3.2 Weathering
What are the four main climatic regions?
Glacial/ periglacial Temperate Arid/ semi-arid Humid tropical
How does relief influence/ affect the type and rate of weathering?
Gradient of slope Steep slopes - less weathering because water may flow over the surface. Shallow slopes - less weathering because removal might not occur (weathered material needs to be removed for weathering to continue). Although water may accumulate on shallow slopes providing water for chemical weathering. Shelter or exposure/direction slope is facing May be temperature differences between south- and north-facing slopes. Only important if the temperature differences are around a critical temperature (e.g. 0 degrees C for freeze-thaw weathering). In northern hemisphere - rates of physical weathering greater on north-facing slopes - more freeze-thaw cycles due to lack of direct sunlight Slope processes e.g. landslides may expose, previously unexposed, bear rock that can be weathered
What are the weathering characteristics in glacial/ periglacial climatic regions?
Low precipitation Low temperatures Mechanical weathering - freeze-thaw weathering
What are the weathering characteristics in humid tropical climatic regions?
High rainfall often seasonal Long periods of high temperatures Moisture availability high Intense deep weathering Organic content high but decomposition high High precipitation Long periods of high temperatures Strong chemical weathering
How does climate influence the type and rate of weathering?
Physical weathering - occurs more in cold climates In general, frost-shattering increases as the number of freeze-thaw cycles increases. Chemical weathering - increases as temperatures and rainfall increase - rocks in hot and wet climates experience faster rates of chemical weathering than do rocks in cold, dry climates.
What are the weathering characteristics in temperate climatic regions?
Precipitation and evaporation fluctuate Mechanical and chemical weathering occur Chemical weathering more dominant Increased precipitation, lower temperatures, reduced evaporation Organic content moderate to high, breakdown moderate Moderate precipitation Temperatures change More chemical weathering (physical weathering occurs as well)
How does rock composition influence the type and rate of weathering?
Rocks formed of resistant minerals - weather slowly Granite is resistant to weathering because it has resistant minerals such as quartz, muscovite and feldspar. Rocks formed of weaker minerals - weather rapidly Minerals which form at higher temperatures are the least stable at surface temperatures - minerals in dark-coloured rocks (basalt) weather faster than those in pale-coloured rocks (granite).
What is weathering?
The decomposition and disintegration of rocks in situ (not moving). Decomposition refers to the chemical process and creates altered rock substances whereas disintegration or mechanical weathering produces smaller, angular fragments of the same rock.
What are some examples of other biological effects which assist chemical processes?
The release of humic acids by decaying vegetation - encourages hydrolysis. The release of carbon dioxide by plants - encourages carbonation. A blanket of vegetation traps water - encourages a variety of chemical processes.
How does vegetation influence/ affect the type and rate of weathering?
2 main ways vegetation weathers rocks Chemical - secretion of organic acids Biological/physical - growth of roots Influence of vegetation is linked with: type of climate and nature of soil - moisture content - root depth - acidity of humus More vegetation = an increase in the rate of chemical weathering Increased level of carbon dioxide from plant respiration forms carbonic acid when dissolved in water and increases rates of carbonation. More vegetation = a decrease in the rate of physical weathering Thermal insulation of the vegetation decreases frost action and thermal effects. More vegetation = increases direct biological weathering Growth of plant roots into joints and along bedding planes and wedging rocks apart.
What is hydrolysis?
A chemical reaction between the minerals found in the rock, forming a weaker material. Example 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.
How does rock texture influence the type and rate of weathering?
Course-grained rocks - weather faster than fine-grained rocks - have more air spaces, more permeable Fine-grained rocks - greater surface area for weathering - highly susceptible to weathering Igneous rocks are more resistant to physical disintegration than sedimentary rocks - greater strength of interlocking crystalline textures in comparison with granular ones
What is the difference between weathering and erosion?
Erosion is the process by which soil and rock particles are worn away and moved elsewhere by wind, water or ice. Weathering involves no moving agent of transport.
How does climate influence the type and rate of weathering? What do Peltier's diagrams show?
How weathering is related to moisture availability (average annual rainfall) and average annual temperature. It shows how the relationship between average annual rainfall and average annual temperature influences the type of weathering. Different types and rates of weathering occur in different climatic zones because of the different temperature and amount of precipitation. The type and rate of weathering vary with climate. Rainfall and temperature can affect the type and rate at which weathering occurs.
What is vegetation root action?
It is when the roots and seeds of trees, grasses and other plants find their way into small natural cracks in the rock. When these roots and seeds grow, they exert pressure on the rock around them, causing the cracks to widen and deepen, eventually breaking the rock.
What is freeze-thaw weathering (frost shattering)?
It is when water continually seeps into cracks, freezes and expands, eventually breaking the rock apart. 1. Water enters cracks and joints in the rock. 2. When temperatures drop, the water freezes and expands (due to pressure) causing the crack to widen. 3. The ice melts and water makes its way deeper into the cracks. 4. The process repeats itself until the rock splits entirely. Occur in environments where moisture is plentiful and there are frequent fluctuations about and below freeing point e.g. periglacial and alphine regions.
What are the differences between limestone and granite?
Limestone - sedimentary rock - formed by sedimentation - made up of calcium carbonate - contains at least 50% calcium carbonate - mainly white, although it may be tinted by impurities - easily weathered - dissolve in weak acids - strength is more varied - made from dead bodies of millions of tiny sea creatures - formed with calcite and aragonite - does contain planes of weakness Granite - igneous rock - forms from magma that cools and solidifies after a volcanic eruption - grainy appearance and can be pink or varying shades of grey - between 20% and 60% of the volume of the rock is made by quartz and 35% feldspar - weathers slowly - hard for water to penetrate - impacted greatly from chemical weathering (hydrolysis) and mechanical weathering (freeze thaw) - consists mainly of quartz, mica, and feldspar - quartz is very resistant to weathering and this forms within the rock - rock creates a tor - an isolated mass of bare rock - overall process of cooling takes a long time - rock is usually found in large deposits - does not contain planes of weakness
How does rock type influence the type and rate of weathering?
Limestone (sed.) Carbonate minerals weathered by carbonation. Insoluble clay minerals left as a residue. Joints - allow water penetration and deeper weathering Granite (ign.) Quartz and muscovite in granite - resist chemical weathering - left as a residue. Feldspar and other minerals - broken down by hydrolysis to produce clay minerals. Joints - allow water penetration and deeper weathering Sandstone (sed.) Granular disintegration Resistance depends on the nature of the sandstone, e.g. Quartz - most resistant CaCO3 - prone to carbonation Joints and bedding planes - provide lines of weakness for physical and chemical weathering Shale (sed.) Clay minerals in shale are weathered products of other rocks - resistant to chemical weathering Slate (met.) Similar chemical reactions to those in shale Resistant to chemical weathering Cleavage may allow water penetration and allow freeze-thaw action to produce 'flat' scree fragments Basalt (ign.) Rich in minerals - less resistant to decay by hydrolysis Highly weathered Joints allow water penetration encouraging chemical and physical decay leading to block disintegration Dolerite (ign.) Same chemical weathering as basalt Spherodial weathering - controlled by the joint pattern and is a sub-surface process
What are the weathering characteristics in arid/semi-arid climatic regions?
More evaporation than precipitation Rainfall low Temperature high, seasonal Organic content low Mechanical weathering, salt weathering, granular disintegration, dominant is direst areas Low organic input compared to decomposition Semi-arid areas - increased precipitation, decreased evaporation Low precipitation High temperatures, seasonal Slight chemical weathering
How does rock structure influence the type and rate of weathering?
More weaknesses (joints, bedding planes, cleavage) = an increase in physical and chemical weathering - allows water to penetrate through the rock Weaknesses control the size and shape of the weathered fragments. Examples Dolerite - spheroidal weathering Granite - formation of tors and scree slopes
What are the three different types of weathering?
Physical (mechanical) - rocks breaking apart due to rain, wind, or temperature Freeze-thaw weathering (frost shattering) Heating/ cooling (disintegration/exfoliation) Salt crystal growth Pressure release (dilatation) Chemical - a process which changes the composition of rocks (changes the molecular structure of rocks) Carbonation Hydrolysis Hydration (and dehydration) Biological - breakdown of rock caused by living/ once-living organisms such as plants and animals Vegetation root action
What is pressure release (dilatation)?
This is the process whereby overlaying rocks are removed by erosion causing underlying rocks to expand (because pressure has been released) and fracture parallel to the surface (pseudo-bedding planes). The removal of a great weight such as a glacier will have the same effect. Where horizontal pressure is released by rock falls on cliff face, vertical cracks may develop. This process can be caused by human activities such as quarrying.
What is salt crystal growth?
This occurs in two ways: In areas where temperatures fluctuate around 26-28 degrees C, soduim sulphate and sodium carbonate expand by 300%, putting pressure on the rock causing it to crack. When water evaporates, salt crystals may be left behind. As the temperature rises, the salts expand and exert pressure on the rock causing it to crack. Frequent in hot desert regions where there is rainfall and high temperatures.
What is heating/cooling (disintegration/exfoliation)
This process is found in hot deserts where there is a large diurnal temperature range. Rocks heat up and expand during the day and cool and contract by night. As rock is a poor conductor of heat, stresses occur only in the outer layers and cause peeling or exfoliation to occur.
What is carbonation?
This process occurs on rocks containing calcium carbonate, e.g. chalk and limestone. Rainwater contains weak acid (carbonic acid) which forms as the rain absorbs atmospheric carbon dioxide. Carbonic acid then attacks the carbonate materials. Calcium carbonate reacts with an acid water and forms calcium bicarbonate, or calcium hydrogen carbonate, which is soluble and removed by percolating water.
What is hydration (and dehydration)?
When minerals absorb water, expand and change (form a new substance). Wetting and drying can cause the addition or removal of water from the molecules of some minerals, causing expansion or contraction. During expansion, the volume of the rock increases, which can put stress on the rock and make it more vulnerable to other types of weathering (including physical weathering processes). Anhydrate converts to gypsum when exposed to water, which creates a volume increase that destroys any planar features in the beds. Anhydrate can be created when gypsum in dehydrated.
