Geology Reference
In-Depth Information
contact with the hot rocks. Because the water near the bot-
tom of the fracture system is under higher pressure than the
water near the top, it must be heated to a higher temperature
before it will boil. Thus, when the deeper water is heated to
near the boiling point, a slight rise in temperature or a drop
in pressure, such as from escaping gas, will instantly change
it to steam. The expanding steam quickly pushes the water
above it out of the ground and into the air, producing a gey-
ser eruption. After the eruption, relatively cool groundwater
starts to seep back into the fracture system where it heats to
near its boiling temperature and the eruption cycle begins
again. Such a process explains how geysers can erupt with
some regularity.
Hot spring and geyser water typically contains large
quantities of dissolved minerals because most minerals dis-
solve more rapidly in warm water than in cold water. Because
of this high mineral content, some believe that the waters of
many hot springs have medicinal properties. Numerous spas
and bathhouses have been built at hot springs throughout the
world to take advantage of these supposed healing properties.
When the highly mineralized water of hot springs or gey-
sers cools at the surface, some of the material in solution is pre-
cipitated, forming various types of deposits. The amount and
type of precipitated minerals depend on the solubility and com-
position of the material that the groundwater fl ows through.
If the groundwater contains dissolved calcium carbonate
(CaCO
3
), then
travertine
or
calcareous tufa
(both of which are
varieties of limestone) are precipitated. Spectacular examples
of hot spring travertine deposits are found at Pamukhale in
Turkey and at Mammoth Hot Springs in Yellowstone National
Park (
Figure 13.23a). Groundwater containing dissolved
silica will, upon reaching the surface, precipitate a soft, white,
hydrated mineral called
siliceous sinter
or
geyserite
, which can
accumulate around a geyser's opening (Figure 13.23b).
◗
◗
Figure 13.24
The Geysers, Sonoma County, California Steam
rising from one of the geothermal power plants at The Geysers
in Sonoma County, California. Steam from wells drilled into this
geothermal region, about 120 km north of San Francisco, is piped
directly to electricity-generating turbines to produce electricity that is
distributed throughout the area.
Geothermal energy
is any energy produced from Earth's
internal heat. In fact, the term
geothermal
comes from
geo
,
“Earth,” and
thermal
, “heat.” Several forms of internal heat
are known, such as hot dry rocks and magma, but so far,
only hot water and steam are used.
Approximately 1-2% of the world's current energy
needs could be met by geothermal energy. In those areas
where it is plentiful, geothermal energy can supply most, if
not all, of the energy needs, sometimes at a fraction of the
cost of other types of energy. Some of the countries currently
using geothermal energy in one form or another are Iceland,
the United States, Mexico, Italy, New Zealand, Japan, the
Philippines, and Indonesia.
In the United States, the first commercial geothermal
electricity-generating plant was built in 1960 at The Gey-
sers, about 120 km north of San Francisco, California. Here,
wells were drilled into the numerous near-vertical fractures
underlying the region. As pressure on the rising ground-
water decreases, the water changes to steam, which is piped
directly to electricity-generating turbines and generators
(
Figure 13.24).
As oil reserves decline, geothermal energy is becoming an
attractive alternative, particularly in parts of the western United
States, such as the Salton Sea area of southern California, where
geothermal exploration and development have begun.
◗
Search WWH ::
Custom Search