Geoscience Reference
In-Depth Information
Hydrologic Cycle and Groundwater
Now that the concept of groundwater has been discussed, go
to the
Geo Media Library
and open
Hydrologic Cycle and
Groundwater
. This animation reviews the processes that have
been described so far in this chapter. It begins with the hydro-
logic cycle and demonstrates how water moves from one reser-
voir to another. It also animates the relationship among evapo-
ration, precipitation, and the way that groundwater is stored
and moves. Review how the water table fluctuates and how
groundwater depletion impacts the water table. A final exercise
tests your understanding of groundwater terminology. Once
you complete the animation, be sure to answer the questions at
the end to assess your knowledge of this topic.
maybe you explored an unknown cave in southern Indiana,
Tennessee, or Kentucky. If so, you explored how groundwater
can shape the subterranean landscape over a long period of
time. These effects are visible not just beneath the ground, but
also at the surface in the form of depressions and sometimes
even hills.
A region that contains a lot of caves and other soluble rock
features is said to be a landscape with
karst topography
. Karst
topography is most closely associated with extensive, thick de-
posits of limestone. As discussed in Chapter 14, these rocks dis-
solve easily through the weathering process of carbonation. This
erosion produces a complex system of joints that allows water to
flow from the surface into the underlying rock. Vegetation also
plays a role because it provides organic acids that enhance the
solution process. When these conditions occur in humid regions,
such as the eastern part of the United States and southeastern
China, karst landscapes can be widespread (Figure 15.10). They
also occur in some regions that are now arid but were humid
at some point in the geologic past, such as New Mexico and
northwestern Africa.
kEY CONCEPTS TO REMEMBER ABOuT
GROuNDWATER
1.
Groundwater refers to water that is stored within the
ground.
2.
Some water is stored in soil pore spaces to form the
soil-water belt. When the soil is dry, water is nevertheless
held tightly as hygroscopic water. With increasing pre-
cipitation, water fills more of the pores as capillary wa-
ter. If the soil and associated pores are saturated, they
reach field capacity and water is free to move down-
ward under the force of gravity.
3.
When water percolates downward, it first encounters
the unsaturated zone. At some point in its downward
flow, this water encounters the aquifer, which is a layer
of rock fully saturated with water.
4.
The top of the saturated zone is called the water table.
In places where the groundwater supply is sufficiently
large to be a source of water for people, this zone is
called an aquifer.
Caves and Caverns
Perhaps the best-known karst landforms are
caves
, which con-
sist of underground voids in rock that are sufficiently large for
people to enter and explore. Although most caves are small,
some cave systems are enormous and contain numerous pas-
sageways and large chambers. Such a large cave system is
called a
cavern
. To give you an idea of how large chambers can
be, consider that
The Big Room
at Carlsbad Caverns is about
110,000 m
2
(1,184,030 ft
2
) in area.
People have been aware of caves and caverns for a long
time. In Europe and Asia, for example, caves provided shelter
for prehistoric people during the Pleistocene Epoch when gar-
gantuan glaciers covered much of the landscape. Evidence for
these occupations can be seen in Europe in the number of beau-
tiful paintings (known as
cave art
) that adorn the cave walls.
5.
The High Plains Aquifer is the largest aquifer in North
America, underlying much of the Great Plains. This
aquifer is being extensively mined to irrigate agricultural
crops. Consequently, a significant decrease in water
table elevation is occurring in many places.
Karst Landforms
and Landscapes
Have you ever been to a cool cave system, such as Mammoth
Cave in Kentucky or Carlsbad Caverns in New Mexico? Or
Karst topography
Terrain that is underlain by soluble rocks,
such as limestone and dolomite, where the landscape evolves
largely through the dissolution of rock.
Cave
A cavity in rock, produced by the dissolution of calcium
carbonate, that is large enough for someone to enter.
