Springs:
Not until the middle of the seventeenth century did the French physicist Pierre Perrault invalidate the age-old assumption that precipitation could not adequately account for the amount of water emanating from springs and flowing in rivers. Over several years, Perrault computed the quantity of water that fell on France’s Seine River basin. He then calculated the mean annual runoff by measuring the river’s discharge. After allowing for the loss of water by evaporation, he showed that there was sufficient water remaining to feed the springs. Thanks to Perrault’s pioneering efforts and the measurements by many after ward, we now know that the source of springs is water from the zone of saturation and that the ultimate source of this water is precipitation.
Whenever the water table intersects Earth’s surface, a natural outflow of groundwater results, which we call a spring (Figure 17.18).
Springs such as the one pictured in Figure 17.18 form when an aquitard blocks the downward movement of groundwater and forces it to move laterally. Where the permeable bed crops out, a spring results.which shows a perched water table intersecting a slope.
Not until the middle of the seventeenth century did the French physicist Pierre Perrault invalidate the age-old assumption that precipitation could not adequately account for the amount of water emanating from springs and flowing in rivers. Over several years, Perrault computed the quantity of water that fell on France’s Seine River basin. He then calculated the mean annual runoff by measuring the river’s discharge. After allowing for the loss of water by evaporation, he showed that there was sufficient water remaining to feed the springs. Thanks to Perrault’s pioneering efforts and the measurements by many after ward, we now know that the source of springs is water from the zone of saturation and that the ultimate source of this water is precipitation.
Whenever the water table intersects Earth’s surface, a natural outflow of groundwater results, which we call a spring (Figure 17.18).
Springs, however, are not confined to places where a perched water table creates a flow at the surface. Many geologic situations lead to the formation of springs because subsurface conditions vary greatly from place to place. Even in areas underlain by impermeable crystalline rocks, permeable zones may exist in the form of fractures or solution channels. If these openings fill with water and intersect the ground surface along a slope, a spring results.
Hot Springs:
There is no universally accepted definition of hot spring. One frequently used definition is that the water in a hot spring is 6° to 9° C (10° to 15° F) warmer than the mean annual air temperature for the locality where it occurs (fig 17.9)
In the United States alone, there are more than 1000 such springs.
Temperatures in deep mines and oil wells usually rise with increasing depth, an average of about 25° C per kilometer, a figure known as the geothermal gradient. Therefore, when groundwater circulates at great depths, it becomes heated. If the hot water rises rapidly to the surface, it may emerge as a hot spring. The water of some hot springs in the eastern United States is heated in this manner. The springs at Warm Springs, Georgia, the presidential retreat of Franklin Roosevelt (Figure 17.19B), are one example. The temperature of these hot springs is always near 32°C (90°F). Another example is Hot Springs National Park, Arkansas, where water temperatures average about 60° C (140° F)
The great majority (more than 95 percent) of the hot springs (and geysers) in the United States are found in the West. A glance at (fig 17.20)
reinforces this fact.The reason for such a distribution is that the sources of heat for most hot springs are magma bodies and hot igneous rocks, and it is in the West that igneous activity has occurred more recently. The hot springs and geysers of the Yellowstone region are well known examples.
Geysers:
Geysers are intermittent hot springs or fountains in which columns of water are ejected with great force at various intervals, often rising 30 to 60 meters (100 to 200 feet) into the air. After the jet of water ceases, a column of steam rushes out, usually with a thunderous roar. Perhaps the most famous geyser in the world is Old Faithful in Yellowstone National Park (fig 17.21)
. The great abundance, diversity, and spectacular nature of Yellowstone’s geysers and other thermal features undoubtedly was the primary reason for its becoming the first national park in the United States. Geysers are also found in other parts of the world, notably New Zealand and Iceland. In fact, the Icelandic word geysa, meaning “to gush,” gives us the name geyser.
How Geysers Work:
Geysers occur where extensive underground chambers exist within hot igneous rocks. How they operate is shown in FIG 17.22
. As relatively cool groundwater enters the chambers, it is heated by the surrounding rock. At the bottom of the chambers, the water is under great pressure because of the weight of the overlying water. This great pressure prevents the water from boiling at the normal surface temperature of 100° C (212° F). For example, water at the bottom of a 300-meter (1000-foot) water-filled chamber must attain nearly 230° C (450° F) before it will boil. The heating causes the water to expand, with the result that some is forced out at the surface. This loss of water reduces the pressure on the remaining water in the chamber, which lowers the boiling point. A portion of the water deep within the chamber quickly turns to steam, and the geyser erupts (Figure 17.22). Following eruption, cool ground water again seeps into the chamber, and the cycle begins a new.
Geyser Deposits:
When groundwater from hot springs and geysers flows out at the surface, material in solution is often precipitated, producing an accumulation of chemical sedimentary rock. The material deposited at any given place commonly reflects the chemical makeup of the rock through which the water circulated. When the water contains dissolved silica, a material called siliceous sinter, or geyserite, is deposited around the spring. When the water contains dissolved calcium carbonate, a form of limestone called travertine, or calcareous tufa, is deposited. The latter term is used if the material is spongy and porous. The deposits at Mammoth Hot Springs in Yellowstone National Park are more spectacular than most others (FIG 17.23)
. As the hot water flows upward through a series of channels and then out at the surface, the reduced pressure allows carbon dioxide to separate and escape from the water. The loss of carbon dioxide causes the water to become supersaturated with calcium carbonate, which then precipitates. In addition to containing dissolved silica and calcium carbonate, some hot springs contain sulfur, which gives water a poor taste and unpleasant odor. Undoubtedly, Rotten Egg Spring, Nevada, is such a situation.
No comments:
Write comments