Stephen Ott
Geology 111
The Geysers of Yellowstone National Park
This paper is a report for the Physical Geology Course on the subject of the geysers in
Yellowstone National Park. In this report, I will describe for the reader what a geyser is,
including what the necessary conditions are for the existence of a geyser and the different types
of geysers that exist. After geysers are described, I will explain the basics of how a geyser works.
Finally, I will use Old Faithful as an example to show how many geysers have individual
characteristics that make each geyser unique.
The word geyser is from an old Icelandic word which means “gushing” (Smith & Siegel,
2000). The first geysers which were known world-wide were from the island of Iceland. A
geyser is described as a hot spring that periodically ejects water – usually hot – vertically into the
air. Most geysers are found in about 45 geyser basins throughout the word, but Yellowstone
National Park contains approximately 500 geysers, which is about 55% of all existing geysers. In
Yellowstone National Park itself, the Upper Geyser Basin contains about 200 of those geysers in
an area of approximately 1 ½ square miles (Bryan, 1995). This is the largest concentration of
geysers found anywhere in the world.
Geysers belong to the larger family of geothermal features found at Yellowstone National
Park, including hot pools, fumaroles, mud pots and spring terraces. Geysers are distinguished
from the others in that they spray water into the air, sometimes sometime several hundred feet
high. Although there are six generic types of geysers (Rinehart, 1980, p. 51), they mostly fall
into two categories: those that erupt out of an empty columnar tube and those that erupt from
inside a pool of water.
Because of the unusual conditions necessary to create a geyser, they are much less common
than the other types of geothermal features that are mentioned above (Bryan, 1995). Those
specific conditions necessary for the existence of a geyser are as follows: first, there must be an
abundant supply of water; second, there must be a sufficient heat source; and third, there must be
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a special underground plumbing system (Bryan, 1995; Rinehart, 1980). This third condition
largely controls whether if a geyser fill form instead of one of the other more common geologic
features. I will describe each of these specific conditions more fully in the paragraphs that
follow.
A geyser’s water source comes from underground. Many geysers are located near rivers, so
it is suspected that the underground water aquifer from the rivers is usually the main source of
water (Rinehart, 1980). Other source water comes from deep underground, where rainwater or
snow melt has percolated for possibly even hundreds of years (Bryan, 1995). Geysers usually
develop in geologic regions where a very porous rock, such as rhyolite, is underground to collect
and store the available water.
The necessary heat for the geyser eruption comes indirectly from a magma source that is
close to the surface of the earth, called a “hotspot.” The earth contains about thirty active
hotspots around the globe. The Yellowstone hotspot exists about 3-5 miles below the surface of
the earth. It is approximately 45 miles long by 30 miles wide, which makes it the largest hotspot
lying underneath a continent (Smith & Siegel, 2000). Two interesting bits of information can
give the reader an idea about the enormous amount of heat given off by the Yellowstone hotspot.
First of all, it was found that the hottest borehole temperatures have been measured at about
238ºC (water boils at 100ºC). Also, the amount of energy the hotspot emits totals to about 5
gigawatts – enough energy to fill the needs of a city of more than 2 million people (Smith &
Siegel, 2000).
The plumbing, or network of tubes and caverns in the rocks below the surface, must be
strong enough to withstand blasts of high pressure generated by the geyser as it erupts and it must
have a constricted opening that prevents the water source in the reservoir from circulating easily.
Most Yellowstone National Park geysers are lined with geyserite, a silica rock that is strong
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enough to resist the intense pressures described (Bryan, 1995). Because of this special plumbing,
the activity of geysers can be disrupted by the influence of earthquakes, which can create new
cracks and fissures or seal old ones. This “re-plumbing” can alter to activity of geysers to the
extent that current geysers can completely cease any eruptive activity or new geysers can be
created.
Geysers get their eruptive activity from the tremendous explosive force that exists when a
liquid boils into a gas. The process for this explosive boiling begins approximately 1.5 to 3 miles
below the surface of the earth, where the hotspot magma heats up a brine (salty) solution, which
in turn heats up the fresh water reservoir that erupts out of the geyser opening (Smith & Siegel,
2000). The process of eruption occurs when the water in the lower section of the reservoir is
heated to its boiling point. Because of the pressure from the water above, and because the boiling
bubbles are cooled and reabsorbed as they rise to the surface, the deep water in the reservoir
becomes superheated above the normal boiling point of water. When the water is heated
sufficiently that the boiling bubbles make it almost all the way to the surface, the bubbles expand
and explode, pushing up a large quantity of water through the geyser opening. This expulsion of
the surface water decreases the pressure on the water below, which likewise explodes into a boil,
and a chain reaction occurs to expel tremendous amounts of water through the geyser opening.
(Smith & Siegel, 2000; Byran, 1995; Science, 1997)
Although most geysers do not have predictable eruptions, it is evident from the regular
geysers that there are factors that can influence a geyser’s eruptions. One of these factors is the
barometric pressure due to the weather (Rinehart, 1980). This effect on geyser eruptions comes
as a result of the decrease in boiling point of water at lower atmospheric pressure. Another
interesting factor that effects geyser eruptions is the tidal forces, because the gravity of the moon
and sun attract both the water in the geyser and the surface of the earth. The tidal effect is
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significant enough that it can alter the interval between geyser eruptions by as much as 30
minutes.
Underground, the heated water
reservoir, which frequently reaches
temperatures of 205ºC, dissolves some of the
silica (quartz) in the rhyolite rock that it
passes through. This silica is deposited as
geyserite, also called sinter, when the geyser
water cools on the surface of the ground
Castle Geyser
(Bryan, 1995). Thousands of years of these sinter deposits are responsible for forming the
interesting mounds that give many geysers their names, such as Castle Geyser or Beehive Geyser
(Smith & Siegel, 2000). Also, because the water is so hot and so acidic, it can dissolve much
more mineral material than normal aquifer water. The result is that geyser water carries as much
as 1.5 g/L of dissolved minerals (Rinehart, 1980). This much dissolved minerals deposits up to
352,000 kg of solid per day in Yellowstone National Park (Rinehart, 1980)
The cooling geyser water, full of these dissolved minerals, becomes a breeding ground for
algae and thermophilic cyanobacteria. Geyser water streams are lined with brilliant yellows,
oranges, browns, and greens. Each different color is a result of different bacteria that can tolerate
different temperature ranges. One writer noted that “you can tell the approximate temperature of
the stream by the color of its cyanobacteria” (Bryan, 1995). Some of this bacteria is so unique
that it has been used in various scientific processes such as DNA replication (Satchell, 1996;
Wilkinson, 1997).
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The most famous geyser in
Yellowstone National Park, and probably in
the entire world, is Old Faithful Geyser in the
Upper Geyser Basin. Some of the
characteristics of Old Faithful are insightful
to mention in this report as added explanation
to a description of geysers.
As already explained about geysers,
Old Faitherful Geyser
Old Faithful has a special plumbing system
beneath its vent that allows for the hot water to simmer and then boil explosively. Old Faithful is
famous for its regularity. From data I received from a National Park volunteer (Stephens, 2000),
I observed that the average interval between eruptions ranged from about 80 to 90 minutes in
August 2000. A typical eruption from Old Faithful reaches about 55 meters high, lasts about 2 to
5 minutes and expels 3,700 to 8,500 gallons of boiling water (Smith & Siegel, 2000; Perkins,
1997). Although is not the most predictable geyser in the park, it is the most predictable of the
large geysers.
To study the workings of Old Faithful, National Park officials have lowered a miniature,
watertight camera through the geyser opening. The camera revealed that the opening constricts
from 20 cm to only 11 cm wide at a depth just 7 meters below the surface. Below that, the shaft
opens to “wildly boiling mixture of steam, water, and what look like carbon dioxide bubbles” in a
cavity about the size of a large automobile (Science, 1997; Bryan, 1995). The water temperature
measured 117ºC, which is 25ºC higher than the normal boiling temperature of 92ºC at elevations
as high as Yellowstone National Park. The researchers reported that the action resembled a
seething “liquid tornado of unbelievable violence” (Bryan, 1995). Down to about 10.5 m, jets of
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superheated boiling water were observed rushing in nearly horizontally from the shaft walls
(Science, 1997).
As mentioned, Old Faithful is known for its regularity. Most geysers are not predictable.
However, another one relatively unknown feature about Old Faithful is that it is bimodel. This
means that the time intervals between eruptions are clustered statistically around two averages,
one long and one short. When Old Faithful will demonstrate a short eruption interval and when it
will demonstrate a long eruption interval has been difficult to predict, but it has been found that
shorter eruptions occur after short intervals and long eruptions occur after long intervals.
(Rinehart, 1980) One possible explanation for the bimodality is given by Rinehart (1980), who
writes that there is the possibility of a large “side pool” beneath the geyser which is sometimes
filled and participates in the eruption.
The geysers of Yellowstone National Park are a fascinating feature of the geology of this
earth, and I feel very fortunate that I am close enough to enjoy them frequently.
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Bibliography
Bryan, T. Scott (1995). The Geysers of Yellowstone, Third Edition (Niwot, Colorado:
University Press of Colorado).
Perkins, Sid, (1997).“Inside Old Faithful: Scientists Look down the throat of a geyser,”
Science News, Vol. 152, Issue 15, p. 232.
Rinehart, John S. (1980). Geysers and Geothermal Energy (New York: Springer-Verlag).
Satchell, Michael (1996). “”There’s Gold in Them Thar Pools. Tourists like the sights.
Biotech companies are mining Yellowstone’s attractions for profit.” U.S. News & World Report,
Vol. 121, Issue 22, p. 81.
“Secrets of Old Faithful,” (1997). Science, Vol. 278, Issue 5337, p. 393.
Smith, Robert B. and Siegel, Lee J. (2000). Windows into the Earth: The Geologic Story
of Yellowstone and Grand Teton National Parks (New York, New York: Oxford University
Press).
Wilkinson, Todd (1997) “Yellowstone’s Geysers Open for Prospecting. Historic deal
signed this month gives company the rights to search thermal waters for microbial ‘gold’.”
Christian Science Monitor, Vol. 89, Issue 192, p. 1.
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