Page 74 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 57

(a clay mineral), and silicic acid, H 4 SiO 4 : graze them (e.g. Yatsu 1988, 285–397; Spencer 1988;
Trenhaile 1987, 64–82).This process is particularly effec-
2HAlSi 3 O 8 + 9H 2 O → Al 2 Si 2 O 5 (OH) 4 + 2H 4 SiO 4 tive in tropical limestones. Boring organisms include
bivalve molluscs and clinoid sponges. An example is the
The silicic acid is soluble in and removed by water blue mussel (Mytilus edulis). Grazing organisms include
leaving kaolinite as a residue, a process termed desili- echinoids, chitons, and gastropods, all of which dis-
cation as it involves the loss of silicon. If the solution place material from the rock surface. An example is the
equilibrium of the silicic acid changes, then silicon West Indian top shell (Cittarium pica), a herbivorous
dioxide (silica) may be precipitated out of the solution: gastropod.
Under some conditions, bacteria, algae, fungi, and
H 4 SiO 4 → 2H 2 O + SiO 2 lichens may chemically alter minerals in rocks. The
boring sponge (Cliona celata) secretes minute amounts
Weathering of rock by hydrolysis may be complete or of acid to bore into calcareous rocks. The rock miner-
partial (Pedro 1979). Complete hydrolysis or alliti- als may be removed, leading to biological rock erosion.
zation produces gibbsite. Partial hydrolysis produces In an arid area of southern Tunisia, weathering is con-
either 1 : 1 clays by a process called monosiallitiza- centrated in topographic lows (pits and pans) where
tion, or 2 : 1 and 2 : 2 clays through a process called moisture is concentrated and algae bore, pluck, and etch
bisiallitization (cf. pp. 159–60). the limestone substrate (Smith et al. 2000).
Humans have exposed bedrock in quarries, mines,
Chelation and road and rail cuts. They have disrupted soils by det-
onating explosive devices, and they have sealed the soil
This is the removal of metal ions, and in particular in urban areas under a layer of concrete and tarmac.
ions of aluminium, iron, and manganese, from solids Their agriculture practices have greatly modified soil
by binding with such organic acids as fulvic and humic and weathering processes in many regions.
acid to form soluble organic matter–metal complexes.
The chelating agents are in part the decomposition
products of plants and in part secretions from plant
roots. Chelation encourages chemical weathering and SEDIMENT TRANSPORT
the transfer of metals in the soil or rock.
A river in flood demonstrates sediment transport,
the dirty floodwaters bearing a burden of mate-
Biological weathering
rial derived from the land surface. As well as the
Some organisms attack rocks mechanically, or chemi- visible sediment, the river also carries a load of material
cally, or by a combination of mechanical and chemical in solution. Geomorphologists often distinguish between
processes. sediment transport, which is essentially mechanical, and
Plant roots, and especially tree roots, growing in bed- solutional transport, which is essentially chemical; they
ding planes and joints have a biomechanical effect –as also discriminate between processes involving a lot of
they grow, mounting pressure may lead to rock fracture. sediment moving en masse – mass movement – and sed-
Dead lichen leaves a dark stain on rock surfaces.The dark iment moving as individual grains more or less dispersed
spots absorb more thermal radiation than the surround- in a fluid – fluid transport (cf. Statham 1977, 1). In
ing lighter areas, so encouraging thermal weathering. mass movement, the weight of sediment is a key con-
A pale crust of excrement often found below birds’ nests trolling factor of motion, whereas in fluid transport the
on rock walls reflects solar radiation and reduces local action of an external fluid agency (wind or water) is
heating, so reducing the strength of rocks. In coastal the key factor. However, the distinction blurs in case
environments, marine organisms bore into rocks and of slow mass movements, which resemble flows, and in