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weathering

weathering, collective term for the processes by which rock at or near the earth's surface is disintegrated and decomposed by the action of atmospheric agents, water, and living things. Some of these processes are mechanical, e.g., the expansion and contraction caused by sudden, large changes in temperature, the expansive force of water freezing in cracks, the splitting caused by plant roots, and the impact of running water; others are chemical, e.g., oxidation, hydration, carbonization, and loss of chemical elements by solution in water. Weathering is important because it aids in the formation of soil and prepares materials for erosion. Construction materials for buildings and roads are subject to weathering by water, carbon dioxide, aerosol gases, freeze-thaw cycles, and salt (see also formation of potholes). New techniques in road construction, allowing a minimum of weathering, more weather-resistant aggregates, and better building materials have lowered costs for maintenance and repair.

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weathering

weathering The breakdown of rocks and minerals at and below the Earth's surface by the action of physical and chemical processes. Essentially it is the response of Earth materials to the low pressures, low temperatures, and presence of air and water that characterize the near-surface environment, but which were not typical of the environment of formation. There are several varieties of rock breakdown (see also MECHANICAL WEATHERING). Simple disintegration may occur, resulting in the production of coarse, angular blocks, of peels or skins (the process of ‘desquamation’), of sands, and of silts. Minerals may be removed in solution, and chemical weathering may form new, often easily eroded substances. See also CARBONATION; EROSION; FROST WEDGING; HYDRATION; HYDROLYSIS; and THERMOCLASTIS.

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weathering

weathering The breakdown of rocks and minerals at and below the Earth's surface by the action of physical and chemical processes. Essentially it is the response of Earth materials to the low pressures, low temperatures, and presence of air and water that characterize the near-surface environment, but which were not typical of the environment of formation. There are several varieties of rock breakdown. Simple disintegration may occur, resulting in the production of coarse, angular blocks, of peels or skins (the process of ‘desquamation’), of sands, and of silts. Minerals may be removed in solution, and chemical weathering may form new, often easily eroded substances.

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weathering

weathering Breakdown and chemical disintegration of rocks and minerals at the Earth's surface by physical and chemical processes. In physical weathering in cold, wet climates, water seeping into cracks in the rock expands on freezing, so causing the rock to crack further and to crumble. Extreme temperature changes in drier regions, such as deserts, also cause rocks to fragment. Chemical weathering can lead to a weakening of the rock structure by altering the minerals of a rock and changing their size, volume, and ability to hold shape. Unlike erosion, weathering does not involve transportation.

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weathering

weathering.
1. Inclination given to any upper surface, e.g. off-set.

2. Process of undergoing change caused by action of weather. In some instances the effect of time on a building may be beneficial, giving the surface a beauty that cannot be artificially applied (e.g. limestone ashlar).

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Weathering

Weathering


Weathering refers to the group of physical, chemical, and biological processes that change the physical and chemical state of rocks and soils at and near the surface of the earth. Weathering is primarily a result of climatic forces. Because the effects of climate occur at the earth's surface, the intensity of weathering decreases with depth, with most of the effects exhibited within the first meter of the surface. The most important climatic force is water, as it moves in and around rocks and soil .

Physical weathering is the disintegration of rock into smaller pieces by mechanical forces concentrated along rock fractures. Abrasion of rocks occurs when wind or water carry particles that wear away rocks. Physical weathering due to frost is referred to as frost shattering or frost wedging. Because water expands when it freezes, it can break rocks apart from the inside when it seeps into cracks in a rock or soil. The specific volume (volume/unit mass) of water increases by 9% during freezing, which produces a stress that is greater than the strength of most rocks. Frost action is the most common physical weathering process, as frost is widespread throughout the world. Frost even occurs in the tropics at high elevations, and as a weathering force, is most effective in coastal arctic and alpine environments, where there are hundreds of frost (freeze-thaw) cycles per year.

Exfoliation is the breaking off of rocks in curved sheets or slabs along joints that are parallel to the ground surface. Exfoliation occurs when a rock expands in response to the removal of adjacent rock. Most commonly the release of stress upon a rock occurs when overlying rock is eroded away (i.e., when the pressure of deep burial is removed). The rock breaks apart along expansion fractures that increase in spacing with depth.

Another type of physical weathering is salt wedging. Most water as it moves through the earth contains dissolved salts; in some areas the salt content may be high, with possible sources being seawater or chemical weathering of marine sediments. As the saline water moves into rock fractures and subsequently evaporates, salt crystals form. As the process continues, the crystals grow until they act as a wedge and crack and break the rocks. Salt wedging most commonly occurs in dry landscapes where the groundwater is near the surface.

Hydration is a physical process that also results in weathering. Soil aggregates and fine-grained rocks can disintegrate due to wetting and drying cycles and the expansion and contraction associated with the cycles. Also air that is drawn into pores under dry conditions and then trapped as water returns to the soil or rock can cause fracturing.

Thermal weathering is another physical process. Repeated daily heating and cooling of rock results in expansion during heating and contraction during cooling. Different materials expand and contract at different rates, resulting in stresses along mineral boundaries.

Chemical weathering of rocks or soils occurs through chemical reactions when rocks or soils react with water, gases, and solutions. During these chemical reactions, minerals are added or removed or are decomposed into other materials such as clay minerals .

Carbon dioxide , a chemical weathering agent, dissolves in rain and forms a weak carbonic acid . This weak acid, through the process of carbonation, can dissolve rocks such as limestone and feldspar. Carbonation of limestone can result in the formation of karst topography that may include caves, disappearing streams, springs, and sinkholes .

In chemical oxidation weathering, rocks are transformed through reactions with oxygen dissolved in water. Iron, often found in silicate minerals, is the most commonly oxidized mineral element, when ferrous iron (Fe+2)) is oxidized to ferric iron (Fe+3). Color changes often indicate when oxidation has occurred, such as the "rusting" seen with the oxidation of iron. Other readily oxidized minerals include magnesium, sulfur, aluminum , and chromium.

Hydrolysis is the most common weathering process, where mineral cations in a rock or soil mineral are replaced by hydrogen (H+) ions. Pure water is a poor hydrogen donor, but carbon dioxide dissolved in water, which produces carbonic acid, acts as a source of hydrogen ions. Weathering products formed include clay minerals.

Biological weathering occurs when organisms aid in the breakdown of rocks and minerals. Plants such as lichens and mosses produce a weak acid that dissolves geological materials. Plant roots growing in the cracks of rocks, through the process of root pry, can make the crack larger and may loosen other types of materials.

[Judith L. Sims ]


RESOURCES

BOOKS

Ollier, Cliff, and Colin Pain. Regolith, Soils, and Landforms. New York: John Wiley & Sons Ltd., 1996.

Rolls, David, and Will J. Bland. Weathering: An Introduction to the Basic Principles. London: Edward Arnold, 1998.

Spickert, Diane Nelson, and Marianne D. Wallace. Earthsteps: A Rock's Journey Through Time. Golden, CO: Fulcrum Publishing, 2000.

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Weathering

Weathering

Physical (mechanical) weathering

Temperature and moisture

Chemical weathering

Biological weathering

Rates of weathering

Resources

Weathering is the process by which rocks and minerals are decomposed into simpler materials by means of physical (mechanical), chemical, and biological processes. Weathering is an important phenomenon for the human species because it is the mechanism by which one of the planets most important natural resourcessoilis formed.

The exact way in which weathering occurs in any particular situation depends primarily on two factors: the type of rock and the environmental conditions to which the rock is exposed. For example, rocky formations along a seacoast are likely to be exposed to the mechanical action of waves and tides. But rocks beneath a humid rainforest are more likely to be weathered by chemical reactions made possible by abundant water.

Physical (mechanical) weathering

During physical weathering, a large piece of rock is broken down into smaller and smaller pieces. This process can come about as the result of a number of natural processes. For example, gravity may cause a

large boulder to break loose from the top of a mountain and fall. When the boulder strikes solid ground, it may break apart into many smaller pieces.

Ground movements can also result in physical weathering. As overlying rocks and soils are removed by natural or human-caused forces, underlying rocks may work their way to Earths surface. As pressure on these rocks is relieved, they may begin to expand outward, often breaking into thin sheets by a process known as exfoliation.

Abrasion can also cause physical weathering. Imagine a windstorm blowing across a broad expanse of sandy desert. Tiny particles of sand are carried along by the wind, a current of air that acts like sandpaper on rocks that stand in its pathway. The wind-sandpaper scours off pieces of grit and sand from these rocks, contributing to their physical weathering.

Temperature and moisture

In many locations, changes in temperature and moisture content of the environment cause significant physical weathering. When rock is warmed, it expands; when it cools, it contracts. In some regions, rocks are heated to relatively high temperatures during the day and then cooled to much lower temperatures during the night. The constant expansion and contraction of the rocks may result in breakage and mechanical weathering. This effect is likely to be more pronounced if water is present. If water fills the cracks in a rock during the day and the temperature falls below freezing at night, the water will freeze, expand, and crack the rock.

Chemical weathering

Chemical weathering is the process by which changes take place in the chemical composition of rocks. Chemical weathering represents a second stage of rock disintegration in which small pieces of rock produced by physical weathering are then further decomposed by chemical processes.

Three chemical reactions in particular are effective in bringing about the weathering of a rock: acid reactions, hydrolysis, and oxidation. Acids form readily in the soil. One of the most common such reactions occurs when carbon dioxide in the air reacts with water to form a weak acid, carbonic acid. Carbonic acid has the ability to attack many kinds of rocks, changing them into other forms. For example, when carbonic acid reacts with limestone, it produces calcium bicarbonate, which is partially soluble in water. Caves are formed when underground water containing carbonic acid travels through blocks of limestone, dissolves out the limestone, and leaves empty pockets (caves) behind.

Acids produced by human activities can also produce chemical weathering. For example, the conversion of metallic ores to the pure metals often results in the formation of sulfur dioxide. When sulfur dioxide combines with water, it forms the weak acid sulfurous acid and, eventually, the stronger sulfuric acid. In other cases, groundwater percolating through natural occurrences of sulfur-rich rock can produce acid drainage. Both of these acids are capable of attacking certain kinds of rocks in much the way that carbonic acid does.

Hydrolysis is a chemical reaction by which a compound reacts with water to form one or more new substances. A number of rock-forming minerals readily undergo hydrolysis, especially in acidic conditions. For example, the common mineral feldspar will undergo hydrolysis to produce a clay-type mineral known as kaolinite and silicic acid. Both of these new compounds are much more soluble in water than is feldspar. Hydrolysis of the mineral results, therefore, in the degradation of any rocks in which it may occur.

Oxidation occurs when the metallic part of a mineral reacts with oxygen in the air (or from some other source) to produce a new substance that is different in structure or more soluble than the original mineral. The spectacular red, orange, and yellow color of certain natural rock formationssuch as those in Utahs Bryce Canyonare an indication that an oxide of iron has been produced during the chemical weathering of the rock formations.

Biological weathering

The presence of living organisms can also cause weathering, for example when roots grow into small cracks and increase in size. Microbes can also be important in geological processes, and the field of geobiology expanded greatly in the early twenty-first century as geologists realized the importance of biological organisms in geologic and geochemical processes.

Rates of weathering

The rate at which rocks disintegrate depends both on the type of rock involved and the external

KEY TERMS

Abrasion The mechanism by which one material rubs against another material, sometimes producing weathering in the process.

Chemical reaction Any change that takes place in which one substance is changed into one or more new substances.

Exfoliation The process by which skinlike layers form on the outer surface of a rock and, in some cases, eventually peel off.

Oxidation The chemical reaction by which a substance reacts with oxygen.

forces to which the rock is exposed. As an example, most sandstone tends to weather easily compared to granite, which is very resistant to weathering. The presence of moisture, high temperatures, large temperature variations, and air movement also tend to increase the rate at which weathering takes place. Human activities can also affect the rate of weathering. For example, large quantities of gaseous oxides are produced by electrical power generating plants. When these oxides react with water vapor in the air, they form acid rain. When acid rain falls to Earths surface, it may attack rocky materials in much the same way that natural acids like carbonic acid do.

Resources

BOOKS

Blatt, H., R. Tracy, and B. Owens. Petrology: Igneous, Sedimentary, and Metamorphic. New York: Freeman, 2005.

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, New Jersey: Prentice Hall, 2004.

Walther, J.V. Essentials of Geochemistry. Sudbury, Mass.: Jones and Bartlett, 2005.

David E. Newton

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Weathering

Weathering

Weathering is the process by which rocks and minerals are broken down into simpler materials by means of physical (mechanical), chemical, and biological processes. Weathering is an extremely important phenomenon for the human species since it is the mechanism by which one of the planet's most important natural resources—soil—is formed.

The exact way in which weathering occurs in any particular situation depends primarily on two factors: the type of rock and the environmental conditions to which the rock is exposed. For example, rocky formations along a seacoast are likely to be exposed to the mechanical action of waves and tides . But rocks buried underground are more likely to be attacked by chemical reactions made possible by water that runs through them.


Physical (mechanical) weathering

During physical weathering, a large piece of rock is broken down into smaller and smaller pieces. This process can come about as the result of a number of natural processes. For example, the force of gravity may cause a large boulder to break loose from the top of a mountain and fall. When the boulder hits solid ground, it may break apart into many smaller pieces.

Ground movements can also result in physical weathering. As overlying rocks and soils are removed by natural or human-caused forces, underlying rocks may work their way to Earth's surface. As pressure on these rocks is relieved, they may begin to expand outward, often forming an flaky appearance known as exfoliation.

Abrasion can also cause physical weathering. Imagine a wind storm blowing across a broad expanse of sandy desert . Tiny particles of sand are carried along by the wind, a current of air that acts like sandpaper on rocks that stand in its pathway. The wind-sandpaper scours off pieces of grit and sand from these rocks, contributing to their physical weathering.


Temperature and moisture

In many locations, changes in temperature and moisture content of the environment cause significant physical weathering. When rock is warmed, it expands; when it cools, it contracts. In some regions, rocks are heated to relatively high temperatures during the day and then cooled to much lower temperatures during the night. The constant expansion and contraction of the rocks may result in pieces being broken off.

This effect is likely to be more pronounced if water is present. Suppose that water fills the cracks in a rock during the day. At night, if the temperature drops far enough, that water will freeze and form veins of ice in the rock. But water expands as it freezes. Therefore, the veins of ice are likely to break apart pieces of the rock, a process that is repeated day after day and night after night when temperatures follow the pattern described above.


Chemical weathering

Chemical weathering is the process by which changes take place in the very chemical structure of rocks themselves. Chemical weathering represents a second stage of rock disintegration in which small pieces of rock produced by physical weathering are then further broken apart by chemical processes.

Three chemical reactions in particular are effective in bringing about the weathering of a rock: acid reactions, hydrolysis , and oxidation. Acids form readily in the soil . One of the most common such reactions occurs when carbon dioxide in the air reacts with water to form a weak acid, carbonic acid. Carbonic acid has the ability to attack many kinds of rocks, changing them into other forms. For example, when carbonic acid reacts with limestone, it produces calcium bicarbonate, which is partially soluble in water. Caves are formed when underground water containing carbonic acid travels through blocks of limestone, dissolves out the limestone, and leaves empty pockets (caves) behind.

Acids produced by human activities can also produce chemical weathering. For example, the conversion of metallic ores to the pure metals often results in the formation of sulfur dioxide . When sulfur dioxide combines with water, it forms the weak acid sulfurous acid and, eventually, the stronger acid sulfuric acid . Both of these acids are capable of attacking certain kinds of rocks in much the way that carbonic acid does.

Hydrolysis is a chemical reaction by which a compound reacts with water to form one or more new substances. A number of rock-forming minerals readily undergo hydrolysis, especially in acidic conditions. For example, the common mineral feldspar will undergo hydrolysis to produce a clay-type mineral known as kaolinite and silicic acid. Both of these new compounds are much more soluble in water than is feldspar. Hydrolysis of the mineral results, therefore, in the degradation of any rocks in which it may occur.

Oxidation occurs when the metallic part of a mineral reacts with oxygen in the air (or from some other source) to produce a new substance that is different in structure or more soluble than the original mineral. The spectacular red, orange, and yellow color of certain natural rock formations—such as those in Utah's Bryce Canyon—are an indication that an oxide of iron has been produced during the chemical weathering of the rock formations.


Biological weathering

The presence of living organisms can also cause weathering. Imagine that a seed falls into a small crevice in a rock and begins to germinate. As the plant continues to grow and send down roots, it will work its way into the rock and eventually make the crevice grow in size. Eventually the plant's roots may actually tear the rock apart.


Rates of weathering

The rate at which rocks disintegrate depends both on the type of rock involved and the external forces to which the rock is exposed. As an example, sandstone tends to weather rather easily, while granite is quite resistant to weathering. The presence of moisture, high temperatures, large temperature variations, and air movement also tend to increase the rate at which weathering takes place. Human activities can also affect the rate of weathering. For example, large quantities of gaseous oxides are produced by electrical power generating plants. When these oxides react with water vapor in the air, they form "acid rain." When acid rain falls to the earth's surface, it may attack rocky materials in much the same way that natural acids like carbonic acid do.


Resources

books

Hamblin, W.K., and E.H. Christiansen. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001.

Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology. 4th ed. John Wiley & Sons, 2000.


David E. Newton

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Abrasion

—The mechanism by which one material rubs against another material, sometimes producing weathering in the process.

Chemical reaction

—Any change that takes place in which one substance is changed into one or more new substances.

Exfoliation

—The process by which skinlike layers form on the outer surface of a rock and, in some cases, eventually peel off.

Oxidation

—The chemical reaction by which a substance reacts with oxygen.

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