WEATHERING AND EROSION

WEATHERING  is the breaking down of rocks, soil and minerals as well as wood and artificial materials through contact with theEarth's atmosphere, waters and biological organisms. Weathering occurs in situ (on site), that is, in the same place, with little or no movement, and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity and then being transported and deposited in other loc

PHYSICAL WEATHERING/MECHANICAL WEATHERINGPhysical weathering, also recognized as mechanical weathering, is the class of processes that causes the disintegration of rocks without chemical change. The primary process in physical weathering is abrasion (the process by which clasts and other particles are reduced in size). However, chemical and physical weathering often go hand in hand. Physical weathering can occur due to temperature, pressure, frost etc. Physical weathering is also called mechanical weathering or disaggregation.

HEATING AND COOLINGThermal stress weathering (sometimes called insolation weathering)[2] results from the expansion and contraction of rock, caused by temperature changes. For example, heating of rocks by sunlight or fires can cause expansion of their constituent minerals eventually cause the rock to crack apart. Thermal stress weathering comprises two main types, thermal shock and thermal fatigue. Thermal stress weathering is an important mechanism in deserts, where there is a large diurnal temperature range, hot in the day and cold at night.[3]

 The repeated heating and cooling exerts stress on the outer layers of rocks,

EXPOLIATION is weathring of rocks due to release of pressure on rock surface . It is also known as sheeting shearing . Before expoliation rocks are under tremendous pressure because of the overlying rock material and atmosphere. 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, a process known as exfoliation. Exfoliation due to pressure release is also known as "sheeting“………………………….

ABRASIONThe primary process in physical weathering is abrasion . It is the process by which clasts and other particles are reduced in size. Abrasion by water, ice, and wind processes loaded with sediment can have tremendous cutting power, as is amply demonstrated by the gorges, ravines, and valleys around the world. In glacial areas, huge moving ice masses embedded with soil and rock fragments grind down rocks in their path and carry away large volumes of material.

BIOLOGICAL WETHERING

Biological effects on mechanical weatheringLiving organisms may contribute to mechanical weathering (as well as chemical weathering, see 'biological' weathering below). Lichensand mosses 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.

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CHEMICAL WEATHERINGYou have probably noticed that no tworock look exactly the same. Some look like they have been carved by a sculptor. Some look like they have been painted red, and others have been hollowed out to form caves.One of the reasons rocks look so varied in their appearance is because they are subjected to chemical weathering, which is the process by which rocks are broken down by chemical reactions. In this lesson, you will learn about the different types of chemical weathering

and how exposure to things such as water, oxygen, carbon dioxide and acids can alter the minerals found in rocks.We mentioned that chemical weathering is a way in which rocks get broken down. But it is important to emphasize That because there is an actual chemical change taking place, rocks are not just crumbled into smaller rocks; they are actually being chemically altered. In other words, after chemical weathering, we end up with a different substance than the one we started with.

HydrolysisThere are different types of chemical weathering. Let's start with a discussion of hydrolysis, which is the chemical breakdown of a substance when combined with water. You can recall this term by remembering that the prefix 'hydro' means 'water' and the suffix 'lysis' means 'to break down.'With chemical weathering of rock, we see a chemical reaction happening between the minerals found in the rock and rainwater. The most common example of hydrolysis is feldspar, which can be found in granite changing to clay. When it rains, water seeps down into the ground and comes in contact with granite rocks. The feldspar crystals within the granite react with the water and are chemically altered to form clay minerals, which weaken the rock.

OxidationAnother type of chemical weathering is oxidation. Oxidation is the reaction of a substance with oxygen. You are probably familiar with oxidation because it is the process that causes rust. So just like your car turns to rust through oxidation, rocks can get rusty if they contain iron.You may have noticed that rusted metal on your car is somewhat fragile; you could even poke your finger through a rust patch if it's big enough. This is because when iron reacts with oxygen, it forms iron oxide, which is not very strong. So when a rock gets oxidized, it is weakened and crumbles easily, 

DISSOLUTION AND CARBONATION

Dissolution and carbonation[edit]A pyrite cube has dissolved away from host rock, leaving gold behindRainfall 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 sulfur 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 solubility (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. Carbonation is therefore a large feature of glacial weathering.The reactions as follows:CO2 + H2O → H2CO3carbon dioxide + water → carbonic acidH2CO3 + CaCO3 → Ca(HCO3)2carbonic acid + calcium carbonate → calcium bicarbonate

Erosion and TransportErosionErosion is the process by which soil and rock particles are worn away and moved elsewhere by gravity, or by a moving transport agent – wind, water or ice. 

Transport refers to the processes by which the sediment is moved along – for example, pebbles rolled along a river-bed or sea shore, sand grains whipped up by the wind, salts carried in solution.

EROSION BY GRAVITY

Mass–wasting is the down-slope movement of loose rock and soil due to gravity. Whilst processes such as soil creep are very slow, landslides can be very sudden, and may cause loss of life.

Mass-wasting is often aided by water, so landslides and mudflows frequently occur after heavy rain

EROSION BY WINDWind erosion and transport is a serious environmental problem in the driest parts of the world, removing soil from farmland and covering whole towns with sand and dust.

A strong breeze (wind speed 20 km/h) can easily pick up dry dust and fine sand. The lightest particles are swirled up into the air and carried in suspension for hundreds of kilometres, whilst sand grains bounce along the surface by saltation. Small pebbles can be moved along with the sand, a process called creep.

EROSION BY WATERAround the world, moving water picks up and transports millions of tonnes of sediment every day, along rivers, coasts, and even in the deep oceans.Sediment, whether picked up by flowing water or by waves, is moved along in one of four ways: Traction is the rolling or dragging of large grains along a river bed or shore, aided by the push of the smaller grains (below).Saltation is the bouncing of sand grains as they are picked up, carried along, and dropped repeatedly by flowing water. Fine particles (silt and clay) are carried in Suspension in the water – they will only settle out if the water is still. Soluble salts are carried in Solution in the water – the sea is obviously salty, but rivers contain dissolved salts, too.

EROSION BY ICE OR GLACIER

A glacier is a river of ice formed from compacted snow. They are most common in the Polar regions (e.g.Antarctica) and in mountain regions like the Alps and Himalayas. Glaciers move slowly down valleys (at speeds up to a few metres per day), but have enormous erosive power. Glaciers erode the surface of the Earth.