Geology Reference
In-Depth Information
freezing. The reason frost action is so effective is that water
expands by about 9% when it freezes, thus exerting great
force on the walls of a crack, widening and extending it by
frost wedging
(
◗
Figure 6.3a). Repeated freezing and thawing
dislodge angular pieces of rock from the parent material that
tumble downslope and accumulate as
talus
(Figure 6.3b).
Frost action is most effective in high mountains, even dur-
ing the summer months, but it has little or no effect where
the temperature rarely drops below freezing, or where earth
materials are permanently frozen.
Some rocks form at depth and are stable under tre-
mendous pressure. Granite crystallizes far below the surface,
so when it is uplifted and eroded, its contained energy is
released by outward expansion, a phenomenon known as
pressure release
. The outward expansion results in the ori-
gin of fractures called
sheet joints
that more or less parallel
the exposed rock surface. Sheet-joint-bounded slabs of rock
slip or slide off the parent rock, leaving large, rounded masses
known as
exfoliation domes
(
◗
Figure 6.2
Differential Weathering The spectacular scenery
at Bryce Canyon National Park in Utah resulted from differential
weathering and erosion of the 40- to 50-million-year-old Wasatch
Formation that was deposited in ancient lakes. Weathering and
erosion that took place along closely spaced fractures yielded this
panorama of spires, pillars, gullies, and ravines.
Figure 6.4a).
That solid rock expands and produces fractures might
be counterintuitive, but is nevertheless a well-known
phenomenon. In deep mines, masses of rock detach from the
sides of the excavation, often explosively. These
rock bursts
and less violent
popping
pose a danger to mine workers, and
in South Africa they are responsible for approximately
20 deaths per year. In some quarries for building stone,
excavations to only 7 or 8 m exposed rocks in which sheet
joints formed (Figure 6.4b), in some cases with enough
force to throw quarrying machines weighing more than a
ton from their tracks.
During
thermal expansion and contraction,
the vol-
ume of rocks changes as they heat up and then cool down.
The temperature may vary as much as 30°C a day in a des-
ert, and rock, being a poor conductor of heat, heats and
expands on its outside more than its inside. Even dark
minerals absorb heat faster than light-colored ones, so dif-
ferential expansion takes place between minerals. Surface
expansion might generate enough stress to cause fractur-
ing, but experiments in which rocks are heated and cooled
repeatedly to simulate years of such activity indicate that
thermal expansion and contraction are of minor impor-
tance in mechanical weathering.
The formation of salt crystals can exert enough force
to widen cracks and dislodge particles in porous, granu-
lar rocks such as sandstone. And even in rocks with an
interlocking mosaic of crystals, such as granite,
salt crys-
tal growth
pries loose individual minerals. It takes place
mostly in hot, arid regions, but also probably affects rocks
in some coastal areas.
Animals, plants, lichens, and bacteria all participate in the
mechanical and chemical alteration of rocks (
◗
enriches other resources by removing soluble materials. In
fact, some sediments and sedimentary rocks are resources
in their own right, or they are the host rocks for petroleum
and natural gas.
ALTERED?
Weathering is a surface or near-surface process, but the rocks it
acts on are not structurally and compositionally homogeneous
throughout, which accounts for
differential weathering
. That
is, weathering takes place at different rates even in the same
area, so it commonly results in uneven surfaces. Differential
weathering and
differential erosion
—that is, variable rates of
erosion—combine to yield some unusual and even bizarre
features, such as hoodoos, spires, and arches (Figure 6.2) (see
Geo-inSight on pages 136 and 137).
The two recognized types of weathering,
mechanical
and
chemical
, both proceed simultaneously on parent mate-
rial, as well as on materials in transport and those deposited
as sediment. In short, all surface or near-surface materials
weather, although one type of weathering may predominate
depending on such variables as climate and rock type.
Mechanical weathering
takes place when physical forces
break earth materials into smaller pieces that retain the
composition of the parent material. Granite, for instance,
might be mechanically weathered and yield smaller pieces
of granite or individual grains of quartz, potassium feld-
spars, plagioclase feldspars, and biotite (Figure 6.1). Sev-
eral physical processes are responsible for mechanical
weathering.
Frost action
involving water repeatedly freezing and
thawing in cracks and pores in rocks is particularly effective
where temperatures commonly fluctuate above and below
◗
Figure 6.5a).
Burrowing animals, such as worms, reptiles, rodents, termites,
and ants, constantly mix soil and sediment particles and bring
material from depth to the surface where further weathering
occurs. The roots of plants, especially large bushes and trees,
wedge themselves into cracks in rocks and further widen them
(Figure 6.5b).
Search WWH ::
Custom Search