Mudpots are among the most fascinating and interesting of the
Yellowstone thermal features. They are also a type of hot spring, but
one for which water is in short supply. Whatever water is available
becomes thoroughly mixed with clay and other fine undissolved mineral
matter. The mud is generally gray, black, white, or cream colored, but
some is tinted pale pink and red by iron compounds (fig. 43); hence, the
picturesque term “paint pots” is commonly used.
Mudpots form in places where the upflowing thermal fluids have
chemically decomposed the surface rocks to form clay. Such small amounts
of water are involved, however, that the surface discharge is not great
enough to flush the clay out of the spring. Caldrons of mud of all
consistencies result, from the very thin soupy material in many mudpots
to the almost hard-baked material in the less active features. Some
mudpots expel pellets of very thick viscous mud which build up circular
cones or mounds; this type is commonly called a “mud volcano” (fig. 53).
[Illustration: SEISMIC GEYSER, showing rock rubble blown out during
an explosive thermal eruption. Note the trees that have been killed
by the heat and eruptive activity. According to George D. Marler of
the National Park Service, this geyser developed from cracks caused
by the Hebgen Lake Earthquake of August 17, 1959. (Fig. 51)]
[Illustration: OLD FAITHFUL IN FULL ERUPTION. The interval between
eruptions averages about 65 minutes, but it varies from 33 to 96
minutes. The time lapse between eruptions can be predicted rather
closely, mainly on the basis of the length of time involved in the
previous eruption. If an eruption lasted 4 minutes, for example,
this means that a certain amount of water emptied from the geyser’s
chambers and that a certain length of time will be necessary to
recharge the system for the next eruption. But if the previous
eruption lasted only 3 minutes, less time will be needed for
recharge, and the next eruption will occur sooner. (The above
discussion is based primarily on many years of observation and study
of Old Faithful by George D. Marler and other observers of the
National Park Service; photograph courtesy of Sgt. James E. Jensen,
U.S. Air Force.) (Fig. 52)]
Mudpot activity differs from season to season throughout the year
because of the varying amounts of rain and snow that fall upon the
surface to further moisten the mud. Accordingly, mudpots are commonly
drier in late summer and early fall than they are from winter through
early summer.