Geology Reference
In-Depth Information
which may be thousands of kilometres from the point
of entrainment. Indeed, dust particles can be carried
around the world (in less than 80 days!) (p. 37).
Dust
is
a somewhat loose term but can be taken as a suspension
of solid particles in the air (or a deposit of such parti-
cles, familiar to anyone who has done housework). Most
atmospheric dust is smaller than 100 micrometres and
a large portion is smaller than 20 micrometres.
the surface and come to rest. Coarse grains are often
associated with erosional surfaces, as the fine grains are
winnowed by the wind. Fine grains tend to occur on
depositional surfaces. Coarse particles may also move to
the ground surface from below.
COASTAL PROCESSES
Coastal landforms are fashioned by weathering, by sedi-
ment erosion and transport associated with wave action
and tides, and by sediment deposition. For expediency,
it is helpful to distinguish degradational processes from
aggradational processes.
Wind deposition
Wind moves much sediment around the globe, although
by no means so much as the sediment moved by rivers.
Some of this sediment, representing 10 per cent of that
carried by rivers, is delivered to the oceans. The rest
falls on land. In Israel, the average fall is 0.25 kg/m
2
/yr
but falls of as much as 8.3 kg/m
2
/yr are recorded after
storms.
Wind deposition may take place in three ways
(Bagnold 1941): (1) sedimentation, (2) accretion, and
(3) encroachment.
Sedimentation
occurs when grains
fall out of the air or stop creeping forward. For sand
grains, this happens if the air is moving with insufficient
force to carry the grains forward by saltation or to move
other grains by creep. For silt and clay, this happens if par-
ticles are brought to the ground by air currents or if the
air is still enough for them to settle out (
dry deposition
),
or if they are brought down by rain (
wet deposition
).
Wet deposition appears to be significant where dust
plumes pass over humid regions and out over the oceans.
It is the main processes bringing down Saharan dust in
the Mediterranean region (Löye-Pilot and Martin 1996).
Wet deposition may give rise to blood rains and red
rains. Measured deposition rates on land range from
3.5 t/km
2
/yr to 200 t/km
2
/yr (Goudie 1995; Middleton
1997).
Accretion
occurs when grains being moved by
saltation hit the surface with such force that some grains
carry on moving forward as surface creep, but the major-
ity come to rest where they strike. Accretion deposits are
thus moulded by the combined action of saltation and
surface creep.
Encroachment
takes place when deposi-
tion occurs on a rough surface. Under these conditions,
grains moving as surface creep are held up, while saltat-
ing grains may move on. Deposition by encroachment
occurs on the front of a dune when grains roll down
Degradational processes
Shoreline weathering
The same weathering processes act upon shore environ-
ments as upon land environments, but the action of
seawater and the repeated wetting and drying of rocks
and sediments resulting from tides are extra factors with
big effects. Direct chemical attack by seawater takes place
on limestone coasts: solution of carbonate rocks occurs,
but as seawater is normally supersaturated with respect
to calcium carbonate it presumably takes place in rock
pools, where the acidity of the seawater may change. Salt
weathering is an important process in shoreline weath-
ering, being most effective where the coastal rocks are
able to absorb seawater and spray. As tides rise and fall,
so the zone between the low-water mark and the highest
limit reached by waves and spray at high tide is wet-
ted and dried. Water-layer weathering is associated with
these wetting and drying cycles. Biological erosion, or
bioerosion
, is the direct action of organisms on rock.
It is probably more important in tropical regions, where
wave energy is weak and coastal substrates are home to
a multitude of marine organisms. Tactics employed by
organisms in the erosive process are chemical, mechan-
ical, or a mixture of the two. Algae, fungi, lichens, and
animals without hard parts are limited to chemical attack
through secretions. Algae, and especially cyanobacteria,
are probably the most important bioerosional instru-
ments on rock coasts. Many other animals secrete fluids
