Geoscience Reference
In-Depth Information
3.
Tides are daily adjustments that occur with respect to the
water level along any given coastline. Tidal processes are
driven largely by the gravitational pull of the Moon and, to
a lesser extent, the Sun. Other important factors are the
centrifugal force of the combined Earth-Moon rotation
and the geometric relationship of the Sun and Moon.
4.
Waves are the dominant factor that shapes coastlines.
Waves are disturbances in bodies of water and form as a
result of frictional forces related to wind. They generate
rotating bodies of water that migrate obliquely toward
the shore. At the point where the wave base interacts
with the ocean bottom in shallow water, the oscillations
become deformed, ultimately causing waves to crash.
5.
Littoral processes refer to the way that sediment is
transported and deposited in the shore zone. An impor-
tant feature is a longshore current that develops along
a coastline due to the deflected force of oblique waves
striking the shore. In a related process, water runs up
the beach and back into the ocean or sea in processes
called swash and backwash, respectively.
Figure 19.13 Headlands along the southern coast of Tasma-
nia in Australia. Headlands are prominent coastal features that
project out into the water. Note that at least three headlands ap-
pear in this image.
rock such as limestone or granite. Where the rock is relatively
soft, perhaps shale or sandstone, waves can erode sediment
more effectively and a bay forms.
Once the pattern of headlands and intervening bays devel-
ops, the erosive power of waves begins to concentrate on the
headlands. This change occurs because approaching waves ini-
tially begin to slow down in front of the headlands, where they
first encounter shallow water. This friction causes the waves to
pivot around the headlands in a process called wave refraction .
As you can see in Figures 19.14a and b, waves bend around a
headland, which results in the energy of the waves being ex-
pended on three sides of the landform. In this way, headlands
are more vigorously eroded than the intervening bays. The bays
become zones of deposition, forming features called pocket
beaches , because sediment is funneled into them from the
eroded headlands (Figure 19.14c).
The basic coastal erosional landform is the wave-cut bluff,
which is a vertical face that commonly forms along rocky
shorelines. Excellent examples of large wave-cut bluffs occur
on many parts of the coast of Scotland (Figure 19.15), as well
asĀ  the coasts of England, Australia, and California. Wave-cut
bluffs evolve in a predictable sequence of events as shown in
Figure 19.16. This sequence begins with waves that are pound-
ing against a noneroded shoreline (Figure 19.16a). With time
(FigureĀ 19.16b), the waves cut a vertical face into the rock out-
crop, and at the same time they plane a more horizontal surface
Coastal Landforms
Let's now turn to the various landforms created along shore-
lines. Recall that fluvial, glacial, and eolian landforms can be
subdivided into erosional and depositional categories. Coastal
landforms can be viewed in the same way. This discussion be-
gins with erosional coastlines.
Erosional Coastlines
Most coastal erosion is accomplished through the unceasing
pounding of the shoreline by waves. Waves have tremendous
power after they break, with the associated spray moving as fast
as 113 km/h (70 mi/h). Given that water has a very high density,
the rapidly moving water has a lot of power and is responsible
for the majority of coastal erosional landforms.
Although wave erosion can occur along any part of the
coast, it tends to be focused on a particular part of the landscape
called a headland. A headland is a promontory that juts into
the ocean or sea and thus is surrounded on three sides by water
(Figure 19.13). Headlands tend to form along shorelines where
bands of rock with alternating resistance run perpendicular to
the coast. The headlands are associated with the more resistant
Headland A portion of the coast that extends outward into a
large body of water.
Wave refraction The process through which waves are
focused and bent around headlands.
 
 
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