Short Essay on Mechanical Weathering

The chief agents of mechanical weathering, or disintegration, are frost action, temperature changes, unloading, crystal growth, and the wedging action of plant roots.

Many different factors influence the efficiency of these agents in causing disintegration. The composition and texture of the rocks being weathered and the presence of joints, fractures, and voids clearly affect the rate at which solid rock can be reduced to rubble.

Mechanical weathering is also affected by climate, topography, and the length of time over which weathering agents have been operating. In general mechanical weathering predominates over chemical weathering in regions. By definition, mechanical weathering involves the physical disintegration of solid masses of rock into loose fragments. There is little chemical change in the rock itself. Thus, a chemical analysis of the disintegrated rock would be similar to that of the parent rock.

Water expands by about 9 percent when it freezes. If the water freezes in a confined space, pressure caused by the expansion may cause rock masses to be pushed apart and ruptured.

Such frost action has an important effect in mechanical weathering in that the freezing water is capable of exerting thousands of pounds of pressure pier square inch. Many of us have become aware of the effects of frost action during severe winter when concrete roads and side-walks are cracked by alternate freezing and thawing. The way freezing water disrupts solid rock is similar to the manner in which it breaks up streets and side­walks.

After water has filled a crack in the rock, the water at the lip of the crack may freeze. Once this has occurred, the water deeper in the crack is sealed off, and as it begins to freeze and expand, it pushes against the walls of the fracture, and thereby widens the space between those walls. In a subsequent thaw, fragments of rock may slip down into the crack and act as wedges to hold it open.

These processes are most prevalent in areas where water is abundant and where temperatures drop to below freezing at night and then warm during the day. Mountainous regions in temperate zones have this kind of daily temperature change, and in such rugged regions frost action is an important weathering process.

Insolation

Rocks, like many other solids, expand when they are warmed and contract when they are cooled. The heat is provided by the sun and so geologist refers to the weathering that may result as insolation weathering.

It would seem logical that repeated day-time heating and night time cooling of rocks (with concurrent expansion and contraction) would weaken the boundaries between grains and eventually cause them to separate from the rock mass. In laboratory experiments conducted by David T. Griggs at Harvard University, however, repeated heating and cooling did not cause fragmentation of the rock.

Unfortunately, the experiments were unable to assess the amount of strain induced by millions of expansions and contractions over thousands of centuries. Most geologists believe that insolation is only a minor contributor to mechanical weathering. Its effect is usually obscured by other weathering processes. Perhaps in desert regions where there are large-scale fluctuations in temperature, insolation may have some effect in weathering rocks.

Unloading

Quarrymen and miners are well aware that when the heavy weight of rock is removed from a mine with consequent loss of support to the surrounding rock, the outside pressure may sometimes cause a veritable explosion of rock fragments into the exacavations. In nature, erosion may similarly removed large volumes of surface rock, thus lightening the load on deeper rock and allowing it to expand.

Because the mass of rock is still confined on all sides, it can only respond to the release of pressure by expanding upwards. As it expands it will rupture and form joints that are roughly parallel to the surface topography. The process is called unloading, and the joint systems are referred to as sheeting. Sheeting is often seen in granite and other massive crystalline rocks that have been laid bare by glaciation or running water.

It can also be readily observed along the upper walls of quarries, where it may even facilitate the removal of the stone. Sheeting provides planar passageways along which solution and frost action may proceed vigorously. Half Dome at Yosemite National Park and Stone Mountain, Georgia, are examples of mountains whose shape has been controlled, at least partially, by sheeting.

Saline Crystal Growth

The growth of crystals, especially crystals of sodium chloride, calcium sulphate, or magnesium sulphate have been found to cause scaling in exposed rock surfaces and to dislodge grains and crystals from their parent rock.

The process is particularly effective in porous rocks in which crystals exert large expansive stresses as they grow. Disintegration of building stone because of the growth of crystals in pore spaces has been a vexing problem for architects involved in the preservation of historically important buildings.

The Temples at Luxor, Egypt, for example, have been seriously damaged by alternate solution and crystallization of salt. During dry spells, magnesium (and calcium) sulphate have been known to crystallize along zones of weakness in the dolomitic building stones of London’s Houses of Parliament, thereby causing spalling and crumbling at a rate that has caused considerable anxiety among restoration specialists. The magnesium sulfate is formed as a result of a chemical reaction between coal smoke and the magnesium in the dolomite.

Root Wedging

When one observes the growth of a plant from a seed, it is apparent that even a frail seedling is capable of exerting sufficient force to push aside relaively firm soil. As they grow into rock fractures and expand, the roots of large plants such as trees can exert correspondingly greater forces and are capable of widening cracks and accelerating the rate of disintegration by significant amounts. When the plant dies and the roots decay, they leave openings in which freezing water may accumulate and further widen the void space. Plants also react with rocks chemically, as will be described in the following section.