Weathering is the breaking down of Earth's rocks In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids, soils Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics and minerals A mineral is a naturally occurring solid chemical substance that is formed through geological processes and that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not have a specific chemical composition through direct contact with the planet's atmosphere An atmosphere is a layer of gases that may surround a material body of sufficient mass,and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low. Some planets consist mainly of various gases, but only their outer layer is their. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion Erosion is the process of weathering and transport of solids in the natural environment or their source and deposits them elsewhere. It usually occurs due to transport by wind, water, or ice; by down-slope creep of soil and other material under the force of gravity; or by living organisms, such as burrowing animals, in the case of bioerosion, which involves the movement of rocks and minerals by agents such as water, ice, wind, and gravity.

In addition, weathering is the effect of atmospheric exposure to man-made structures and materials.

Two important classifications of weathering processes exist — physical and chemical weathering. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and pressure. The second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals (also known as biological weathering) in the breakdown of rocks, soils and minerals.^[1]

The materials left over after the rock breaks down combined with organic material creates soil Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics. The mineral content of the soil is determined by the parent material Parent material, in soil science, means the underlying geological material in which soil horizons form. Soils typically get a great deal of structure and minerals from their parent material. Parent materials are made up of consolidated or unconsolidated mineral material that has undergone some degree of physical or chemical weathering, thus a soil derived from a single rock type can often be deficient in one or more minerals for good fertility, while a soil weathered from a mix of rock types (as in glacial An "ice age" or, more precisely, "glacial age" is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in an expansion of continental ice sheets, polar ice sheets and alpine glaciers. Within a long-term ice age, individual pulses of extra cold climate are termed &, aeolian Aeolian processes pertain to the activity of the winds and more specifically, to the winds' ability to shape the surface of the Earth and other planets. Winds may erode, transport, and deposit materials, and are effective agents in regions with sparse vegetation and a large supply of unconsolidated sediments. Although water is a much more powerful or alluvial Alluvium is loose, unconsolidated (not cemented together into a solid rock), soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel. When this loose alluvial material sediments) often makes more fertile soil.

Physical weathering

Physical weathering is the class of processes that causes the disintegration of rocks without chemical change. The primary process in physical weathering is abrasion Abrasion is the mechanical scraping of a rock surface by friction between rocks and moving particles during their transport by wind, glacier, waves, gravity, running water or erosion. After friction, the moving particles dislodge loose and weak debris from the side of the rock. These particles can be dissolved in the water source (the process by which clasts and other particles are reduced in size). However, chemical and physical weathering often go hand in hand. For example, cracks exploited by physical weathering will increase the surface area exposed to chemical action. Furthermore, the chemical action at minerals in cracks can aid the disintegration process.

Thermal stress

Thermal stress weathering (sometimes called insolation weathering)^[2] results from expansion or contraction of rock, caused by temperature changes. Thermal stress weathering comprises two main types, thermal shock Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients. However, they are used in many high temperature applications due to their high melting and thermal fatigue. Thermal stress weathering is an important mechanism in deserts A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Deserts are defined as areas with an average annual precipitation of less than 250 millimetres per year, or as areas where more water is lost by evapotranspiration than falls as precipitation. In the Köppen, where there is a large diurnal Diurnal temperature variation is a meteorological term that relates to the variation in temperature that occurs from the highs of the day to the cool of nights temperature range, hot in the day and cold at night.^{[citation needed]} The repeated heating and cooling of exerts stress In continuum mechanics, stress is a measure of the average force per unit area of a surface within a deformable body on which internal forces act. In other words, it is a measure of the intensity of the internal forces acting between particles of a deformable body across imaginary internal surfaces . These internal forces are produced between the on the outer layers of rocks, which can cause their outer layers to peel off in thin sheets. Although temperature changes are the principal driver, moisture can enhance thermal expansion Thermal expansion is the tendency of matter to change in volume in response to a change in temperature. When a substance is heated, its particles begin moving and become active thus maintaining a greater average separation. Materials which contract with increasing temperature are rare; this effect is limited in size, and only occurs within limited in rock. Forest fires A wildfire is any uncontrolled, non-structure fire that occurs in the wilderness, wildland, or bush. Other names such as wildland fire, forest fire, brush fire, vegetation fire, grass fire, peat fire, bushfire , and hill fire are commonly used. The name wildfire was once a synonym for Greek fire as well as a word for any furious or destructive and range fires are also known to cause significant weathering of rocks In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids and boulders exposed along the ground surface. Intense, localized heat can rapidly expand a boulder, causing its surface to exfoliate or spall Spall are flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure . Spalling and spallation both describe the process of surface failure in which spall is shed.^{[citation needed]}

Frost weathering

Frost weathering or cryofracturing is the collective name for several processes where ice is present. This processes include frost shattering, frost-wedging and freeze-thaw weathering. This type of weathering is common in mountain areas where the temperature is around the freezing point of water. Certain frost-susceptible soils expand or heave Frost heaving is the process by which the freezing of water-saturated soil causes the deformation and upward thrust of the ground surface. This process can damage plant roots through breaking or desiccation, cause cracks in pavement, and damage the foundations of buildings, even below the frost line. Moist, fine-grained soil at certain upon freezing as a result of water migrating via capillary action Capillary action, or capillarity, refers to certain phenomena associated with the behavior of liquids in thin tubes or in porous materials. Liquids, such as water, will tend to move "up-hill" which does not normally occur in large containers. The interface between liquids, or a liquid and a gas, can form a meniscus or crescent shape to grow ice lenses near the freezing front.^[3] This same phenomena occurs within pore spaces of rocks. The ice accumulations grow larger as they attract liquid water from the surrounding pores. The ice crystal growth weakens the rocks which, in time, break up.^[4] It is caused by the expansion of ice Ice, technically, is one of the 15 known crystalline phases of water. In non-scientific contexts, the term usually means ice Ih, which is known to be the most abundant of these solid phases. It can appear transparent or opaque bluish-white colour, depending on the presence of impurities or air inclusions. The addition of other materials such as when water Water is a chemical substance with the chemical formula H2O. Its molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state, water vapor or steam freezes, so putting considerable stress on the walls of containment.

Freeze induced weathering action occurs mainly in environments where there is a lot of moisture, and temperatures frequently fluctuate above and below freezing point—that is, mainly alpine Alpine climate is the average weather for a region above the tree line. The climate becomes colder at high elevations—this characteristic is described by the lapse rate of air: air tends to get colder as it rises, since it expands. The dry adiabatic lapse rate is 10 °C per km of elevation or altitude. Therefore, moving up 100 meters on a and periglacial Periglacial is an adjective originally referring to places in the edges of glacial areas but has later on been widely used in geomorphology to describe any place where geomorphic processes related to freezing of water occur. In its original meaning a periglacial area was at the time in question, the area was not buried by glacial ice but was areas. An example of rocks susceptible to frost action is chalk Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find chert or flint, which has many pore spaces for the growth of ice crystals. This process can be seen in Dartmoor Dartmoor is an area of moorland in the centre of Devon, England. Protected by National Park status, it covers 954 square kilometres where it results in the formation of tors A tor is a rock outcrop formed by weathering, usually found on or near the summit of a hill. In the South West of England, where the term originated, it is also a word used for the hills themselves – particularly the high points of Dartmoor in Devon and Bodmin Moor in Cornwall. When water that has entered the joints freezes, the ice formed strains the walls of the joints and causes the joints to deepen and widen. When the ice thaws, water can flow further into the rock. Repeated freeze-thaw cycles weaken the rocks which, over time, break up along the joints into angular pieces. The angular rock fragments gather at the foot of the slope to form a talus slope (or scree Scree, also called talus, is a term given to an accumulation of broken rock fragments at the base of crags, mountain cliffs, or valley shoulders. Landforms associated with these materials are sometimes called scree slopes or talus piles. These deposits typically have a concave upwards form, while the maximum inclination of such deposits slope). The splitting of rocks along the joints into blocks is called block disintegration. The blocks of rocks that are detached are of various shapes depending on rock structure.

Pressure release

In pressure release, also known as unloading, overlying materials (not necessarily rocks) are removed (by erosion, or other processes), which causes underlying rocks to expand and fracture parallel to the surface. Often the overlying material is heavy, and the underlying rocks experience high pressure under them, for example, a moving glacier A glacier is a perennial mass of ice which moves over land. A glacier forms in locations where the mass accumulation of snow and ice exceeds ablation over many years. The word glacier comes from French via the Vulgar Latin glacia, and ultimately from Latin glacies meaning ice. The corresponding area of study is called glaciology. Pressure release may also cause exfoliation to occur.

Intrusive igneous rocks (e.g. granite Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granites usually have a medium to coarse grained texture. Occasionally some individual crystals (phenocrysts) are larger than the groundmass in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is sometimes known as a porphyry) are formed deep beneath the Earth's surface. They are under tremendous pressure because of the overlying rock material. When erosion removes the overlying rock material, these intrusive rocks are exposed and the pressure on them is released. The outer parts of the rocks then tend to expand. The expansion sets up stresses which cause fractures parallel to the rock surface to form. Over time, sheets of rock break away from the exposed rocks along the fractures. Pressure release is also known as "exfoliation" or "sheeting"; these processes result in batholiths A batholith is a large emplacement of igneous intrusive (also called plutonic) rock that forms from cooled magma deep in the earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock-types, such as granite, quartz monzonite, or diorite (see also granite dome) and granite domes, an example of which is Dartmoor.

Hydraulic action

This is when water (generally from powerful waves) rushes into cracks in the rockface rapidly. This traps a layer of air at the bottom of the crack, compressing it and weakening the rock. When the wave retreats, the trapped air is suddenly released with explosive force. The explosive release of highly pressurized air cracks away fragments at the rockface and widens the crack itself.

Salt-crystal growth

Salt crystallization, otherwise known as haloclasty, causes disintegration of rocks when saline (see salinity Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates. Salinity in Australian English and North American English may also refer to the salt content of soil) solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals A crystal or crystalline solid is a solid material, whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography. The process of crystal formation via mechanisms of crystal growth is called behind. These salt crystals expand as they are heated up, exerting pressure on the confining rock.

Salt crystallization may also take place when solutions decompose Decomposition or rotting is the process by which tissues of a dead organism break down into simpler forms of matter. The process is essential for new growth and development of living organisms because it recycles the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death. It is a rocks (for example, limestone Limestone is a sedimentary rock composed largely of the mineral calcite . Like most other sedimentary rocks, limestones are composed of grains; however, most grains in limestone grains are skeletal fragments of marine organisms such as coral or foraminifera. Other carbonate grains comprising limestones are ooids, peloids, intraclasts, and and chalk Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find chert or flint) to form salt solutions of sodium sulfate In inorganic chemistry, a sulfate is a salt of sulfuric acid or sodium carbonate Sodium Carbonate , Na2CO3, is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate; and is domestically well known for its everyday use as a water softener. It has a cooling alkaline taste, and can be extracted from the ashes of many plants. It is, of which the moisture evaporates to form their respective salt crystals.

The salts which have proved most effective in disintegrating rocks are sodium sulfate, magnesium sulfate Magnesium sulfate is a chemical compound containing magnesium, sulfur and oxygen, with the formula MgSO4. In its hydrated form, the pH is 6.0 (5.5 to 6.5). It is often encountered as the heptahydrate, MgSO4·7H2O, commonly called Epsom salt. Anhydrous magnesium sulfate is used as a drying agent. Since the anhydrous form is hygroscopic (readily, and calcium chloride Calcium chloride, CaCl2, is a salt and the compound of calcium and chlorine. It behaves as a typical ionic halide, and is solid at room temperature. It has several common applications such as brine for refrigeration plants, ice and dust control on roads, and in concrete. The anhydrous salt is also widely used as a desiccant, where it will adsorb. Some of these salts can expand up to three times or even more.

It is normally associated with arid A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life. As a result, environments subject to arid climates tend to lack vegetation and are called xeric or desertic climates where strong heating causes strong evaporation and therefore salt crystallization. It is also common along coasts. An example of salt weathering can be seen in the honeycombed stones in sea wall A seawall is a form of hard and strong coastal defense constructed on the inland part of a coast to reduce the effects of strong waves. Honeycomb is a type of tafoni Tafoni are small cave-like features found in granular rock such as sandstone, with rounded entrances and smooth concave walls. They often occur in groups that can riddle a hillside, cliff, or other rock formation. They can be found in all climate types, but are most abundant in intertidal areas and semi-arid and arid deserts. Currently favored, a class of cavernous rock weathering structures, which likely develop in large part by chemical and physical salt weathering processes.

Biological Weathering

Living organisms may contribute to mechanical weathering (as well as chemical weathering, see 'biological' weathering below). Lichens Lichens are composite organisms consisting of a symbiotic association of a fungus (the mycobiont) with a photosynthetic partner (the photobiont or phycobiont), usually either a green alga (commonly Trebouxia) or cyanobacterium (commonly Nostoc). The morphology, physiology and biochemistry of lichens are very different from those of the isolated and mosses Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems. At certain times mosses produce spore capsules which may appear as beak-like grow on essentially bare rock surfaces and create a more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface microlayer of the rock. On a larger scale seedlings sprouting in a crevice and plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration. Burrowing animals and insects disturb the soil layer adjacent to the bedrock surface thus further increasing water and acid infiltration and exposure to oxidation processes.

Chemical weathering

Chemical weathering changes the composition of rocks, often transforming them when water interacts with minerals to create various chemical reactions. Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near surface environment. New or secondary minerals develop from the original minerals of the rock. In this the processes of oxidation Redox describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed. This can be either a simple redox process, such as the oxidation of carbon to yield carbon dioxide (CO2) or the reduction of carbon by hydrogen to yield methane (CH4), or a complex process such as the oxidation of sugar(C6H12O6) in the and hydrolysis Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations (H+) (conventionally referred to as protons) and hydroxide anions (OH−) in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by step-growth polymerization. Such polymer are most important.

Dissolution

Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as sulphur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. Sulfur dioxide, SO₂, comes from volcanic eruptions or from fossil fuels, can become sulfuric acid within rainwater, which can cause solution weathering to the rocks on which it falls.

Some minerals, due to their natural soluability (e.g. evaporites), oxidation potential (iron-rich minerals, such as pyrite), or instability relative to surficial conditions (see Goldich dissolution series) will weather through dissolution naturally, even without acidic water.

One of the most well-known solution weathering processes is carbonation, the process in which atmospheric carbon dioxide leads to solution weathering. Carbonation occurs on rocks which contain calcium carbonate, such as limestone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid which reacts with calcium carbonate (the limestone) and forms calcium bicarbonate. This process speeds up with a decrease in temperature, not because low temperatures generally drive reactions faster, but because colder water holds more dissolved carbon dioxide gas.^{[citation needed]} Carbonation is therefore a large feature of glacial weathering.

The reactions as follows:

CO₂ + H₂O => H₂CO₃

carbon dioxide + water => carbonic acid

H₂CO₃ + CaCO₃ => Ca(HCO₃)₂

carbonic acid + calcium carbonate => calcium bicarbonate

Carbonation on the surface of well-jointed limestone produces a dissected limestone pavement which is most effective along the joints, widening and deepening them.

Hydration

Mineral hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral.

When rock minerals take up water, the increased volume creates physical stresses within the rock. For example iron oxides are converted to iron hydroxides and the hydration of anhydrite forms gypsum.

Hydrolysis on silicates and carbonates

Hydrolysis is a chemical weathering process affecting silicate and carbonate minerals. In such reactions, pure water ionizes slightly and reacts with silicate minerals. An example reaction:

Mg₂SiO₄ + 4H⁺ + 4OH^- ⇌ 2Mg²⁺ + 4OH^- + H₄SiO₄

olivine (forsterite) + four ionized water molecules ⇌ ions in solution + silicic acid in solution

This reaction results in complete dissolution of the original mineral, assuming enough water is available to drive the reaction. However, the above reaction is to a degree deceptive because pure water rarely acts as a H⁺ donor. Carbon dioxide, though, dissolves readily in water forming a weak acid and H⁺ donor.

Mg₂SiO₄ + 4CO₂ + 4H₂O ⇌ 2Mg²⁺ + 4HCO₃^- + H₄SiO₄

olivine (forsterite) + carbon dioxide + water ⇌ Magnesium and bicarbonate ions in solution + silicic acid in solution

This hydrolysis reaction is much more common. Carbonic acid is consumed by silicate weathering, resulting in more alkaline solutions because of the bicarbonate. This is an important reaction in controlling the amount of CO₂ in the atmosphere and can affect climate.

Aluminosilicates when subjected to the hydrolysis reaction produce a secondary mineral rather than simply releasing cations.

2KAlSi₃O₈ + 2H₂CO₃ + 9H₂O ⇌ Al₂Si₂O₅(OH)₄ + 4H₄SiO₄ + 2K⁺ + 2HCO₃^-

Orthoclase (aluminosilicate feldspar) + carbonic acid + water ⇌ Kaolinite (a clay mineral) + silicic acid in solution + potassium and bicarbonate ions in solution

Oxidation

Within the weathering environment chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe²⁺ (iron) and combination with oxygen and water to form Fe³⁺ hydroxides and oxides such as goethite, limonite, and hematite. This gives the affected rocks a reddish-brown coloration on the surface which crumbles easily and weakens the rock. This process is better known as 'rusting'. Many other metallic ores and minerals oxidize and hydrate to produce colored deposits, such as chalcopyrites or CuFeS₂ oxidizing to copper hydroxide and iron oxides.

Biological

A number of plants and animals may create chemical weathering through release of acidic compounds, i.e. moss on roofs is classed as weathering.

The most common form of biological weathering is the release of chelating compounds, i.e. acids, by plants so as to break down aluminium and iron containing compounds in the soils beneath them. Decaying remains of dead plants in soil may form organic acids which, when dissolved in water, cause chemical weathering.^{[citation needed]} Extreme release of chelating compounds can easily affect surrounding rocks and soils, and may lead to podsolisation of soils.

Building weathering

Buildings made of any stone, brick or concrete are susceptible to the same weathering agents as any exposed rock surface. Also statues, monuments and ornamental stonework can be badly damaged by natural weathering processes. This is accelerated in areas severely affected by acid rain.

Gallery

See also

• Aeolian processes
• Biorhexistasy
• Case hardening of rocks
• Decomposition
• Environmental chamber
• Eluvium
• Erosion
• Pedogenesis
• Soil production function
• Space weathering
• Spheroidal weathering
• Weather testing of polymers

References

1. ^ http://facstaff.gpc.edu/~pgore/geology/geo101/weather.htm
2. ^ Hall, K. The role of thermal stress fatigue in the breakdown of rock in cold regions, Geomorphology, 1999.
3. ^ Taber, Stephen (1930), "The mechanics of frost heaving", Journal of Geology 38: 303–317, http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA247424&Location=U2&doc=GetTRDoc.pdf
4. ^ Goudie, A.S.; Viles H. (2008). "5: Weathering Processses and Forms". in Burt T.P., Chorley R.J., Brunsden D., Cox N.J. & Goudie A.S.. Quaternary and Recent Processes and Forms. Landforms or the Development of Gemorphology. 4. Geological Society. pp. 129–164. ISBN 1862392498, 9781862392496. http://books.google.co.uk/books?id=wg0Rl7dY5ZYC&pg=PA137&dq=frost-shattering&ei=IMwWS5q7CaWGzASK34j7Dw#v=onepage&q=frost-shattering&f=false. Retrieved 2009-12-02.

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