The process of weathering deals with the breaking down of minerals, soils and rocks as well as synthetic materials as a result of contact with the atmosphere, biological entities and water. Weathering is slightly different from erosion, because it does not involve movement. Erosion involves the movement of water, wind, snow or other agents across minerals or rocks. Weathering simply takes place “in situ.” There are two significant types of weathering, chemical and physical. Each can involve a biological component, but the difference is that physical weathering has to do with soils and rocks breaking down as a result of directly contacting atmospheric phenomena like water, heat, pressure and ice. Chemical weathering refers to the effect that biologically produced or atmospheric chemicals have on soils, rocks and minerals.
After the weathering process breaks rocks down, and it combines with surrounding organic materials, the result is soil. The parent material of the rock often determines the soil’s material content. This means that a soil that comes from one type of rock is often deficient when it comes to the necessary nutrients for fertility, while soils that come from several rock types weathering together, as in the sediments that underlie alluvial or glacial plains, often are more fertile for cultivation. Many of the major landscapes and landforms on Earth have come from extended weathering processes.
Several processes lead to physical weathering, referring to rocks disintegrating without any sort of chemical change. The primary mechanism is abrasion, which reduces particles in size. One of the most common processes is thermal stress. This happens when rock contracts and expands as a result of changes in the temperature. Thermal fatigue and thermal shock are the two main forms of thermal stress. This is a common occurrence in deserts, which feature a wide range of temperatures between the chill of the night and the extreme heat of the day. This repeated oscillation causes stress on the external rock layers, leading them to peel away in sheets in a process known as exfoliation. Moisture can also lead to thermal expansion in rock. When range and forest fires take place, the boulders and rocks in the area undergo significant thermal stress, as intense heat can quickly cause a boulder to expand. Thermal shock happens when thermal differences lead different parts of a particular object to expand in different degrees. When this stress is greater than the material’s strength, a crack forms, ultimately dooming the structural integrity.
Frost weathering, or frost edging, refers to a number of processes in which ice influences the development of rocks. When this takes the form of shattering, frost has made rock shatter into small fragments known as scree, which you will often see at the base of mountainous areas or near slopes. When the temperature stays very close to the freezing point of water, frost weathering is very common, as it forms and melts when temperatures fluctuate above and below that point. When the ice crystals grow, they weaken the rocks, because of the expansion that happens to ice when water hits that freezing point. You will see frost weathering in environments with a significant amount of moisture, particularly in areas near glaciers and alpine region. Just one example of a rock particularly vulnerable to frost weathering is chalk, which features many porous spaces that permit ice crystals to grow. Repeating the cycle of freezing and thawing weakens the rocks, causing them ultimately to break into angular sections.
Pressure release is another weathering mechanism. When igneous rocks like granite form deep under the surface of the Earth, they have a lot of pressure from the rock above them. When erosion takes those top layers away, the lower rocks are revealed, and they lose that pressure, making the outer portions expand. This causes stress to the rest of the rock, causing parallel fracture lines. As time goes by, sheets of rock fall away from the exposed pieces in a process called exfoliation.
The growth of salt crystals is another mechanism that leads to physical weathering. When saline liquids flow into cracks inside rocks, salt crystals end up forming in the interior of the rocks, disintegrating them from within. As the salt crystals receive heat, they expand, increasing pressure. This also takes place when a saline solution causes rocks to decay from the outside in, as happens with chalk and limestone, forming solutions of sodium carbonate or sodium sulfate. This generally happens in dry climates where heating extremes lead to higher levels of evaporation and a resulting salt crystallization. This also happens frequently along the seashore. If you look at the stones in sea wall formations, their honeycombed patterns form a type of weathering.
Organisms contribute to the physical weathering process as well. Mosses and lichens start to grow on rock surfaces that are generally bare, upping the humidity. When the organisms connect to the rock, the surface layer of the rock begins to break down more quickly than it would have otherwise. Seedlings that sprout in niches and send roots further in also bring physical pressure to bear, allowing water and other substances to infiltrate the rock.
Chemical weathering also ends up breaking rocks up into smaller pieces, but through compositional transformations rather than through physical force. As a rock’s mineralogy changes over time as a result of its exposure to the environment, it chemically weathers, and secondary minerals end up developing. The processes of hydrolysis and oxidation are particularly important for this process. Just one example is mountain block uplift, which gives new strata of rock exposure. The resulting moisture permits significant chemical weathering, as well as the release of calcium ions and other charged particles into surface waters.
Carbonation and dissolution mark another one of the chemical weathering processes. Rainfall has acid in it because carbon dioxide in the atmosphere dissolves in it, generating a weak form of carbonic acid. Environments without pollution actually feature a pH of approximately 5.6 in the rain. “Acid rain” results when nitrogen dioxide and sulfur dioxide, products of industrial work, are also in the atmosphere, reducing the pH of rain as low as 3.0. When these solutions hit rock below, the weathering is often significant. Some minerals are high in natural solubility, instability, or potential for oxidation. These weather quite a bit naturally, even without pollutants in the rain. Carbonation takes place when carbon dioxide in the atmosphere weathers rocks that have calcium carbonate, like chalk and limestone. Rain mixes with the carbon dioxide or another organic acid to create a weak acid that reacts with the calcium carbonate in the rock to make calcium bicarbonate. When the temperature is lower, this reaction is faster, because water contains more carbon dioxide gas dissolved at lower temperatures. With limestone, this occurs most rapidly at the joints, making them deeper and wider.
Hydrolysis is another chemical weathering process, specific to carbonate and silicate minerals. These reactions feature the ionization of water before reaction with silicate minerals. In theory, the reaction leads to the total dissolution of the original rock, as long as there is enough water. However, in nature, pure water only rarely donates positive hydrogen ions. Carbon dioxide dissolves in the water, serving as the source of the positive hydrogen ions necessary for the reaction. Another hydrolysis reaction features carbonic acid being used up in silicate weathering, leading to more alkaline solutions. This controls the amount of atmospheric carbon dioxide and can ultimately influence climate.
When a metal is present in a rock, chemical weathering can also take the form of oxidation. When water is present, iron oxidizes to form positive iron hydroxides and such oxides as hematite, limonite and goethite. This makes the rocks look red-brown on the surface and crumble without much impact. Another word for this process is “rusting,” although the process is different from the rusting of the purely metallic iron.
Weathering is a process that occurs in nature, whether or not humanity adds pollutants to the air. Particularly in the case of acid rain and other pollutants, human activity can speed up the weathering process. However, over the course of millennia, weathering takes on a significant role, ensuring that rocks will continue to break down into soil over the course of decades and centuries. The simple continuation of the water cycle is just one process that ensures that weathering is inevitable and infinite.
Works Cited
Hogan, C. Michael. “Calcium,” in A. Jorgenson and C. Cleveland (eds.), Encyclopedia of Earth.
Washington DC: National Council for Science and the Environment, 2010.
Paradise, T.R. “Petra Revisited: An Examination of Sandstone Weathering Research in Petra,
Jordan.” Special Paper 390: Stone Decay in the Architectural Environment.” Doi: 10.1130/0-8137-2930-6.39.
Taber, Stephen. “The Mechanics of Frost Heaving.” Journal of Geology 38(4): 303-315.
Uroz, S., Calvaruso, C., Turpault, M-P., and Frey-Klett, P. “Mineral Weathering by Bacteria:
Ecology, Actors and Mechanisms.” Trends in Microbiology 17(9): 378-387.
Zambell, C.B., Adams, J.M., Gorring, M.L., and Schwartzman, D.W. "Effect of lichen
colonization on chemical weathering of hornblende granite as estimated by aqueous elemental flux".Chemical Geology 291: 166.
