Hot springs occur where the rising hot waters of a thermal system issue
from the ground-level openings of the feeder conduits (fig. 45). By far
the greatest numbers discharge water and steam in a relatively steady
noneruptive manner, although they vary considerably in individual
behavior. Depending upon pressure, water temperature, rate of upflow,
heat supply, and arrangement and size of underground passages, some hot
springs boil violently and emit dense clouds of vapor whereas in others
the water quietly wells up with little agitation from escaping steam. In
some hot springs, however, the underground channels are too narrow or
the upflow of very hot water and steam is too great to permit a steady
discharge; periodic eruptions then result. These special kinds of
springs are called “geysers” (from the Icelandic word _geysir_, meaning
to “gush” or “rage”). At least 200 geysers, of which about 60 play to a
height of 10 feet or more, occur in Yellowstone National Park; this is
more than in any other region of the world.
How does a geyser work? We cannot, of course, observe the inner plumbing
of a geyser, except for that part which is seen by looking into its
uppermost “well.” Deeper levels directly below the “well” can be probed
by scientific instruments to some extent, and research drilling in some
parts of the geyser basins also provides much useful information. The
available information suggests that the plumbing system of a geyser (1)
lies close to the ground surface, generally no deeper than a few hundred
feet; (2) consists of a tube, commonly nearly vertical, that connects to
chambers, side channels, or layers of porous rock, where substantial
amounts of water can be stored; and (3) connects downward through the
central tube and side channels to narrow conduits that rise from the
deepwater source of the main thermal system.
Considering a geyser system as described above and applying what is
known about the behavior of water and steam, we can understand what
causes a natural thermal eruption. Figure 50 shows diagrammatically the
succession of events believed to occur during the typical eruptive cycle
of a geyser such as Old Faithful.
[Illustration: A GEYSER IN ACTION. Photographs of successive stages
in the eruption of Old Faithful illustrate what probably happens
during a natural geyser eruption. The underground plumbing is
diagrammatic and does not reflect any specific knowledge of Old
Faithful’s system. Direction of flow of water is shown by arrows.
(Based on information supplied by D. E. White, L. J. P. Muffler, R.
O. Fournier, and A. H. Truesdell.) (Fig. 50)
Stage 1 (Recovery or recharge stage). After an eruption, the partly
emptied geyser tubes and chambers fill again with water. Hot water
enters through a feeder conduit from below, and cooler water
percolates in from side channels nearer the surface. Steam bubbles
(with some other gases such as carbon dioxide and hydrogen sulfide)
start to form in upflowing currents, as a decrease in pressure
causes a corresponding decrease in boiling temperature. At first the
bubbles condense in the cooler, near-surface water that is not yet
at boiling temperature, but eventually all water is heated enough
that the bubbles will no longer condense or “dissolve.”]
[Illustration: Stage 2 (Preliminary eruption stage). As the rising
gas bubbles grow in size and number, they tend to clog certain parts
of the geyser tube, perhaps at some narrow or constricted point such
as at A. When this happens, the expanding steam abruptly forces its
way upward through the system and causes some of the water to
discharge from the surface vent in preliminary spurts. The deeper
part of the system, however, is not yet quite hot enough for
“triggering.”]
[Illustration: Stage 3 (Full eruption stage). Finally, a preliminary
spurt “unloads” enough water (with resulting reduction in pressure)
to start a chain reaction deeper in the system. Larger amounts of
water in the side chambers and pore spaces begin to flash into
steam, and the geyser rapidly surges into full eruption.]
[Illustration: Stage 4 (Steam stage). When most of the extra energy
is spent, and the geyser tubes and chambers are nearly empty, the
eruption ceases. Some water remains in local pockets and pore
spaces, continuing to make steam for a short while. Thereafter the
system begins to fill again, and the eruptive cycle starts anew.]
No two geysers have the same size, shape, and arrangement of tubes and
chambers. Also, some geysers, such as Great Fountain, have large surface
pools not present in cone-type geysers such as Old Faithful. Hence, each
geyser behaves differently from all others in frequency of eruption,
length of individual eruptions, and amount of water discharged. Geysers
may also vary in their own behavior as their plumbing features change
through the years. The great amount of energy that builds up in some of
them from time to time creates enough explosive force to shatter parts
of the plumbing system, thereby causing a change in their eruptive
behavior. In fact, some geyser eruptions have been so violent that large
chunks of rock have been exploded out of the ground and scattered around
the surrounding area (fig. 51). With time, the precipitation of minerals
may partly seal a tube or chamber, gradually altering the eruptive
mechanism.
Despite all the variable factors involved in geyser eruptions, and all
the changes that can take place from time to time to alter the pattern
of those eruptions, several of the Yellowstone geysers function
regularly, day after day, week after week, and year after year. Within
this group of regulars is the most famous feature of all—Old
Faithful—which has not missed an eruption in all the many decades that
it has been under close observation (fig. 52). We can only conclude that
nature has provided this incredible geyser with a stable plumbing system
that is just right to trigger delightfully graceful eruptions at
close-enough time intervals to suit the convenience of all Park
visitors.