Yellowstone


Since no distinct volcanic ediface is evident, many visitors to Yellowstone National Park are not immediately aware that they are spending much of their time within the caldera of what has been recently coined a Supervolcano. Whilst this term has no accepted meaning within the scientific community, it serves to emphasize the immense eruptive potential that Yellowstone Caldera (and several other such structures around the world) possesses. The caldera was formed when the massive magma chamber was largely emptied during a catastrophic eruption, resulting in subsidence of its overlying roof. At present, the underlying magma chamber manifests itself by the heating of groundwater, resulting in the worlds largest collection of geothermal features, including the many Geysers to which the Parks popularity can be largely attributed.


Riverside Geyser Castle Geyser

Riverside Geyser

Castle Geyser


Yellowstone volcano is the most recent in a succession of eruptive centers resulting from a hot-spot in the earths mantle. The North American Plate is gradually shifting in a southwestern direction over a stationary hotspot. Over the last about 17 million years, episodic activity in the region of the hotspot has resulted in a series of calderas which form a recognizable track in the surrounding topography with the most recently active Yellowstone caldera(s) being at its northeastern end. This track is referred to as the Snake River Plain. Whilst many "classic" volcanic features may not be immediately apparent in many parts of the park, a number of sites expose parts of the complex geological record of the area. Yellowstone Canyon cuts deep into the caldera floor, exposing many layers of volcanic deposits. The waterfalls mark the boundaries of erosion-resistant layers of rhyolitic lava. Also, for example at Sheepeaters cliff, exposed deposits of more recently erupted basaltic lava that have cooled to produce hexagonal columns can be easily viewed.


Grand Canyon of the Yellowstone Grand Canyon of the Yellowstone Lower Falls, Grand Canyon of the Yellowstone

Grand Canyon of the Yellowstone

Grand Canyon (View Artists Point)

Lower Falls


Sheapeater Cliff, Basalt Columns, Yellowstone Sheapeater Cliff, Basalt Columns, Yellowstone

Sheepeaters Cliff, Basalt Column

Closeup, Basalt Column


Geological records reveal that the Snake River Plain has been responsible for extremely large and devastating eruptions. Indeed, the Bruneau-Jarbridge and Twin Falls calderas which were active between 13 and 8 million years ago appear to have displayed extremely unusual eruption characteristics. Based on studies of their eruption deposits, it has been proposed to name the extremely high-temperature silicic super-eruptions attributable to these eruptive centers "Snake River (SR)-Type" eruptions (Brannley et al., 2008. Bull. Volcanol. 70, p.293-314). These eruptions were collossal and involved rhyolite lava erupted at unusually high temperatures, resulting in thick and extensive deposits of extensively welded (almost lava-flow like) rhyolitic ignimbrites largely devoid of pumice, extensive parallel-laminated relatively coarse ash-deposits (deposited by massive pulsating pyroclastic density currents) and finer ashes deposited in large volumes many hundreds of kilometers from the eruptive centers, Unusually large and low-aspect ratio rhyolitic lava flow fields are also found, along with basaltic flows which were probably emplaced late in the eruption or in subsequent eruptive periods. Indeed the SRP is today largely covered with basaltic material. Rhyolitic lava is usually highly viscous due to its high silica content, yet it seems that eruption temperature and maybe magma composition (e.g. high fluorine) reduced silicate polymerization, leading to improved flow properties.

Eruptions at Yellowstone caldera do not conform to the SR-type, since deposits are pumice-rich and conform more closely to those of conventional explosive rhyolitic volcanism. This suggests that as location of the hot-spot under the surface has changed, the eruption mechanism has evolved. This is probably due to the fact that the archetypal SR-type eruptions occurred in a region of continental rifting during formation of the western Snake River Rift. Further, the lithosphere (earths crust) differs along the SR track. The lithosphere contributes to magma composition since basaltic magmas rising in the region of the hot-spot partially melt it, incorporating it in the developing magma. Differing magma chemistry may have an effect on eruption dynamics due to influence on e.g. fluidity of the eruption products. Nevertheless, whilst only retaining few SR-type characteristics, eruptions at Yellowstone have been similarly devastating to large swathes of surrounding landscape.


Snake River Hotspot Track

(Sketch based on Fig.1 Branney et al. 2008, Bull. Volcanol. 70, p293-324)

Yellowstone Annotated Map

(Sketch based on USGS and NPS maps)


The Yellowstone Plateau has evolved due to the formation of three overlapping calderas in the last 2 million years (Christiansen 2001. USGS Prof. Paper 729-G). These are the massive Island Park (IP) Caldera (2.1 Ma), the smaller Henry's Fork Caldera (1.3 Ma) which lies which lies outside the National Park Boundaries in the WSW corner of the IP Caldera and the more recent Yellowstone Caldera (640000 years old) which covers most of the eastern half of the IP Caldera and is about 52 by 75 km wide. It is thought that during the eruption of Yellowstone Caldera approximately 1000 cubic kilometers of material was erupted causing a succession of massive pyroclastic density currents covering thousands of cubic km of the surrounding landscape. The caldera resulting from collapse of the eruption chamber during the eruption is hardly recognizable today since largely filled with materials erupted in smaller post-caldera forming eruptions, with further modifications resulting from large hydrothermal explosions and extensive erosion. Significant Postcaldera eruptive events emplaced largely rhyolitic lavas. These occurred between 516-479 thousand years (kA) ago at the ring-fracture zone boundary at the perimeter of the caldera, about 260 kA ago, and most recently during several episodes 170, 150, 115, 100 and 75 kA ago when activity was located along two NW trending zones located along the SW flank of the caldera and in its center along a line passing through Norris Basin to the north of the caldera, respectively. These "recent" episodes are responsible for much of the topography of the area as we see it today, emplacing volumes of between 10 and 150 cubic km of rhyolitic lavas in at least 17 major flow units. These have covered much of the floor of the caldera and completely buried its SW rim. Comparison of the composition of these lavas reveals a gradual cooling trend of the post-caldera magma-reservoir with increasing magma fractionation, whilst some recharging continues to periodically occur (Vazquez et al. 2009. J. Volc. Geotherm. Res. (In Press)). Indeed, influx of fresh magma into the chamber is probably responsible for the ground swelling which is continually monitored in the park. The system is clearly still active and has the potential for further huge eruptions some time in the future. Seismic and gravitational studies are able to detect the plume of basaltic magma (hotspot) which still underlies the Yellowstone area (see Leeman et al. 2009. J. Volc. Geotherm. Res. (In Press) and references therein). This plume continues to supply magma to the crust where it incorporates crust material and differentiates into largely rhyolitic material. Basaltic eruptions appear to have followed the rhyolitic phases at most of the SR Track sites (Bonnichsen et al. 2008. Bull. Volcanol. 70, p.315-342). It is thought that Yellowstone will also eventually follow this trend.


Of particular interest for visitors is the geothermal activity manifested by the over 10000 hot springs, mud-pots, fumaroles or geysers present in the caldera and in its vicinity. The underlying magma reservoir is thought to supply about 5500 MegaWatts of energy to this system. The geothermal system is driven by liquid dominated reservoirs in the western portion of the caldera, whilst the eastern portion is driven by steam-dominated reservoirs (Fournier 1989. Ann. Rev. Earth Planet. Sci. 17, p.13-53). Most of Yellowstones geothermal systems lie within the caldera itself, yet the Norris Geyser Basin and Mammoth Hot Springs lie along a complex subsidence structure extending northwards from the caldera which may be underlain by independent magma bodies in the vicinity of the two geothermal sites (Kharaka et al. 2000. J. Geochem. Explor. 69/70, p.201-205 and ref. therein). A number of rhyolitic and basaltic eruptive episodes after caldera formation were aligned along this structure (Hildreth et al. 1991. J. Petrol. 32, p.63-128).

Hot Springs and Geysers are formed where geothermally heated waters rise to the surface. The difference between these features largely lies in their flow rate and the morphology of the system feeding them. Geysers usually require chambers in which waters can collect and pressurize before being expelled to the surface. Many hot springs may erupt as geysers if heat input to the system is increased.


Flow of liquids / steam within the system is through areas of rock with high permeability and through interconnected channels such as fractures, veins and hollows. This is particularly evident following strong seismic events which can cause extensive formation of new cracks, thus modifying and often opening flow channels. Following the powerful Hebgen Lake Earthquake in 1959, numerous hot springs started to erupt as geysers, presumably due to increased pressure release from the deep geothermal reservoir and accompanying faster flow of ascending superheated waters (Marler and White, 1975. Geol. Soc. Am. Bull. 86, p.749-759). On the other hand, channels may gradually by blocked by deposition of silica (silicification) or other minerals (Dobson et al. 2003. J. Volc. Geotherm. Res. 123, p.313-324). In fact numerous factors appear to have an influence on geyser activity. There are indications that rainfall, barometric pressure or gravitational forces (relating to positioning of earth, sun and moon) may influence the behaviour of some geysers. Rainfall levels and indeed climatic trends over years have been demonstrated to influence the frequency of eruption of some Yellowstone geysers (Hurwitz et al. 2008. Geology 36(6), p.451-454). Increased gravitational forces may close channels and modify water flow into a geyser, thus changing activity. A similar "ground tension related effect" could account for changes in geyser activity observed in California in the days prior to several earthquakes (Silver and Valette-Silver 1992. Science 257, p.1363-1368).


How do geysers work ? The exact plumbing systems of different geysers show great variation, whilst the basic principles underlying activity are relatively conserved. Geysers can be classified in a number of ways. A broad distinction is often made between dome geysers which erupt through a narrow vent, and fountain geysers which erupt from a broad pool. Geyser plumbing systems have been further classified into 6 basic types (A-F) according to shape and complexity of their plumbing systems (Rhinehart "Geysers and Geotherm. Energy" Springer-Verlag). Whilst this is an extreme oversimplification it nevertheless serves to illustrate some working models. Type A broadly corresponds to a classic dome geyser. Fountain geysers usually have types D-F.

White Dome Geyser Great Fountain Geyser

Dome Geyser (White Dome)

Fountain Geyser (Great Fountain)


Geyser Types Geyser Types

Geyser Types ABC

Geyser Types DEF


Basically, a geyser requires a heat source (cooling magma), a source of water, permeable rocks through superheated waters can rise and a pressure-tight (i.e. non-permeable) chamber or series of chambers where pressure can build up prior to eruption. Most geysers are found in highly silicic rhyolitic rocks. Superheated waters dissolve silica from these rocks at temperatures of around 300'C which exist in the high-pressure environment deep under the surface. Obsidian clast rich deposits are particularly susceptible to silica dissolution. Silica exsolves again and is deposited when the superheated waters boil or cool as pressure and temperature fall, respectively as they rise towards the surface. The deposited "geyserite" (largely hydrated silicon dioxide (a form of opal)) not only seals the geysers plumbing system, allowing it to pressurize. Basically, superheated waters rise into the geysers chamber(s), whilst cooler near-surface waters (may) also enter the chamber from above. These initially effectively cap the hotter water body below keeping it pressurized and allowing it to maintain temperatures well above boiling point under atmospheric pressure conditions. As the chamber fills, the superheated water body transfers heat to the cooler water above. Eventually, the upper water body reaches boiling point and bubbles of water vapour may rise through it to the surface. As heating continues the bubbling becomes more vigorous and water may start to be pushed out of the vent. The overflow and / or increasing concentration of bubbles decreases the mass of the upper water body, thus reducing the pressure acting on the body of superheated water below. This depressurization causes violent "run-away" vaporization of the water below, resulting in water and steam being violently expelled and leading to further depressurization which continues to drive the eruption.

Some geysers (e.g. Great Fountain) erupt in a number of consecutive bursts. This is largely accounted for by complex plumbing systems where pressure is consecutively released from a number of different chambers, although the exact mechanism is not known.

The superheated waters at yellowstone are thought to be of meteoric origin. It is estimated that these waters are at least 500 years old (in terms of time in the system) when they are expelled from the parks geysers. The fact that many of Yellowstones geyser basins lie alongside rivers suggests that these provide a good supply of cooler surface waters for the geysers.


Many geysers transport large amount of silica to the surface as evidenced by large geyserite deposits (e.g. Castle, Lone Star, Great Fountain, Grotto). Exsolution at the surface is enhanced by evaporation. Microorganisms are also thought to play a role in silica deposition. The large cones built up by some geysers are testimony to a long history of activity. Castle geyser was indeed thought to be thousands of years old until recent radiocarbon dating revealed an age of about 500 years. Nevertheless, most geysers appear to last for much shorter periods of time.

Yellowstone not only features significant deposits of geyserite, but also has a complex of travertine terraces at the Mammoth Hot Springs. Here, the underlying rocks differ from those in the geyser basins and include sandstone and limestone strata. Unlike in the geyser basins, the geothermal waters are not at boiling point at the surface and they contain high levels of carbonates, sulphates and magnesium. Consequently the precipitate travertine (calcareous sinter), not silicious sinter (geyserite). There are also rare examples of largely ferric sinter at the Chocolate Pots along Gibbon River south of Norris Basin. The pots comprise about 60% iron, aluminium, nickel and manganese oxides, with only about 17% silica. The high iron oxide content accounts for their rusty colour.


Grotto Geyser, Siliceous Sinter, Geyserite, Yellowstone Palette Spring, Mammoth Hot Springs, Yellowstone Chocolate Pots, Ferrous Hot Springs ,Gibbon River, Yellowstone

Grotto Geyser, Siliceous Sinter (Geyserite) Cone

Palette Spring, Travertine (Calcareous Sinter) Terraces

Chocolate Pots, largely Ferrous Sinter


Hot Springs may be less dynamic than geysers, yet due to an often more steady flow rate can provide a home to a variety of often colourful microorganisms. The distribution of these organisms depends on temperature gradients meaning that organisms of a particular colour may be located in a spring, whereas others requiring lower temperatures may be located at its periphery or in its run-off areas. This explains the various colours that may be observed around a single spring. The thermophilic organisms are an important source of heat-tolerant enzymes for biotechnological applications since some have evolved to withstand and grow in temperatures near to boiling point.


Grand Prismatic Spring Sinter Terraces, Cyanobacterial Mats, Yellowstone Chromatic Pool, Yellowstone Microbial Foam, Mammoth Hot Springs, Yellowstone

Cyanobacterial mat-covered sinter terraces, Grand Prismatic Spring

Chromatic Pool - Multiple Colours thanks to several microorganisms

Floating layer of microbes at Mammoth Hot Springs


Readers with a general interest in hot springs and geysers may also wish to view the sections on the brine springs of Dallol, or the Wai-O-Tapu geothermal area in New Zealand.


Geothermal Highlights at Yellowstone:

Upper Geyser Basin

(Link to Upper Geyser Basin Sub-Page)


Grand Geyser, Upper Geyser Basin, Yellowstone Grand Geyser, Upper Geyser Basin, Yellowstone Daisy Geyser, Upper Geyser Basin, Yellowstone

Grand Geyser

Grand Geyser

Daisy Geyser


Grotto Geyser, Upper Geyser Basin, Yellowstone Castle Geyser, Steam Phase, Rainbow, Upper Geyser Basin, Yellowstone Castle Geyser, Steam Phase, Upper Geyser Basin, Yellowstone

Grotto Geyser

Castle Geyser, Steam Phase

Castle Geyser, Steam Phase


Grotto Geyser, Upper Geyser Basin, Yellowstone Plume Geyser, Upper Geyser Basin, Yellowstone Sawmill Geyser, Upper Geyser Basin, Yellowstone

Grotto Geyser

Plume Geyser

Sawmill Geyser


Riverside Geyser, Upper Geyser Basin, Yellowstone Riverside Geyser, Upper Geyser Basin, Yellowstone Old Faithful Geyser, Upper Geyser Basin, Yellowstone

Riverside Geyser

Riverside Geyser

Old Faithful Geyser


Heart Spring, Hot Spring, Upper Geyser Basin, Yellowstone Anemone Geyser, Upper Geyser Basin, Yellowstone

Heart Spring

Anemone Geyser


Lion Geyser, Upper Geyser Basin, Yellowstone Sawmill Geyser, Upper Geyser Basin, Yellowstone Beehive Geyser, Upper Geyser Basin, Yellowstone

Lion Geyser

Sawmill Geyser

Beehive Geyser


Morning Glory Pool, Hot Spring, Upper Geyser Basin, Yellowstone Morning Glory Pool, Hot Spring, Upper Geyser Basin, Yellowstone Chromatic Pool, Hot Spring, Upper Geyser Basin, Yellowstone

Morning Glory Pool

Morning Glory Pool

Chromatic Pool


Midway Basin


Grand Prismatic Spring, Midway Geyser Basin, Yellowstone Sinter Terrace, Grand Prismatic Spring, Midway Geyser Basin, Yellowstone Sinter Terrace, Grand Prismatic Spring, Midway Geyser Basin, Yellowstone

Grand Prismatic Spring

Grand Prismatic Spring

Grand Prismatic Spring


Steam Rising over Midway Geyser Basin, Firehole River Excelsior Geyser Overflow into Firehole River

Steam Rising over Midway Geyser Basin

Excelsior Geyser Overflow into Firehole River



Lower Geyser Basin (incl. Features near Firehole Lake)


Great Fountain Geyser, Lower Geyser Basin, Yellowstone White Dome Geyser Erupting, Rainbow, Lower Geyser Basin, Yellowstone Great Fountain Geyser, Lower Geyser Basin, Yellowstone

Great Fountain Geyser

White Dome Geyser

Great Fountain Geyser


Boiling Mud Pots, Fountain Paint Pot Area, Lower Geyser Basin, Yellowstone Clepsydra Geyser, Fountain Paint Pot Area, Lower Geyser Basin, Yellowstone

Boiling Mud Pots, Fountain Paint Pot Area

Clepsydra Geyser


Biscuit Basin (part of Upper)


Hot Springs, Biscuit Basin, Yellowstone Shell Geyser, Biscuit Basin, Upper Geyser Basin, Yellowstone

Hot Springs, Biscuit Basin

Shell Geyser

Jewel Geyser, Biscuit Basin, Upper Geyser Basin, Yellowstone Jewel Geyser, Biscuit Basin, Upper Geyser Basin, Yellowstone

Jewel Geyser

Jewel Geyser


Black Sand Basin (part of Upper)


Cliff Geyser Erupting, Black Sand Basin, Upper Geyser Basin, Yellowstone Cliff Geyser Erupting, Black Sand Basin, Upper Geyser Basin, Yellowstone Spouter Geyser, Upper Geyser Basin, Black Sand Basin, Yellowstone

Cliff Geyser

Cliff Geyser

Spouter Geyser


Lone Star (Third) Geyser Basin


Lone Star Geyser, Geyserite Cone, Yellowstone Lone Star Geyser, Geyserite Cone, Yellowstone Lone Star Geyser, Steam Phase, Geyserite Cone, Yellowstone

Lone Star Geyser

Lone Star Geyser

Lone Star Geyser, Steam Phase


Mud Volcano / Sulphur Cauldron Area


Mud Volcano, Yellowstone Dragons Mouth Spring, Mud Volcano Area, Yellowstone Mud Volcano, Yellowstone

Mud Volcano

Dragons Mouth Spring

Mud Volcano


Churning Caldron, Mud Volcano Area, Yellowstone Churning Caldron, Mud Volcano Area, Yellowstone

Churning Caldron

Churning Caldron


Sulfur Caldron, Yellowstone Sulfur Caldron, Yellowstone Sulfur Caldron, Bubbles, Yellowstone

(Left) Sulfur Caldron

(Right) Sulfur Caldron

Bubbles on Surface of (right) Sulfur Caldron


West Thumb Basin


West Thumb Geyser Basin, Yellowstone Fishing Cone, West Thumb Geyser Basin, Yellowstone

Hot Springs

Fishing Cone


Mammoth Hot Springs

(Link to Mammoth Hot Springs Sub-Page)


Cleopatra Terrace, Mammoth Hot Springs, Yellowstone Cleopatra Terrace, Mammoth Hot Springs, Yellowstone Orange Spring Mound, Mammoth Hot Springs, Yellowstone

Cleopatra Terrace

Cleopatra Terrace

Orange Spring Mound


Mammoth Hot Springs, Yellowstone Liberty Cap, Mammoth Hot Springs, Yellowstone Main Terrace Overflow, Mammoth Hot Springs, Yellowstone

Mammoth Hot Springs, Overview

Liberty Cap, Exposed Spring Deposit

Main Terrace Overflow


Main Terrace, Mammoth Hot Springs, Yellowstone Angel Terrace, Mammoth Hot Springs, Yellowstone Trees embedded in Main Terrace, Mammoth Hot Springs, Yellowstone

Main Terrace

Angel Terrace

Main Terrace


Norris Geyser Basin


Steamboat Geyser Minor Activity, Norris Geyser Basin, Yellowstone Norris Geyser Basin, Yellowstone

Steamboat Geyser, Minor Activity

Norris Geyser Basin, Overview northern section


Additional Features


Artists Paint Pots Area, Yellowstone Artists Paint Pots Area, Bubbling Mud Pool, Yellowstone Chocolate Pots, Ferrous Hot Springs ,Gibbon River, Yellowstone

Artists Paint Pots Area

Boiling Mudpots, Artists Paint Pots Area

Chocolate Pots




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