22
2 WEATHERING AND THE SEDIMENTARY CYCLE
bouncing lethargically over many millions of years on a conveyor belt. The sand grains
history is one major cyclic event composed of many subsidiary ones. We now examine
the sedimentary cycle in more detail.
2.2 THE SEDIMENTARY CYCLE
This section looks more closely at the sedimentary cycle in the smaller scale, leaving
aside the major cycle of plate formation and destruction. Classically the sedimentary
cycle consists of the phases of weathering, erosion, transportation, deposition, lithifaction, uplift, and weathering again (Fig. 2.1).
Weathering is the name given to the processes that break down rock at the earth's
surface to form discrete particles (Ollier, 1969). Erosion is the name given to the processes that remove newly formed sediment from bedrock. This is followed generally
by transportation and finally, when energy is exhausted, by deposition. The processes
and products of weathering are examined more closely in the next section. It is sufficient at this point to state that weathering is generally divided into biological, chemical,
and physical processes. Chemical weathering selectively oxidizes and dissolves the constituent minerals of a rock. Physical processes of weathering are those that bring about
its actual mechanical disaggregation. Biological weathering is caused by the chemical
and physical effects of organic processes on rock.
Erosion, the removal of new sediment, can be caused by four agents: gravity, glacial action, running water, and wind. The force of gravity causes the gradual creep of
sediment particles and slabs of rock down hillsides, as well as the more dramatic avalanches. Glacial erosion occurs where glaciers and ice sheets scour and abrade the face
of the earth as they flow slowly downhill under the influence of gravity. Moving water
is a powerful agent of erosion in a wide spectrum of geomorphological situations ranging from desert flash flood to riverbank scouring and sea cliff undercutting. The erosive
action of wind, on its own, is probably infinitesimal. Wind, however, blowing over a dry
desert, quickly picks up clouds of sand and sandblasts everything in its path for a height
of a meter or so. Eolian sandblasting undercuts rock faces, carving them into weird
shapes, and expedites the erosion of cliffs by gravity collapse and rainstorm.
It is important to note that it is an oversimplification to place erosion after weathering in the sedimentary cycle. Weathering processes need time for their effects to be noticeable on a rock surface. In some parts of the earth, notably areas of high relief, erosion may occur so fast that rock is not exposed to the air for a sufficient length of time
to undergo any significant degree of weathering (Fig. 2.2). This point is amplified in the
next section.
Returning to the role of gravity, ice, water, and wind, it is apparent that these are the
agents both of erosion and of subsequent transportation of sediment. The physical processes of these various transporting media are described in Chapter 4. At this point,
however, it is appropriate to point out the role played by these agents in the segregation
of sediments. The products of weathering are twofold: solutes and residua. The solutes
are the soluble fraction of rocks that is carried in water. The residua are the insoluble
products of weathering, which range in size from boulders down to colloidal clay particles. It is interesting to note the competency of the various transporting media to
2 WEATHERING AND THE SEDIMENTARY CYCLE
bouncing lethargically over many millions of years on a conveyor belt. The sand grains
history is one major cyclic event composed of many subsidiary ones. We now examine
the sedimentary cycle in more detail.
2.2 THE SEDIMENTARY CYCLE
This section looks more closely at the sedimentary cycle in the smaller scale, leaving
aside the major cycle of plate formation and destruction. Classically the sedimentary
cycle consists of the phases of weathering, erosion, transportation, deposition, lithifaction, uplift, and weathering again (Fig. 2.1).
Weathering is the name given to the processes that break down rock at the earth's
surface to form discrete particles (Ollier, 1969). Erosion is the name given to the processes that remove newly formed sediment from bedrock. This is followed generally
by transportation and finally, when energy is exhausted, by deposition. The processes
and products of weathering are examined more closely in the next section. It is sufficient at this point to state that weathering is generally divided into biological, chemical,
and physical processes. Chemical weathering selectively oxidizes and dissolves the constituent minerals of a rock. Physical processes of weathering are those that bring about
its actual mechanical disaggregation. Biological weathering is caused by the chemical
and physical effects of organic processes on rock.
Erosion, the removal of new sediment, can be caused by four agents: gravity, glacial action, running water, and wind. The force of gravity causes the gradual creep of
sediment particles and slabs of rock down hillsides, as well as the more dramatic avalanches. Glacial erosion occurs where glaciers and ice sheets scour and abrade the face
of the earth as they flow slowly downhill under the influence of gravity. Moving water
is a powerful agent of erosion in a wide spectrum of geomorphological situations ranging from desert flash flood to riverbank scouring and sea cliff undercutting. The erosive
action of wind, on its own, is probably infinitesimal. Wind, however, blowing over a dry
desert, quickly picks up clouds of sand and sandblasts everything in its path for a height
of a meter or so. Eolian sandblasting undercuts rock faces, carving them into weird
shapes, and expedites the erosion of cliffs by gravity collapse and rainstorm.
It is important to note that it is an oversimplification to place erosion after weathering in the sedimentary cycle. Weathering processes need time for their effects to be noticeable on a rock surface. In some parts of the earth, notably areas of high relief, erosion may occur so fast that rock is not exposed to the air for a sufficient length of time
to undergo any significant degree of weathering (Fig. 2.2). This point is amplified in the
next section.
Returning to the role of gravity, ice, water, and wind, it is apparent that these are the
agents both of erosion and of subsequent transportation of sediment. The physical processes of these various transporting media are described in Chapter 4. At this point,
however, it is appropriate to point out the role played by these agents in the segregation
of sediments. The products of weathering are twofold: solutes and residua. The solutes
are the soluble fraction of rocks that is carried in water. The residua are the insoluble
products of weathering, which range in size from boulders down to colloidal clay particles. It is interesting to note the competency of the various transporting media to