Mechanical Weathering and Erosion
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[edit] Introduction
This article constitutes a brief introduction to the processes of mechanical weathering and erosion.
The distinction between weathering and erosion is that weathering breaks rocks but leaves them in place; whereas erosional processes are capable both of breaking rocks and transporting the broken fragments (clasts). Most, though not all texts will make this distinction; some authors, however, will class mechanical weathering as a form of erosion.
Mechanical weathering is sometimes referred to as physical weathering. In both cases, the purpose of this nomenclature is to distinguish it from the processes collectively known as chemical weathering, in which chemical action breaks down the rocks; we have dealt with this topic in a separate article.
[edit] Mechanical weathering
Mechanical weathering involves mechanical processes that break up a rock while leaving the fragments in place: for example, ice freezing and expanding in cracks in the rock; tree roots growing in similar cracks; expansion and contraction of rock in areas with high daytime and low nighttime temperatures; cracking of rocks in forest fires, and so forth.
Such processes, by increasing the surface area of rocks, make them more susceptible to the processes of chemical weathering and erosion.
[edit] Agents of erosion
The main agents of erosion are gravity; eolian processes (i.e. those caused by the wind); ice in the form of glaciers; and water in the form of rivers, waves, turbidity currents, and runoff caused by rainfall.
The reader will be familiar with most of the processes described, but we should provide a brief introduction to the concept of turbidity currents. A current of water that is turbid (that is, it contains a lot of sediment) is denser than clear water, and will flow along the bottom of a lake or the ocean, often over large distances and at high speeds, before failing and dispersing its load; such currents occur when a turbid river discharges into the clear waters of a lake, or they can be initiated by a mudslide on a continental shelf. A dust storm may be considered the aeolian equivalent of a turbidity current.
[edit] Erosion of rocks
Attrition is the effect these same forces have on the sediments themselves, breaking them into smaller fragments or rounding the clasts into smooth pebbles or rounded grains of sand. The efficiency of this process can be observed anywhere you can find beach glass, which originates as sharp-edged shards; the process of tumble-polishing semi-precious stones artificially emulates this process and will render most pebbles well-rounded in a matter of days.
The simple mechanical force of water or ice can break off chunks of rock, as when glaciers pluck rocks from the surfaces over they move, or when the pounding of waves hammers against a cliff.
Gravity will break off the overhand of a cliff undercut by abrasion and wave pounding, when the rock at the top of the cliff is unable to bear the mechanical strain.
[edit] Transport of sediment
Erosional processes, as we have said, are defined by their ability to transport sediment as well as to create the clasts of which it is composed.
Currents of wind or water can transport sediments in three ways: in suspension, where light particles are carried along in the current above the ground, sea bed, or river bed; by saltation, where particles too heavy to be carried in suspension are bounced along the ground (or river-bed, or whatever); and creep, where particles are rolled along the ground. The size of the particles susceptible to these processes will depend, of course, on the velocity of the current.
By contrast, the size of the clasts that can be carried by glaciers is under no such limitation. One of the characteristic results of glacial action is the transport of huge boulders, up to the size of a house, which could certainly not be transported by wind or water.
Gravity, too, is obviously under no such limitation; it is possible for entire layers of rock to slide down a hillside.
[edit] How do we know?
We are accustomed to ending each article on geology with a section entitled "How do we know?" In this particular case, the exercise seems almost superfluous, for the processes involved are neither hidden nor subtle. we can observe a sandstorm; we can see how the head of a waterfall shifts year on year, or how a river that has shifted its course scours out a new bed; we can see how cliffs crumble and the effects of landslides. The fact that glaciers carry boulders is evident, and the distance they travel each year can be measured; as can the quantity of sediment discharged at the mouth of a river.
A more interesting question is, how do we know that these processes have happened when the agency that caused them is no longer present? How, for example, do we identify the courses of glaciers long since melted, or of rivers that have dried up or shifted their beds? These are questions that we shall review in our further articles on surface processes.
