysers
Changes in the Earth's atmospheric conditions seem to variably affect geyser behavior. Rinehart (1972) studied fluctuations in rainfall, averaged over a year, and deduced that there was little to no effect on geyser behavior. However, analysis of seasonal rainfall and intervals of eruption for the Old Faithful geyser in California show a marked correlation. In this case, increasing rainfall seems to shorten intervals of eruption in a systematic fashion (Silver and Valette-Silver, 1992). It is agreed, however, that such fluctuations definitely affect near-surface features like mudpots or hot springs.
Changes in barometric pressure, even by small amounts, seem to have a significant effect on geyser activity on a year-long scale (Rinehart 1972). Many observers of geyser activity note that the temperature of water in geysers and hot springs increase under reduced pressure conditions, usually during the rainy season. However, many geysers seem to respond erratically to such fluctuations, indicating that there may be conflicting stresses at depth that cause geysers to respond in an irregular fashion.
Figure 4. The
effect of earth tides
on plumbing systems (after Rinehart, 1980)
LOW TIDE HIGH TIDE
Earth tides, like ocean tides, arise from interactions between the gravitational fields among the earth, sun, and moon. These tidal forces can systematically affect geyser behavior. Figure 4 shows a schematic drawing of the effect of tides on plumbing systems. Low tides, which are associated with squeezing, close channel openings and restrict the flow of water into reservoirs. Conversely, dilation associated with high tides opens cracks and channels and allows a faster flux of water into the plumbing system. Many geysers have eruptive schedules that are in sympathy with tides, and appear to be most affected by the 4.4 year cyclic tides, which are due to the difference in inclination of the orbital planes of the earth and moon (Rinehart, 1972, 1980; Figure 5).
Figure 5. The
correlation between tidal force
and the interval between geyser eruptions
(after Rinehart, 1980)
There is a generally a strong correlation between earthquake activity and geyser behavior. This phenomena has been observed for centuries, but only recently have the changes in geyser behavior been documented. The 1959 Hegben Lake earthquake has had the most dramatic effect on Yellowstone geysers of any earthquake since the park's discovery (Rhinehart, 1980). Since the epicenter was so close to Yellowstone (only about 50 km away) the effects were significant. Immediately after the earthquake, all the geysers in the park erupted, and the average temperature in geysers and springs increased by an average of 2°C. Some geysers that were previously dormant became active, ostensibly by the opening of sealed channels along planes of weakness, and several active geysers changed their eruptive behavior. However, geyser response to earthquakes seems somewhat variable. Old Faithful in Yellowstone was the only geyser in the park that did not change its eruptive behavior after the Hegben Lake earthquake (Rhinehart, 1980). However, it has responded to several other earthquakes with epicenters much farther away, such as the magnitude 8.4 earthquake in Alaska in 1964. This indicates that the profound changes in geyser activity due to earthquakes are not due to slip along faults so much as by changes in regional strain. Since geysers record earthquake activity in a reasonably consistent manner, there have been several studies on whether geysers could be used as predictors for earthquake activity. Rhinehart (1980) states that in his detailed studies of geysers, there have been no systematic correlations between earthquakes and geyser activity. A more recent study by Silver and Valette-Silver (1992) showed that some geysers in California show changes in observable activity before nearby earthquake activity. They surmise that, although the response of geysers to tectonic strain is not well understood, there are two basic mechanisms by which seismic activity could affect geyser behavior. In the first, changes in the regional strain field change the volumetric flow velocity into the reservoir, thus affecting the interval between geyser eruptions. In another scenario, the permeability of the plumbing system could change due to strain-induced changes in microfractures in the geyser reservoir.
Although geysers generally record changes in the Earth, it is difficult to clearly isolate the control of any individual event on geyser behavior. Geysers and associated geothermal areas are dynamic by nature and are used most widely for the geothermal energy that can be derived from them. As the technology for subsurface imaging becomes more sophisticated, our understanding of the nature of geyser behavior will significantly improve, and therefore, the use of geysers as a tool for predicting the behavior of the Earth at shallow-crustal levels may become more significant.
Other geyser-related links:
Greater Yellowstone Earth Science Index
Allen, E.T. and Day, A.L. (1935) Hot Springs
of the Yellowstone National Park, Publ. 466. Carnegie Institute of Washington,
Washington, D.C., 525 p.
Barth, T.F.W. (1950) Volcanic Geology: Hot Springs and Geysers
of Iceland, Publ. 587. Carnegie Institue of Washington,
Washington, D.C., 174 p.
Bryan, T. Scott (1995) The Geysers of Yellowstone, Third Edition. University Press of Colorado, 463 p.
Rhinehart, J.S. (1972) Fluctuations in geyser activity caused
by variations in earth tidal forces, barometric pressure, and
tectonic stresses. Jour. Geophys. Res. 77, 342-350.
Rhinehart, J.S. (1972) 18.6-year tide regulates geyser activity. Science 177, 346-347.
Rhinehart, J.S. (1980) Geysers and Geothermal Energy. Springer-Verlag, 223 p.
Silver, Paul G. and Valette-Silver, Nathalie J. (1992) Detection
of Hydrothermal Precursors to Large Northern California
Earthquakes. Science 257, 1363-1368.
White, D.E. (1967) Some principles of geyser activity mainly
from Steamboat Springs, Nevada. Amer. Jour. Sci. 265, 641-684.