Weathering …continued

What are the conditions required for physical weathering? RECAP: What are the conditions required for physical weathering? What kind of conditions would you expect to be required for chemical weathering? Where on this graph would you place physical/ chemical weathering?

Differences between Physical and Chemical Weathering

Chemical Weathering Oxidation, as the name suggests, occurs upon contact of the rock with oxygen, from the air or from water A common effect is the “rusting” of rocks containing iron, as blue-grey becomes reddish-brown when ferrous compounds are oxidised forming iron oxide hematite (Fe2O3) The chemical structure of the rock is altered by oxidation, making it more susceptible to other forms of weathering (4Fe+3 + 3O2 -> 2Fe2O3) 

Carbonation Carbonation is a result of the reactions of rainwater and carbon dioxide to produce carbonic acid (H2CO3), which slowly dissolves any rocks made of calcium carbonate, such as limestone, along lines of weaknesses Gases produced from emissions of carbon dioxide, sulphur dioxide and nitrogen oxide from power stations and car exhaust dissolve in water droplets in the atmosphere, forming dilute acids solutions which fall as weak acidic rain, dissolving limestone and sandstones in a similar way to carbonation. H2O + CO2 -> H2CO3  

Hydrolysis Hydrolysis is an exchange reaction involving minerals and water Free hydrogen (H+) and hydroxide (OH)- ions in water are able to replace mineral ions and drive them into solution As a result, the mineral's atomic structure is changed into a new form. It is a process whereby silicate minerals like potassium feldspar are weathered and a clay mineral is formed. 2KAlSi3O8 + 2H+ + 9 H2O -> Al2Si2O5(OH)4 + 4H4SiO4 + 2 K2+

Hydration Hydration simply involves the absorption of water into the existing minerals of the rock, causing the expansion of the mineral, leading to eventual weakening. It is less severe than hydrolysis

Chelation Chelation is a complex biochemical process, in which chelating agents are released from the decaying humus in the soil and cause a change in the chemical structure of the surrounding rock

Factors Affecting Weathering - A virtual fieldtrip…

Based on the details in the photographs, what are factors affecting weathering?

Factors affecting Weathering Climate Geology Relief Soil/vegetation cover Human activity

Climate Mechanical weathering Requires more specific weather conditions than chemical weathering They all need a constant fluctuation around 0°C, and extreme temperatures either side of this which prevent the constant freezing and thawing will not allow for much mechanical weathering to occur The presence of water is another importance; if the rock is too dry there will be no moisture to freeze in the pores, yet being too dry will lead to vegetation cover, the roots of which tend to hold the rock and soil together. The conditions required for physical weathering, with temperatures fluctuating around zero degrees and a medium level of rainfall, are those typically found in areas of high and middle latitude. Chemical weathering processes are all based upon a range of chemical reactions, each of which requires the presence of water. Since the rate of almost any chemical reaction is speeded up by an increase in temperature, (it is suggested that the rate of chemical weathering doubles with every 10⁰C temperature increase), it follows therefore that the warmer and more moist the environment, conditions typically found in areas of low latitude, the greater the rate of chemical weathering there.

Geology Mineral composition Rocks such as limestone and granite are particularly susceptible to weathering processes due to their mineral composition and thus experience relatively high rates of weathering. Physical structure of rock (presence of joints, bedding planes etc.) Provide lines of weakness which can increase weathering Plant roots are able to exploit cracks and joints in rocks through biological weathering, whilst freeze-thaw processes also exploit cracks, gradually forcing rock fragments apart In turn, the presence of joints, can make rocks, which are otherwise impermeable (e.g. granite), pervious, allowing water to enter the rocks and thus increasing the rate of chemical weathering within the rock The permeability of rocks is also affected by grain size and thus coarse grained rocks tend to experience greater rates of weathering

Relief Slope processes  exposure of previously unexposed, bare rock Lowland areas, unweathered rock may be protected by thick layers of soil and weathered material (although organic acids from denser vegetation may increase chemical weathering) Rainfall totals tend to be higher in upland areas and temperatures colder - again increasing rates of physical weathering such as freeze-thaw. Accumulation of water at base of slopes Temperature variations with respect to the aspect of different slope faces may also impact upon rates of weathering Northern hemisphere: rates of physical weathering are greater on north facing slopes

Soil/Vegetation Cover The presence of vegetation helps to increase rates of weathering. Direct biological weathering, through the growth of plant roots into joints and along bedding planes, wedging rock apart. Vegetation can also increase rates of chemical weathering  release of organic acids (important in processes such as chelation) Increased levels of carbon dioxide from plant respiration which forms weak carbonic acid when dissolved in water and increases rates of carbonation The insulating effects of vegetation can however in some instances reduce weathering rates E.g. By maintaining more constant temperatures (reducing the likelihood of thermal expansion) and therefore also reducing the temperature fluctuations which may have lead to freeze-thaw cycles.

Human Activity Humans have greatly increased rates of weathering by influencing concentrations of chemical pollutants in the atmosphere The increase in gases such as carbon dioxide, sulphur dioxide and nitrogen oxide has lead to the formation of acid rain, where these gases have formed acid in solution with water - e.g. carbonic acid. This acid rain, is able to more readily attack rocks such as limestone through carbonation and increases chemical weathering processes such as hydrolysis. Removal of vegetation (e.g. through deforestation) has conversely resulted in a decrease in chemical and biological weathering