Volcanic Springs

Geysers, Hot Springs,
Mud Pots, and Fumaroles

About geysers and other hot springs

When underground water gets into contact with volcanic heat, different kinds of hydrothermal features may be formed. The most spectacular ones are geysers.
Probably, the ancient Greek philosopher Aristoteles would have described a geyser as "a synthesis of all four elements, water, earth, fire, and air." But it's unlikely that Aristoteles knew geysers, and even if he did, we better look for a more contemporary statement.

How do geysers work?

Geyser activity has some in common with the railroad. Such as a train periodically transports passengers to a destination station, the water of a geyser transports heat from underground to surface at more or less regular intervals. Both, train or geyser, may or may not be on schedule. Transport by a geyser, in other words the eruption, starts if the collected amount of heat exceeds a trigger point. Now the steam pressure in the deep, hot zone is high enough to create steam bubbles, which need more space than liquid water and therefore push out some water onto the surface. The drop in hydrostatic pressure by discharging water causes more superheated water to vaporize instantly, which in turn jets out more water and so on. It's a very quick chain reaction, the lesser remaining water, the lower the pressure, the lower the boiling temperature, the more steam is generated and the more water is ejected. The eruption will continue until no more superheated water is left.

Minute Man Geyser in Yellowstone N.P.:

Minute Man Geyser Shoshone Basin

T. Scott Bryan (The Geysers of Yellowstone, 1995) defines a geyser as a spring, "characterized by intermittent discharge of water ejected turbulently and accompanied by a vapour phase (steam)." This definition holds for heat driven geysers, but can easily be transferred to cold water geysers, where gases such as carbon dioxide are the driving force.

The heart of a geyser is the plumbing system. Ongoing reseach indicates that plumbing systems may feature different complexity, from mostly vertical, nearly unbranched conduits via conduits with bubble trap cavities alongside through to highly contorted, branched conduits with a lot of further sub-structures like fractures, fissures, constrictions, or porous spaces. Contorted piping systems may also have a large lateral extension. The one thing all plumbing systems have in common is a constricting vent near ground surface. Quite often different vents are interconnected by the same plumbing system, or different plumbing systems are interconnected and so geyser eruptions show dependencies between each other. Concerning Yellowstone, academic publications even suggest that all plumbing systems as well as all other thermal springs may be fed by a single giant aquifier in the deep. If this is true, and there really is evidence that it is, any interference with the system by geothermal drilling (even outside the park boundaries) could have desastrous consequences to Yellowstone's geysers.

A geyser eruption is predominantly characterized by height, discharge rate, duration, and interval, where interval stands for the time span between the starts of two subsequent eruptions. Generally speaking, the more isolated a geyser is, the more regular are its intervals. A good example of this is Old Faithful Geyser, even better examples are Riverside Geyser or Lone Star Geyser, all located in Yellowstone. On the other hand, interconnections between plumbing systems allow activity to shift and and lead to more or less variable intervals. If a geyser suspends its eruptions for an unusually long period of time, which may be days or years depending on its former interval, it is called dormant. Whereas a geyser must be regarded as extinct if its plumbing system is irreversibly damaged, has become sealed by geyserite deposits, or if the subterranean hot water supply has ceased. In other cases geysers were buried by a landslide. But in nature much is possible, so even extinct geysers may get a second life.

Are there different types of geysers?

Geysers are divided into two groups: Cone-type geysers and fountain-type geysers.
Cone type geysers eject water through a narrow vent with little or no surface pool. Often, but not always, the opening is embedded in a cone-type sinter structure. The eruption is characterized by steady water jets, which may be of impressive height.

Castle Geyser in Yellowstone N.P., an example of a cone-type geyser:

Castle Geyser Yellowstone

Extremely elongated geyser cones have probably been formed on the bottom of lakes or seas.

Monument Geyser in Yellowstone N.P., an example of a very elongated cone:

Monument Geyser Thermos Bottle Geyser Yellowstone

Cone-type geysers may also have irregularly shaped vents, splitting the water jet up and diverting it in different directions.

Plume Geyser in Yellowstone N.P., erupting through an irregularly shaped vent:

Plume Geyser Geyser Hill Yellowstone

In contrast, the eruption of fountain-type geysers is often shaped more like a dome, rising out of open pools.

Grænihver in Hveravellir / Iceland, an example of a fountain-type geyser:

Grænihver Geyser Hot Spring Hveravellir Iceland


Grand Geyser in Yellowstone N.P., one of the most widely known fountain-type geysers:

Grand Geyser Yellowstone

Are all spouting springs regarded as geysers?

It is still a matter of controversy, if every intermittently spouting spring is a geyser, independently of the height of the eruption or the detectability of a rising vapour phase. If no rising vapour phase can be seen at all or gas/steam bubbles are rising without dragging major parts of the water column along with them, periodically spouting springs are classified as bubble-shower springs. In case an eruption occurs though no vapour phase is rising from the plumbing system, superheated water wells up in an open pool and the decrease in pressure causes violent boiling near the surface only.

Celestine Pool in Yellowstone N.P., an example of a bubble-shower spring erupting by superheated boiling near surface:

Clestine Pool

In case of a poorly developed rising vapour phase, in some springs numerous small bubbles rise at the same time, while in others only a few but large bubbles appear. The "flashing" large steam bubbles out of Firehole Spring's vent are famous and gave rise to its name.

Firehole Spring in Yellowstone N.P., an example of a bubble-shower spring spouting by large steam bubbles:

Firehole Spring

Crested Pool in Yellowstone N.P., an example of a bubble-shower spring spouting by small bubbles:

Crested Pool

All in all, are bubble-shower springs true geysers? I think that the above-described variations are gradual expressions of one and the same "geyser effect". Somewhere in the water column the vapour pressure exceeds the pressure of the liquid water, a vapour phase forms and throws water above the previous surface. For me it is decisive that it is a periodical effect, but not if it happens within the narrow sinuosities of the plumbing system or within the wider pool on top. In this regard a bubble-shower spring can be considered as geyser near utmost low temperature level.

And how does size matter? Well, if the eruption is only a gentle splashing, most observers wouldn't call that feature a geyser. Sometimes also the relation between the height of the eruption and the size of the pool or cone may influence the judgement. Every now and then geysers (for example Splendid Geyser in Yellowstone), which are capable of ejecting impressive water jets, are said to be dormant despite the fact that minor eruptions up to one meter / some feet height still occur. In all these situations a classification as active geyser may be correct, but doesn't match everyones expectation. Apparently, for those borderline cases the acceptance of an intermittently spouting spring as an active geyser lies in the eye of the beholder.

A tiny geyser at Hveravellir / Iceland:

Hot Spring Hveravellir Iceland

If temperature in the deep is just high enough to generate some trapped steam bubbles in the system, but not sufficient to induce periodical boiling of a bubble-shower spring, an external observer will only notice a intermittent rising and falling of the spring's water level. This is due to expansions and contractions of the trapped steam bubbles. True intermittent springs never show any spouting activity or rising bubbles. Using the example of Doublet Pool, the time span between photo 1 and photo 2 was approximately 15 minutes. However, aside from intermittent hot springs there are also cold intermittent springs known, which probably work according to a siphon mechanism.

Doublet Pool at different water levels, Yellowstone N.P. (roll mouse over photo):

Doublet Pool

Also the constantly erupting or violently boiling perpetual spouters are not regarded as geysers. Bedrock with low thermal conductivity supports their formation because it leads to a low temperature difference (gradient) between water of deeper parts of the plumbing system and in the vent near ground surface. A perpetual spouter often is the extreme form of a geyser as opposed to a bubble-shower spring, featuring too high temperatures all along the subterranean tubing to let the systems water return into the liquid state completely. Therefore, if the temperature drops, a perpetual spouter may convert into a geyser, even if intervals remain very short in the range of seconds. So on location it is often hard to distinguish between a true perpetual spouter and a nearly perpetual spouting spring, which has to be classified as geyser.

Steady Geyser in Yellowstone N.P., an example of a perpetually spouting spring:

Steady Geyser Yellowstone

A special type of permanently boiling spring can be observed with the so called "Frying Pan". This is wet soil or at most a very shallow pool, which is boiling extensively.

Frying Pan Bowl in Yellowstone N.P., an example of a frying pan:

Frying Pan Bowl

Finally, to put it straight and simple, if the amount of heat in the underground increases steadily, a quiet spring may develop to an intermittent hot spring, then to a bubble-shower spring, next to a geyser, further to a perpetual spouter, and ultimately to a fumarole. The transitions between each stages are fluent.

Are there other types of hot springs?

The bright colors of thermal pools have always fascinated observers. In contrast to cold waterbodies, floating green, brown, or red microscopic unicelled algae can't survive in most hot springs. Thus, their color does no longer overlay the intrinsic blue color and light scattering of pure water. The deeper the hot spring, the deeper also its brilliant blue tint.

Artemisia Geyser in Yellowstone N.P., showing the deep blue tint of pure water:

Artemisia Geyser Yellowstone

However, water of hot springs is not always transparent clear, but quite often clouded by suspended particles. If these particles are consisting of pure silica, light scattering generates beautiful bluish colors, which are changing with the view angle. Such springs are referred to as opalescent springs. Opalescent springs feature somewhat lower temperatures and discharge rates, otherwise neither the precipitation nor the supply of silica particles from the underground would be sufficient to sustain the optical effect.

Black Opal Pool in Yellowstone N.P., an example of an opalescent spring:

Black Opal Pool Biscuit Basin Yellowstone

Some opalescent hot springs such as Blesi at Haukadalur in Iceland show the dependency of silica precipitation on the water temperature at its best.

Blesi northern pool at lower water temperature (top) and higher temperature (beneath), where colloidal silica dissolves to a higher degree (roll mouse over photo):

More frequently than silica other microscopic small mineral particles are floating in the water. This can be attributed to different processes. Besides the above described precipitation of formely dissolved minerals due to a temperature drop of the water near surface, also a simple washout of particles from soft soil by turbulences may occur. Grey and bluish-grey particles often indicate sulfur compounds, yellow particles consist of pure sulfur, and red particles are often iron minerals. Another process, which may be combined with the others, comprises the etching-out of degraded soil particles by strongly acidic water. Acids are either results of a direct chemical reaction between gases (hydrogen sulfide, carbon dioxide) or minerals (pyrite, marcasite) and water or they are formed in a hot spring via oxidation of dissolved hydrogen sulfide to sulfuric acid, conducted by hyperthermophilic bacteria such as Sulfolobos. Those bacteria may be an additional factor for water clouding. Characteristically, many acidic springs exhibit a muddy appearance, whereas neutral or alkaline springs are transparent clear in most cases. Muddy hot springs are usually quiet or show turbulent currents or local boiling. Muddy perpetual spouters or even muddy geysers (see the acidic Echinus Geyser) do also occur, but at high discharge rates these features often turn into a transparent clear state quite quickly because suspended matters and acids are washed out.

Crater Hills Geyser in Yellowstone N.P., an example of an acidic geyser clouded by particles of elemental sulfur:

Crater Hills Geyser, Sulphur Spring Yellowstone

Lava Spring near Hveragerði / Iceland, an example of a perpetually spouting spring clouded by iron(III) oxide particles:

Lava Spring Gufudalur Hveragerði Iceland

Even more intensely colored by iron(III) oxide are the Tomato Soup Pools in Yellowstone.

Tomato Soup Pool in Yellowstone N.P.:

Tomato Soup Pool Yellowstone

An unusual variety of a particle rich spring is the "chocolate pot". Silica, iron oxides, and to a lesser extent other metal oxides are precipitated in form of a mound, which may reach an impressive height.

Chocolate Pot in Yellowstone N.P.:

Chocolate Pot Yellowstone

Quite rare is a permanent floating of larger particles, which are visible with the naked eye, on the water surface of hot springs. One example is Cinder Pool in Yellowstone National Park, where the black cinders are consisting of elemental sulfur and dispersed iron(II) sulfide. Particles on the surface of East Pyrite Pool in Lassen Volcanic National Park are mainly formed of pyrite (iron(II) disulfide).

East Pyrite Pool in Lassen Volcanic N.P., an example of a spring with floating particles on the surface (roll mouse over photo):

East Pyrite Pool

Often the temperature of the ground is too high or there is not enough liquid water present to form clearwater or muddy hot springs. In this cases, penetrating vapor and hot gases are generating fumaroles. High gas pressure or high volumes are resulting in fumaroles with pronounced vents.

Öskurhóll in Hveravellir / Iceland, an example of a fumarole with pronounced vent:

Öskurhóll Hveravellir Iceland

Fumaroles with sulfur deposits around the vent are also called solfataras. The reason is that the archetype of all sulfur lined fumaroles, the Bocca Grande, is located inside Solfatara volcanic crater on the Phlegraean Fields in Italy. However, recent science does no longer differentiate between both types but uses only the term fumarole.

Bocca Grande at Solfatara volcanic crater / Italy, a fumarole of the solfatara subtype:

Bocca Grande at Solfatara volcanic crater

Lower amounts of vapour or gases may also discharge through a field of barely visible pinholes, depositing very colorful minerals on the ground surface. Academic publications refer sometimes to it simply as "thermally altered ground".

Fumarole at Leirhnjúkur volcano / Iceland, an example of a fumarole (solfatara) with pinhole vents:

Fumarole at Leirhnjúkur volcano Iceland

Fumaroles, which emit acidic gases, are capable of decomposing many types of soil. As a result a more or less highly concentrated slurry of very small degraded soil particles in a limited amount of acid is formed, called mud pot or mud pool.

Fúlipollur at Seltún / Iceland, an example of a mud pot:

Fúlipollur Seltún Iceland

For colorful, other than grey slurries also the term paint pot is used. Again, the differentation between muddy hot springs and mud pots is not sharp. Comparable to springs, mud pots may show different levels of activity, from a slightly bubbling to intermittent bursts (mud geyser) to steady violent boiling.

Fountain Paint Pot in Yellowstone N.P., an example of a paint pot:

Fountain Paint Pot

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