Laboratory Manual for Introductory Geology 4e

LEARNING
OBJECTIVES
■■
Understand how groundwater
infiltrates and flows through
Earth materials
■■
Explain why groundwater
erodes and deposits materials
differently from streams and
glaciers
■■
Learn to recognize landscapes
formed by groundwater and
to interpret groundwater flow
direction from topographic
features
■■
Understand how geologists
carry out groundwater
resource and pollution studies
MATERIALS NEEDED
■■
Specimens of four different
materials, containers, and
graduated cylinders for the
infiltration exercise
■■
Colored pencils
14.1 Introduction
Some rain and snow that falls on the land surface runs off into streams, some evaporates
into the air, and some is absorbed by plants. The remainder sinks into the
ground and is called groundwater. Water moves much more slowly underground
than in streams because it must seep from one pore space to another, rather than
flowing in a channel. Groundwater therefore has much less kinetic energy than
stream water and cannot carry the particles with which streams scrape and wear
away at bedrock. Instead, groundwater erodes chemically, by dissolving soluble rocks
such as limestone, dolostone, and marble, which are made largely of the carbonate
minerals calcite and dolomite.
If groundwater erodes underground, how can it form landscapes at the surface?
Unlike all other agents of erosion, groundwater forms landscapes from below.
As groundwater dissolves rocks underground, it undermines their support of the
surface above, and the land may collapse to produce very distinctive landscapes,
often pockmarked with cavities called sinkholes (FIG. 14.1a). Landscapes in areas of
extreme groundwater erosion are among the most striking in the world, with narrow,
steep-sided towers unlike anything produced by streams or glaciers (FIG. 14.1b).
Groundwater is also unique among erosional agents in that it doesn’t produce any
constructional landforms (i.e., those produced by deposition).
Groundwater is also a vital resource, used throughout the world for drinking,
washing, and irrigating crops. Strict rules govern its use in many places because
groundwater flows so slowly that renewal of the groundwater supply takes a long
time. In addition, its slow flow and underground location make groundwater difficult
to purify once it has been polluted. We examine both roles of groundwater—as
a landscape former and as a resource—in this chapter.
Before exploring either the landscapes created by groundwater or how we make
use of this vital resource, however, we first need to understand how water sinks into
and flows through the ground. Exercise 14.1 explores factors that control the movement
of water underground from the surface and from point to point below ground.
FIGURE 14.1 Landscapes carved by groundwater.
(a) Karst topography, characterized by sinkholes and underground
caves, in southern Indiana.
(b) Karst towers in the Guilin area of southern China.
14.2 Aquifers and Aquitards
Materials that transmit water well are called aquifers, from the Latin aqua (water)
and fero (to carry). To transmit water, a material must have room for water between
its grains. But this property—called porosity—is not necessarily enough to make a
362 CHAPTER 14 GROUNDWATER AS A LANDSCAPE FORMER AND RESOURCE