Geology Reference
In-Depth Information
that weaken the rock before abrading it with teeth and
other hard parts. Grazing animals include gastropods,
chitons, and echinoids (p. 57).
may carry previously eroded beach material or fluvial
sediments from the offshore zone to the littoral zone.
Very severe storm waves, storm surges, and tsunamis may
carry sediments from beyond the offshore zone. During
the Holocene, sediment deposited on exposed continen-
tal shelves and then submerged by rising sea levels has
been carried landwards. In some places, this supply of
sediment appears to have dried up and some Holocene
depositional landforms eroding.
Tides and wave action tend to move sediments
towards and away from shorelines. However, owing to
the effects of longshore currents, the primary sediment
movement is along the coast, parallel to the shore-
line. This movement, called
longshore drift
, depends
upon the wave energy and the angle that the waves
approach the coast. Longshore drift is maximal when
waves strike the coast at around 30 degrees. It occurs
below the breaker zone where waves are steep, or by
beach drift where waves are shallow.
Beach drift
occurs
as waves approaching a beach obliquely run up the
shore in the direction of wave propagation, but their
backwash moves down the steepest slope, normally per-
pendicular to the shoreline, under the influence of grav-
ity (Figure 3.18). Consequently, particles being moved
by swash and backwash follow a parabolic path that
slowly moves them along the shore. Wherever beach
Wave erosion
The pounding of the coast by waves is an enormously
powerful process of erosion. The effects of waves vary
with the resistance of the rocks being attacked and with
the wave energy. Where cliffs plunge straight into deep
water, waves do not break before they strike and cause
little erosion. Where waves break on a coastline, water is
displaced up the shore, and erosion and transport occur.
Plunging breakers produce the greatest pressures on
rocks - up to 600 kPa or more - because air may become
trapped and compressed between the leading wave front
and the shore. Air compression and the sudden impact
of a large mass of water dislodge fractured rock and other
loose particles, a process called
quarrying
. Well-jointed
rocks and unconsolidated or loosely consolidated rocks
are the most susceptible to wave erosion. Breaking waves
also pick up debris and throw it against the shore, causing
abrasion of shoreline materials. Some seashore organisms
erode rocks by boring into them - some molluscs, boring
sponges, and sea urchins do this (p. 57).
Aggradational processes
Sediment transport and deposition
Coastal sediments come from land inland of the shore
or littoral zone, the offshore zone and beyond, and
the coastal landforms themselves. In high-energy envi-
ronments, cliff erosion may provide copious sediment,
but in low-energy environments, which are common
in the tropics, such erosion is minimal. For this rea-
son, few tropical coasts form in bedrock and tropical
cliffs recede slowly, although fossil beaches and dunes are
eroded by waves. Sediment from the land arrives through
mass movement, especially where cliffs are undercut.
Gelifluction is common in periglacial environments.
Nevertheless, the chief sediment source is fluvial erosion.
Globally, rivers contribute a hundred times more sedi-
ment to coasts than marine erosion, with a proportionally
greater contribution in the tropics and lower contribu-
tion at higher latitudes. Onshore transport of sediments
Beach drift
Land
Sand particles
Waves
Longshore current
Figure 3.18
Beach drift.
Source:
After Butzer (1976, 226)
