3.2 Weathering and Rocks Flashcards
What is weathering?
Weathering is the decay, disintegration and decomposition of rocks in situ.
Rocks become adjusted to their surface environments - the products cover the surface as the regolith and form new rocks. Regolith is the layer of unconsolidated dust and rock over the layer of bedrock.
What are some features recognised from it?
Weathering is essential to landscape evolution.
Minerals are formed under high pressure and temperatures as when they cool they become stable
Weathering is irreversible, e.g. when fragments go into plastic or elastic states
Changes the volume, density, grain size, surface area, permeability, consolidation and strength.
Prepares for erosion, creates new landforms, rocks, minerals and solutions.
Can remove, transport, concentrate or consolidate minerals and salts
What are the types of mechanical weathering?
Freeze thaw: water in cracks and joints freeze at 0 in the night, expanding up to 10%. This stress exceeds the resistance of the rock and so the repeated process causes disintegration and fractures in the rock. It is more effective with frequent fluctuations above and below the freezing point - in periglacial and alpine regions, as the strength is determined by more cycles rather than intensity. The rocks disintegrate into scree and felsenmeer (block fields) which are fragments of rock.
Thermal fracture: Large diurnal temperature ranges cause the expansion of rocks in the day (40) to contract at night (just above 0) in deserts. This causes stress fractures and exfoliation which peels at the rock. Exfoliation requires the presence of water, leading to two types of disintegration:
Block disintegration: produces rock fragments in strong deserts and scree slopes at cliffs, etc.
Granular disintegration: breaking down into sand and grain, rates vary depending on colour, size, type and axes of mineral.
Salt crystal growth: in areas of 27 degrees sodium sulphate and carbonates can expand up to 300%. These salts in cracks and pores can expand and when this is evaporated salt water leaves salt crystal behind which can expand and disintegrate rocks. They are common in desert regions as high temperature, low rains allow salt to accumulate below the surface.
Dilation/unloading: overlying rocks being removed by erosion lead to the removal of weight. The upper parts expand, causing cracks or joints at right angles to the surface, creating lines of weakness in the rock. These may be called pseudo bedding planes which vary with distance from the surface.
Vegetation root action: Roots penetrate soil and help break up the underlying bedrock by pressure exerted in the cracks and fissures. As seeds germinate and roots grow, the rock becomes more damaged.
What are the types of chemical weathering?
Hydrolysis: Rocks with feldspar such as granite react with acid water and form clay, silicic acid and potassium hydroxyl. The acid and hydroxyl are removed, leaving behind kaolin (clay), solution occurs as hydroxyl is carbonated. Other minerals in granite are left behind.
Carbonation: rocks with calcium carbonate such as chalk and limestone combine with dissolved CO2 in the rain (carbonic acid). This creates a calcium bicarbonate when it reacts with the rain, which is soluble and removed by water. The effectiveness of this is related to the pH of the water. Clay residue remains.
Hydration: wetting and drying causes the addition/removal of water molecules causing the expansion and destruction of rocks. Usually in anhydrite which becomes gypsum and when expanding by 1600%
Oxidation: rock minerals react with oxygen and loses electrons. When minerals in rocks oxidise, they become less resistant to weathering. This commonly happens in iron and rocks containing iron, known as rusting.
What effects weathering rates?
Climate: Summarised by Peltiers diagram for the relationship between rainfall and temperature. The rate of chemical weathering rises 3x for an increase in 10 degrees due to freeze thaw and salt crystal falling and moisture rising. The depth of weathering also depends on the climate. Chemical action is restricted by cold temperatures although carbon dioxide is more soluble in the cold so carbonation may occur.
What is Peltiers diagram and the depth and climate diagram?
Peltiers diagram shows that at high rainfall, high temperature there is stronger chemical weathering. As rainfall and temperature fall, chemical weathering falls and mechanical weathering occurs as temperature and rainfall continue to fall
The depth and climate diagram shows the amount of precipitation and temperature at different climates and the effect of this on weathering. The most occurs at humid tropical areas, slightly more and temperate areas and least at tropical deserts and polar regions.
How do weathering rates change depending on climate?
Polar areas: freeze-thaw erosion more, depending on cycles. Chemical action is low but carbonation may occur as CO2 is more soluble. Frost is important and can lead to granular disintegration. There is less hydraulic action but high moisture so there is more hydration. Low rates of weathering.
Temperate: Minimal physical, significant chemical due to wet climate, fluctuations. Organic content levels are relatively high.
Arid/semi arid: slowest due to lack of moisture no chemical weathering although may be granular disintegration and thermal fracture due to the diurnal temperature ranges. May be salt crystal growth though lacks water. Evaporation exceeds precipitation
Humid tropical: fastest rates - 40m regolith. Accelerated by hot, moist conditions i.e. ionisation of water, hydrolysis, carbonation, high organic content
How does rock type affect weathering rates?
Chemical composition, nature and joints all factor in.
Sandstone: granular disintegration, weak faults, prone to carbonation but quite resistant
Shale: clay resistant to chemical weathering, hydration, easily weathered and eroded by mechanical weathering
Limestone: carbon ions removed in solution
Granite: high hydrolysis rates
Basalt: rich in hydrolytic minerals - joints allow water penetration
How does rock structure affect weathering?
Joints and bedding planes allow water penetration, increase physical and chemical weathering and can control size. Porosity and permeability can also lead to this.
Coarse rocks have more pore space and so erode faster. Fine rocks have larger surface areas so erode faster
How does vegetation affect weathering?
More vegetation leads to more chemical weathering due to release of acids and CO2 increasing carbonation. This is due to respiration.
Mechanical weathering decreases as thermal insulation decreases the thermal and frosting effects.
Biological weathering will also increase through growth of plants and roots into joints and along bedding planes and wedge rocks apart
How does rock composition affect weathering
Dark coloured rocks erode faster than pale rocks and minerals due to albedo
Examples may include basalt and feldspar opposed to granite and quartz
How does relief affect weathering?
If too shallow, removal may not occur. If too steep, water will just flow over the surface. Rainfall is also higher in upland areas where it is colder so more freeze/thaw. Accumulation of water at the bottom may also provide water for chemical weathering.
Aspect is also important as it determines if it is facing the sun or not which may decrease freeze/thaw cycles.
How does human activity affect weathering?
Pollutants lead to more carbonation and hydrolysis.
The removal of vegetation may also lead to a rise in chemical and biological weathering e.g. through less organic acids.
