Geological History - Yellowstone Treasures - page 366

What makes the different hydrothermal features do what they do? Basically, the great
volume of groundwater is heated by very hot rocks quite near the surface at Yellowstone.
There is a very large amount of old groundwater, at least 60 but perhaps greater than
10,000 years old, just above the magma below Yellowstone. The source of this water may have
been the glaciers that covered the area or rain and snow in the surrounding mountains, 12
to 45 miles (20 to 70 km) distant. Present-day rain and snowmelt seep down and mix with
this old water, become warmed to the boiling point, boil into steam, expand greatly, and find
a way to escape upward. Most of the features occur where faults are common, making it easy
for the heated groundwater and steam to return to the surface.
Geysers such as Old Faithful [GEO.19] are one special kind of steam and hot water emit-
ter. This kind needs a very steady supply of hot groundwater and special plumbing that in-
cludes a narrow fissure that goes deep into the ground and that has even narrower parts along
the way. (For a more specific explanation of geysers, see “How Does a Geyser Work?” on page
88 . )
Changes in the shape of a geyser's plumbing, its water supply, or water temperature can
change the timing, height of eruption, or volume of water. The changes are normally due to
blockage by precipitation of solids onto the fissure walls, minor earthquakes, or eruptions that
are especially violent. Most geysers in Yellowstone erupt repetitively, but the intervals are not
necessarily regular. Old Faithful Geyser is faithful in its timing, because the flow rate and tem-
perature of the very hot water have been very constant.
The 1959 Hebgen Lake earthquake just outside the northwest corner of the park consid-
erably altered conditions in Yellowstone. Some geysers changed their cycles, some stopped
erupting, and dormant ones resumed eruptions. Smaller earthquakes affect Yellowstone's hy-
drothermal features every year.
In other places, where there is also a large supply of very hot groundwater near the surface,
it rises to make a hot pool. These deceptively inviting pools have lovely colors, but their water
is at or near the boiling point. Examples are Grand Prismatic Spring [GEO.20] in Midway
Geyser Basin and Emerald Spring [GEO. 21] in Norris Geyser Basin.
How do the other hydrothermal features work? Steam vents, or fumaroles, are places where
there's a steady flow of steam coming from a fissure in the ground. Fumarole activity is closely
related to current precipitation and occurs where the old hot groundwater is not very near the
surface. In these features a downward seepage of rainwater reaches hot rocks, the water boils,
and the steam is vented through the fissure.