Historical Geology/Mechanical weathering and erosion
In this article we shall present a brief overview of erosion and of mechanical weathering. We can be brief because the erosional processes involved will be discussed at length in subsequent articles, so there is no need to do more than sketch out the topic and its vocabulary.
Geologists make a distinction between weathering and erosion: weathering breaks rocks but leaves them in place; whereas the processes of erosion are capable both of breaking rocks and transporting the broken fragments (clasts). Most, though not all texts will make this distinction. Those that do will still use "erosion" as a catch-all term: that is, when you see a geologist saying that a rock has been "eroded" s/he does not mean to imply that it has not also been weathered.
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. Chemical weathering will be covered in the next article.
Mechanical weathering involves mechanical processes that break up a rock: 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.
Mechanical weathering is probably the least important process we shall mention in this text, in that the history of the Earth and the resulting geological record would probably have been very similar if there was no such thing as mechanical weathering.
The main agents of mechanical erosion are: gravity; aeolian 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, which 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.
Modes of erosion
Abrasion of rocks is caused by the sediments carried by wind and water: waves, for example, can hurl their seaload of sand and shingle against a cliff; sandstorms can literally sand-blast rocks; the sand and silt carried by rivers or turbidity currents have the same effect.
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 quarry rocks from the surfaces they move over, or when the pounding of waves hammers against a cliff.
Gravity can break off the overhang 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. It is also instrumental in causing such things as rockslides and mudflows; such downhill motion is known collectively as mass wasting.
Transport of sediment
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, known as erratic boulders. Gravity, too, is obviously under no such limitation; it is possible for entire layers of rock to slide down a hillside.
How do we know?
In this particular case, asking "how do we know?" 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 shifting 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 subsequent articles.