Geysers, fumaroles (also called _solfataras_), and hot springs are
generally found in regions of young volcanic activity. Surface water
percolates downward through the rocks below the Earth’s surface to
high-temperature regions surrounding a magma reservoir, either active or
recently solidified but still hot. There the water is heated, becomes
less dense, and rises back to the surface along fissures and cracks.
Sometimes these features are called “dying volcanoes” because they seem
to represent the last stage of volcanic activity as the magma, at depth,
cools and hardens.
Erupting geysers provide spectacular displays of underground energy
suddenly unleashed, but their mechanisms are not completely understood.
Large amounts of hot water are presumed to fill underground cavities.
The water, upon further heating, is violently ejected when a portion of
it suddenly flashes into steam. This cycle can be repeated with
remarkable regularity, as for example, at Old Faithful Geyser in
Yellowstone National Park, which erupts on an average of about once
every 65 minutes.
[Illustration: Old Faithful Geyser, Yellowstone National Park, Wyoming.]
Fumaroles, which emit mixtures of steam and other gases, are fed by
conduits that pass through the water table before reaching the surface
of the ground. Hydrogen sulfide (HࠢS), one of the typical gases issuing
from fumaroles, readily oxidizes to sulfuric acid and native sulfur.
This accounts for the intense chemical activity and brightly colored
rocks in many thermal areas.
Hot springs occur in many thermal areas where the surface of the Earth
intersects the water table. The temperature and rate of discharge of hot
springs depend on factors such as the rate at which water circulates
through the system of underground channelways, the amount of heat
supplied at depth, and the extent of dilution of the heated water by
cool ground water near the surface.
[Illustration: Black Growler steam vents (fumaroles), Norris Basin,
Yellowstone National Park, Wyoming.]
[Illustration: Mammoth Hot Springs, Yellowstone National Park, Wyoming.]