KARST TOPOGRAPHY

well-documented sinkhole formed in May 1981 in Winter Park, Florida. During the initial collapse, a three-bed-

Karst topography forms in broad regions underlain by room house, half a swimming pool, and six Porsches in

limestone and other readily soluble rocks. Caverns and

274 CHAPTER 15 G RO U N D WAT E R

Sinkholes Disappearing stream Sinkhole has

enlarged and collapsed

Underground stream in cavern

Figure 15–19 Sinkholes and caverns form in limestone. Streams commonly disappear into sinkholes and flow through the caverns to emerge elsewhere.

sinkholes are common features of karst topography. Kentucky, streams are given names such as Sinking Surface streams often pour into sinkholes and disappear

Creek, an indication of their fate. The word karst is de- into caverns. In the area around Mammoth Caves in

rived from a region in Croatia where this type of topog-

Figure 15–20 This sinkhole in Winter Park, Florida, collapsed suddenly in May 1981, swallowing several houses and a Porsche agency. (AP/Wide World Photos)

Hot Springs and Geysers 275

raphy is well developed. Karst landscapes are found in scend so deep into the Earth, and this type of hot many parts of the world.

spring is uncommon.

2. In regions of recent volcanism, magma or hot igneous rock may remain near the surface and can

䊳 15.7 HOT SPRINGS AND GEYSERS

heat ground water at relatively shallow depths. Hot springs heated in this way are common throughout

At numerous locations throughout the world, hot water western North and South America because these re- naturally flows to the surface to produce hot springs.

gions have been magmatically active in the recent Ground water can be heated in three different ways:

past and remain so today. Shallow magma heats the

1. The Earth’s temperature increases by about 30ºC per hot springs and geysers of Yellowstone National kilometer in the upper portion of the crust. Therefore,

Park.

if ground water descends through cracks to depths

3. Many hot springs have the odor of rotten eggs from of 2 to 3 kilometers, it is heated by 60º to 90º. The

small amounts of hydrogen sulfide (H 2 S) dissolved hot water then rises because it is less dense than

in the hot water. The water in these springs is cold water. However, it is unusual for fissures to de-

heated by chemical reactions. Sulfide minerals, such

Steam

Empty chamber

Vent constricted at narrow neck

Hot pluton Hot pluton (a)

(b)

Figure 15–21 (a) Before a geyser erupts, ground water seeps into underground cham- bers and is heated by hot igneous rock. Foam constricts the geyser’s neck, trapping steam and raising pressure. (b) When the pressure exceeds the strength of the blockage, the constriction blows out. Then the hot ground water flashes into vapor and the geyser erupts.

276 CHAPTER 15 G RO U N D WAT E R

as pyrite (FeS 2 ), react chemically with water to pro- ing the hot water to vaporize, blowing steam and hot wa- duce hydrogen sulfide and heat. The hydrogen sul-

ter skyward (Fig. 15–22).

fide rises with the heated ground water and gives it The most famous geyser in North America is Old the strong odor.

Faithful in Yellowstone Park, which erupts on the average of once every 65 minutes. Old Faithful is not as

Most hot springs bubble gently to the surface from regular as people like to believe; the intervals between cracks in bedrock. However, geysers violently erupt hot eruptions vary from about 30 to 95 minutes. water and steam. Geysers generally form over open

cracks and channels in hot underground rock. Before a geyser erupts, ground water seeps into the cracks and is

15.8 GEOTHERMAL ENERGY

heated by the rock (Fig. 15–21). Gradually, steam bub-

bles form and start to rise, just as they do in a heated teakettle. If part of the channel is constricted, the bub-

Hot ground water can be used to drive turbines and gen- bles accumulate and form a temporary barrier that allows

erate electricity, or it can be used directly to heat homes the steam pressure below to increase. The rising pressure

and other buildings. Energy extracted from the Earth’s forces some of the bubbles upward past the constriction

heat is called geothermal energy. In January 1995, 70 and short bursts of steam and water spurt from the geyser.

geothermal plants in California, Hawaii, Utah, and Nevada This lowers the steam pressure at the constriction, caus-

had a generating capacity of 2500 megawatts, enough to supply over one million people with electricity and equiv- alent to the power output of 2 1/2 large nuclear reactors. However, this amount of energy is minuscule compared with the potential of geothermal energy.

The major problem with current methods of extracting energy from the Earth is that they work only where deep ground water is heated naturally. Unfortunately, only a limited number of “wet” sites exist where abun- dant ground water and hot rock are found together at shallow depths. However, many “dry” sites occur where rising magma has heated rocks close to the surface but little ground water is available. Technology is being developed to harness energy from dry sites. For example, imported water can be circulated through wells drilled in dry, hot rock and then extracted and reused. Easily ac- cessible dry geothermal sites in the United States have enough energy to supply all U.S. consumption for nearly 8000 years.

Scientists and engineers are developing methods for extracting energy from dry Earth heat at a pilot project at Fenton Hill, New Mexico. They drilled two separate wells side by side and pumped water down the injection well to a depth of about 4 kilometers (Fig. 15–23). The pump forces the water into hot, fractured granite at the bottom of the well and then into the extraction well, where it returns to the surface. The scientists have suc- ceeded in pumping water into the well at 20ºC and extracting it 12 hours later at 190ºC.

With 1996 technology, construction of a dry geothermal plant is about 3.6 times as expensive as the cost of a new gas-fired generating plant. A large part of the high cost results from the expense of drilling large- diameter holes 2 or more kilometers into hard rock. As

Figure 15–22 Shallow magma heats ground water, causing long as coal and petroleum prices are low, there is little numerous geyser eruptions in Yellowstone National Park.

incentive to develop geothermal projects to compete with

(Corel Photos)

coal- and oil-fired electric generating plants.

Summary 277

Power plant

Injection well

Production

Figure 15–23

A schematic view of the Fenton Hill, New

well

Mexico, dry geothermal energy plant.

KARST TOPOGRAPHY