Geoscience Reference
In-Depth Information
Figure 14.3 Joints in rocks. Joints are fissures that develop
along horizontal or vertical planes within rock. These columnar
joints formed in basalts now exposed at Devil's Postpile National
Monument in California.
Mechanical Weathering
Mechanical weathering , also called physical weathering ,
involves the destruction of rocks through physical stresses.
When this kind of weathering occurs, rocks do not change
their chemical composition. Instead, they simply break into
progressively smaller pieces of the same kind of rock. As they
do so, the overall surface area of the rock progressively in-
creases, which facilitates even more weathering because more
area within the rock mass is available for additional weather-
ing to occur. This portion of the chapter describes the various
kinds of mechanical weathering.
Figure 14.4 The effects of frost wedging. Frost wedging
occurs when water enters cracks and joints within rocks and
subsequently expands and contracts upon freezing and thawing,
respectively. This process causes these fissures to widen over
time. Frost wedging can cause even large boulders to fracture in
two, such as this specimen in California's Sierra Nevada.
Frost Wedging A common type of mechanical weathering
is frost wedging . It begins when water works its way into
fractures in the rock, such as a joint (Figure 14.3). Joints can
form when rock repeatedly expands and contracts with heating
and cooling, respectively, causing fractures to develop along
horizontal and vertical planes that essentially break the rock
into large blocks. Joints can also form due to regional tectonic
forces that produce stress within the rock mass. Although joint
features may look like faults, they are not because no relative
vertical or horizontal movement is occurring on either side of
the fracture.
Once joints develop, they allow water to flow deeper into
the rock mass at different points. If the rock body lies in a
region that has distinct winter and summer seasons, the water
will tend to go through numerous freeze and thaw cycles dur-
ing the transitional months. When water freezes, it expands its
volume as much as 9%. You can see this in the way that water
in an ice cube tray freezes into cubes that are slightly larger
than the liquid water level. If water subsequently thaws back
to liquid form, it then contracts. (By the way, water is one of
the very few substances that act this way; most materials are
larger in volume as a liquid than as a solid.) Over the course of
time, the stress produced by this freeze-thaw cycle causes the
joints progressively to enlarge so that boulders slowly break
away from each other (FigureĀ  14.4). In places where frost
action is most active, such as high mountain peaks or arctic
Mechanical weathering The breakup of a body of rock into
smaller rocks of the same type.
Joint A crack or fissure along horizontal or vertical planes in
a rock mass that divides the rock into large blocks.
Frost wedging Expansion and contraction of water in rock
cracks due to freezing and thawing.
 
 
Search WWH ::




Custom Search