[Illustration: MOUND OF SINTER at Castle Geyser, Upper Geyser Basin.
Lower part of mound has well-defined layers probably deposited by
normal hot springs. The upper, irregular part resulted from the
vigorous eruptions characteristic of geysers and marks a change in
the local hot-spring activity. (Fig. 47)]
Nearly all geysers and many hot springs build mounds or terraces of
mineral deposits; some are so unusual in form that descriptive names
have been given to them, such as Castle Geyser (fig. 47). These deposits
are generally made up of many very thin layers of rock. Each layer
represents a crust or film of rock-forming mineral which was originally
dissolved in hot water as it flowed through the underground rocks, and
which was then precipitated as the water spread out over the surrounding
ground surface.
[Illustration: TERRACES OF TRAVERTINE at Opal Springs, Mammoth Hot
Springs area. (Fig. 48)]
[Illustration: Closeup view shows layered and porous nature of the
travertine.]
In all major thermal areas of the Park, with the exception of Mammoth
Hot Springs, most of the material being deposited is _sinter_ (the kind
found around geysers is popularly called _geyserite_). Its chief
constituent is silica (the same as in quartz and in ordinary window
glass). At Mammoth, the deposit is _travertine_ (fig. 48), which
consists almost entirely of calcium carbonate. The material deposited at
any given place commonly reflects the predominant kind of rock through
which the hot water has passed during its underground travels. At
Mammoth Hot Springs the water passes through thick beds of limestone
(which is calcium carbonate), but in other areas the main rock type
through which the water percolates is rhyolite, a rock rich in silica.
Through centuries of intense activity, layers of sinter have built up on
the floors of the geyser basins (fig. 44); these deposits are generally
less than 10 feet thick. In one drill hole at Mammoth, deposits of
travertine extend to a depth of 250 feet. Dead trees and other kinds of
vegetation whose life processes have been choked off by the heat, water,
and precipitated minerals of hot-spring activity are a common sight in
many places (fig. 51).
Both travertine and sinter are white to gray. Around active hot springs,
however, the terraces that are constantly under water may be brightly
colored (figs. 43 and 49) because they are coated by microscopic plants
called _algae_. These organisms, which thrive in hot water at
temperatures up to about 170°F, are green, yellow, and brown. Oxides of
iron and manganese also contribute to the coloring in some parts of the
thermal areas. The delicate blue color of many pools, however, results
from the reflection of light off the pool walls and back through the
deep clear water (fig. 43). Other pools are yellow because they contain
sulfur, or are green from the combined influence of yellow sulfur and
“blue” water.
[Illustration: ALGAL-COLORED TERRACES lining the west bank of the
Firehole River at Midway Geyser Basin. Algae are microscopic plants
that grow profusely on rocks covered by hot water at temperatures up
to about 170°F. (Fig. 49)]