30
2 WEATHERING AND THE SEDIMENTARY CYCLE
water freezes, the force of ice crystallization is sufficient to fracture the rock. The two
halves of a fracture do not actually separate until the ice thaws and ceases to bind the
rock together. Freeze-thaw weathering is most active, therefore, in polar climates and
is most effective during the spring thaw. Insolation weathering occurs by contrast in areas with large diurnal temperature ranges. This is typical of hot arid climates. In the Sahara, for example, the diurnal temperature range in winter may be 25~ Rocks expand
and contract in response to temperature. The diverse minerals of rocks change size at
different rates according to their variable physical properties. This differential expansion and contraction sets up stresses within rock. When this process occurs very quickly
the stresses are sufficient to cause the rock to fracture. This is why insolation weathering is most effective in arid desert climates. In the author's personal experience this process was most dramatically experienced when trying to sleep on the slopes of the volcano
Waw en Namus in the Libyan Sahara. Here black basalt sands are cemented by evaporite minerals. Sleep was impossible for several hours after sunset as the rock snapped,
crackled, and popped.
In climatic zones that experience alternate wet and dry seasons, a third process of
physical weathering occurs. Clays and lightly indurated shales alternatively expand with
water and develop shrinkage cracks as they dehydrate. This breaks down the physical
strength of the formation; the shrinkage cracks increase permeability, thus aiding chemical weathering from rainwater, while waterlogged clays may lead to landslides.
The fourth main physical process of weathering is caused by stress release. Rocks
have elastic properties and are compressed at depth by the overburden above them. As
rock is gradually weathered and eroded the overburden pressure decreases. Rock thus
expands and sometimes fractures in so doing. Such fracturing is frequently aided by lateral downslope creep. Once stress-release fractures are opened they are susceptible to
enlargement by solution from rainwater and other processes.
Stress release, insolation, hydration-dehydration, and freeze-thaw are the four main
physical processes of weathering. Stress release is ubiquitous in brittle rocks, insolation
is characteristic of hot deserts, hydration-dehydration is typical in savannah and temperate climates, and freeze-thaw of polar climates.
2.3.3 Chemical Weathering
The processes of chemical weathering rely almost entirely on the agency of water. Few
common rock-forming minerals react with pure water, evaporites excepted. Groundwater, however, is commonly acidic. This is due to the presence of dissolved carbon dioxide from the atmosphere forming dilute carbonic acid. The pH is also lowered by the
presence of humic acids produced by biological processes in soil. The main chemical reactions involved in weathering are oxidation and hydrolysis. Carbonic acid dissolved in
groundwater releases hydrogen ions thus:
H20 + CO 2 -- H2CO 3 = HCO3- + H +.
The released hydrogen may then liberate alkali and alkali-earth elements from complex minerals, such as potassium feldspar:
2KA1Si308 + 2H + + 9H20 = A12Si2Os(OH)4 + 2K + + 4H2SIO4.
2 WEATHERING AND THE SEDIMENTARY CYCLE
water freezes, the force of ice crystallization is sufficient to fracture the rock. The two
halves of a fracture do not actually separate until the ice thaws and ceases to bind the
rock together. Freeze-thaw weathering is most active, therefore, in polar climates and
is most effective during the spring thaw. Insolation weathering occurs by contrast in areas with large diurnal temperature ranges. This is typical of hot arid climates. In the Sahara, for example, the diurnal temperature range in winter may be 25~ Rocks expand
and contract in response to temperature. The diverse minerals of rocks change size at
different rates according to their variable physical properties. This differential expansion and contraction sets up stresses within rock. When this process occurs very quickly
the stresses are sufficient to cause the rock to fracture. This is why insolation weathering is most effective in arid desert climates. In the author's personal experience this process was most dramatically experienced when trying to sleep on the slopes of the volcano
Waw en Namus in the Libyan Sahara. Here black basalt sands are cemented by evaporite minerals. Sleep was impossible for several hours after sunset as the rock snapped,
crackled, and popped.
In climatic zones that experience alternate wet and dry seasons, a third process of
physical weathering occurs. Clays and lightly indurated shales alternatively expand with
water and develop shrinkage cracks as they dehydrate. This breaks down the physical
strength of the formation; the shrinkage cracks increase permeability, thus aiding chemical weathering from rainwater, while waterlogged clays may lead to landslides.
The fourth main physical process of weathering is caused by stress release. Rocks
have elastic properties and are compressed at depth by the overburden above them. As
rock is gradually weathered and eroded the overburden pressure decreases. Rock thus
expands and sometimes fractures in so doing. Such fracturing is frequently aided by lateral downslope creep. Once stress-release fractures are opened they are susceptible to
enlargement by solution from rainwater and other processes.
Stress release, insolation, hydration-dehydration, and freeze-thaw are the four main
physical processes of weathering. Stress release is ubiquitous in brittle rocks, insolation
is characteristic of hot deserts, hydration-dehydration is typical in savannah and temperate climates, and freeze-thaw of polar climates.
2.3.3 Chemical Weathering
The processes of chemical weathering rely almost entirely on the agency of water. Few
common rock-forming minerals react with pure water, evaporites excepted. Groundwater, however, is commonly acidic. This is due to the presence of dissolved carbon dioxide from the atmosphere forming dilute carbonic acid. The pH is also lowered by the
presence of humic acids produced by biological processes in soil. The main chemical reactions involved in weathering are oxidation and hydrolysis. Carbonic acid dissolved in
groundwater releases hydrogen ions thus:
H20 + CO 2 -- H2CO 3 = HCO3- + H +.
The released hydrogen may then liberate alkali and alkali-earth elements from complex minerals, such as potassium feldspar:
2KA1Si308 + 2H + + 9H20 = A12Si2Os(OH)4 + 2K + + 4H2SIO4.