While not perhaps as dramatic as an earthquake or a volcanic eruption, the processes of weathering have shaped the earth over millions of years, bringing rock up-thrust from within the earth into equilibrium with its new surroundings. The students need to know about the Peltier diagram, and have details on the main weathering processes. Weathering is the breakdown of rocks IN SITU. The breakdown of rock by physical and chemical processes.
A lot of the work on Earth Systems is concerned with the ROCK CYCLE, and the basic RECYCLING OF THE EARTH'S CRUST.
Geographical features resulting from Weathering include TORS (there were several theories for their formation including CHEMICAL Weathering and PERIGLACIAL activity).
Tors are normally associated with the granite landscapes of Dartmoor, but they can also be found in Charnwood, Leicestershire, and there is a good discussion on the CHARNWOOD TORS here. (Thanks to Mike Lewis of the Brixworth site for producing these pages....) GRAVESTONES tend to be a useful case study for weathering, and also featured in a recent 'AS' Advanced Geographical Skills paper. You have to be careful with health and safety however, and this has tended to halt this sort of activity. There also has to be a certain amount of caution when dealing with such an environment as people may get offended.
An important aspect of the work on Weathering is the PELTIER DIAGRAM. Climate is one of the main controlling factors of the type and rate of weathering. This relationship between temperature and the availability of water is covered in the PELTIER diagram, which can be found in most AS level textbooks. You need to be familiar with this.
Speed of a chemical reaction increases 2 and a half times with each rise in temperature of 10 degrees Celsius.
PELTIER: an American physicist and climatologist can predict the rate and type of weathering from mean annual temperatures and mean annual rainfall. Chemical weathering is most intense in warm, wet climates. High temperatures promote chemical reactions and heavy rainfall provides the necessary moisture.
An important aspect is the issue of SALT. This is very corrosive. It can corrode cars of course, but also concrete, and alternatives to rock salt are being tried by some councils. I use a resource which considers salt as being a hidden menace.
Could also use RAHN's INDEX OF WEATHERING for gravestones. This shows the various extent to which the stones have been weathered. This is dependent on lithology (rock type), exposure and the time that the stones have been standing.
|
1 |
Unweathered |
| 2 | Slightly Weathered - faint rounding of corners of letters |
| 3 | Moderately Weathered - rough surfaces, letters legible |
| 4 | Badly Weathered - letters difficult to read |
| 5 | Very badly Weathered - letters almost indistinguishable |
| 6 | Extremely Weathered - no letters left, scaling |
CASE STUDY OF WEATHERING
A good example of weathering in action was featured on this BBC news report. It describes the loss of the state symbol of New Hampshire in the USA: a rock formation shaped like a face and called 'The Old Man of the Mountain' - except in the first week of May it fell off. The profile was made out of a series of granite ledges and the bottom section succumbed to the elements. All that's left is a scar rather than the face which once inspired a famous children's story by Nathaniel Hawthorne.
A reminder that nothing lasts for ever. Apparently the OLD MAN OF HOY once had 2 legs rather than being one pillar, and OLD HARRY on the Studland coast of Dorset had a wife, and was also rather stouter in his youth - but then weren't we all...
WEATHERING LESSON PLAN
Defn: "The breakdown and alteration of rocks and minerals at or near the earth’s surface." Occurs because the rocks are not in equilibrium with the environment they find themselves in: different from when they were formed. General term is denudation (includes erosion processes)
PHYSICAL – breakdown of minerals, which remain the same
Block / Granular disintegration – result is blocks of the parent material, or quartz rubble: increases surface area susceptible to attack.-
- Freeze / Thaw action: effect of water in cracks, freezing and expanding in volume by around 10% (in fact 9.05%) - requires cracks and diurnal variation around freezing (force of 150 tons, square foot) – temperatures down to around –22 degrees Celsius – produces angular scree / talus e.g the Great Stone Chute in the Cuillin Hills, Skye, or Wastwater Cumbria.
- The hills which have been affected are left shattered with arêtes and castellation.
- Thermal – exfoliation: key element is changes in temperatures: diurnal range of 50 to 70 degrees Celsius possible in deserts - because rocks are relatively bad conductors of heat only the outside few millimetres are affected. Fragments which break off are platy. Good example is Half Dome in Yosemite N.P, USA, (also known as ‘onion skin’ weathering)- Also known as differential expansion as different colour minerals expand and contract at different rates.
- Assisted by dilation: expansion of rocks (doming) by removal of overburden e.g melting of ice sheets
- Salt weathering: presence of saline solutions – causes crystals to grow, particularly in shady conditions – causes flaking of surface or in small weathering pits – faster in marine locations
CHEMICAL WEATHERING – results in the alteration of the chemical composition of the weathered material due to a reaction which alters:
- the composition of rock minerals
- the volume of the rock
- the strength and coherence of the rock
This is a more selective process, only affecting certain minerals.
Tends to be concentrated at the rock surface or along joints and bedding planes. Block weathering and granular disintegration can also be the result of chemical weathering.
3 products:
- secondary minerals
- resistant minerals e.g quartz
- soluble products
Result of weathering is known as regolith, and in this instance will be composed mainly of unweathered / insoluble residues such as quartz sand and pebbles.
Chemical weathering is capable of penetrating more deeply into the rock than physical weathering. Particularly effective where rock is alternately wetted then dried: for example with seasonal fluctuation in the water table.
Already had an example of this with Carbonation: limestone altered to calcium bicarbonate which is taken into solution and reprecipitated as tufa, or calcite to form stalactites, stalagmites and helictites.
Produces hard water and results in a hummocky terrain known as karst scenery (after a region in former Yugoslavia)
A key element in weathering is the presence of water. Solution / dissolution obviously occurs, also acts as a medium for transporting acids etc.
Hydration: affects rock minerals which have the capacity to take up water. They increase in volume, which sets up stresses within the rock e.g conversion of iron oxides to iron hydroxides. Causes surface flaking, similar to salt weathering.
Hydrolysis: a complex reaction affecting minerals in igneous or metamorphic rocks e.g feldspar in granite – known as rotting – produces potassium hydroxide and alumio-silicic acid. Former is carbonated and removed in solution. A-S acid breaks down into clay minerals, notably kaolinite (china-clay) removed in solution. Produces residual clays.
feldspar + hydrogen ions + water
à clay + dissolved ions4KAlSi3O8 + 4H+ + 2H2O à Al4Si4O10(OH)8 + 4K+ + 8SiO2
Also known as spheroidal weathering, as it rounds off corners and affects statues, gargoyles etc. particularly affected
Oxidation: reactions between rock minerals and oxygen (usually dissolved in water) – changes colour on brickwork
4Fe + 3O2 = 2Fe2O3 (iron oxide or hematite)
also oxidation of pyrite
e.g. FeS2 + O2 + H2O = FeO(OH) + H2SO4
Iron oxides / regolith are red, orange, or brown in color e.g Broken Hill, Australia
Also occurs when mafic basalts are weathered. Can result in water pollution e.g from mine tailings (obvious from water colour)
Reactions speeded up by warmth. Chemist van’t Hoff said speed of reaction increases 2½ times by rise of 10 degrees Celsius, so chemical weathering greater in humid tropical climate. Not always the case: cold water holds more carbon dioxide which speeds up carbonation, and hydration is active in periglacial areas.
Also need to consider importance of evaporation and wind.
BIOLOGICAL – effect of living things
Tree roots: as a tree grows, its roots are extended into the ground. As they grow and thicken, rocks are prised apart. Ivy growing on a building can loosen the bricks: block disintegration. Also occurs on a slower, smaller scale by mosses and lichens.
Decomposition produces humic acid and these can result in the process of Chelation: break down of rock minerals by organic acids. Can also be produced by excreta: e.g weathering of rock occupied by large colonies of seabirds such as gannets. (also affect car paintwork)
Respiration by plant roots increases carbon dioxide in soil and assists the formation of weak acid as rainwater filters through the soil. Trees extract water from soil which can lead to shrinkage and ground subsides.
Burrowing animals e.g moles, fauna break rock, and bring material to the surface where it is exposed to chemical weathering.
Elephants trampling vegetation in game reserves in Zambia and Namibia have led to soil erosion which exposes bedrock which is then affected by weathering.
Crustaceans on rocks at the coast e.g paddocks bore holes in rocks and secretions of shellfish increase rate of erosion.
WEATHERING RATES
Processes of weathering do not occur uniformly over the surface. Main condition affecting the rate is the CLIMATE: in particular 2 factors:
- temperature (absolute values, and seasonal / diurnal fluctuations either side of zero degrees celsius)
- precipitation (type and amount) – because of importance of water in most weathering
Has led to definition of several morphoclimatic regions. Developed since 1950 when L. C Peltier identified 9 such regions:
glacial
periglacial
boreal
maritime
selva
moderate
savanna
semi-arid
arid
See the Peltier diagram which summarises the effect of changes in temperature and rainfall. Remember that climate has altered over time, and varies with altitude.
Can’t therefore conveniently divide up the world.
Illustrated by graveyards. Degrees of weathering can be compared by looking at gravestones and vary greatly.
Other factors influencing the rate of weathering:
ROCK TYPE
Rock strength and hardness
Minerals formed under high temperature and pressure weather most quickly because they are ‘furthest’ from the conditions in which they were formed. Some rocks are ‘harder’ than others.
Chemical composition
The presence of silicate minerals is important. More stable minerals tend to be lighter in colour.
Rock texture
ie. coarse grained or fine grained. In finer grained rocks, the bonding is stronger, but the boundaries between crystals form lines of potential weakness or cleavage. Fine grained rocks often weather quicker.
Joints and bedding planes: allow access by water: limestone is said to have a massive structure.
Vegetation type
Topography: slope angles, and aspect
Useful addresses:
http://www.gpc.peachnet.edu/~pgore/online/physical2.php
http://www.dartmoor-npa.gov.uk/dnp/factfile/tors.pdf
QUESTION
1. "Climate is the most important factor affecting weathering." How
much do you agree with this statement ?
[ 8 marks ]
This critical field of study relates to changes in the shape of the earth. There are some well developed sites for this, which touch on weathering and slopes. Try the LEICESTER UNIVERSITY site, or the English-Polish DYNAMIC GEOMORPHOLOGY.
There is an online lecture on WEATHERING and MASS MOVEMENT at University College, Worcester. I remember a few drunken nights there in the mid 1980's when it was plain old Worcester College. There are links to images related to weathering, and other related 'lectures'.
WEATHERING NOTES
Weathering of Limestone
Limestone is largely made up of calcium carbonate in the form of the mineral calcite. Limestone weathers by the solution of this calcite. This occurs along the joints and bedding planes which are present due to the sedimentary nature of the rock. This causes the surface to become uneven, producing grooves along the surface called lapies or karren. When these develop further they are known as grykes, separating raised sections called clints. (Remember that Clint (Eastwood) is always left standing after a gunfight...)
Sink holes may also form as the ground subsides due to the removal of limestone. These are called dolines, and may be marked on the map as shake holes.
The widening of joints means that more water will end up being drained underground rather than on the surface, and this will then develop cave systems and pot holes. Rocks lying on the surface which are of different geology may end up protecting the limestone beneath them from solution, and end up as perched blocks (the Norber erratics are a good example of this...)
WEBSITES
For details on the geology underlying the scenery of the UK, visit the ESSENTIAL GUIDE TO ROCKS. There's also an opportunity to BE A MOUNTAIN BUILDER here.
An early theory of weathering was proposed by W.M. Davis: for some diagrams of the cycle of erosion go HERE. For an article on the great man himself go HERE.
Go HERE for some cool animations of weathering processes. Try the snails, and chelation. They might give you some ideas of your own. They were made using a programme called KOOL MOVES: a free version is available to download from the website, as is a free handbook and tutorials on how to use it - I've done the download. It promises to be rather fun getting to know it. Just wish I had the time.. Recommended, and well done to Noel once again.
Try EARTHSCIENCEEDUCATION for some useful information on the Dynamic Earth and the Rock Cycle. The site is run by the Earth Science Education Unit at Keele University.
Pamela Gore's WEATHERING site is also good (thanks to Miss Ellis for pointing out that the URL had changed...)
There is an excellent set of materials and resources at the JESEI site. This is the Joint Earth Science Education Initiative, launched at the annual meeting of the Association for Science Education. There are materials from a whole range of organisations. The details of various Earth Science areas are listed on the site. Recommended.
Some of the materials are listed below (check them out on the JESEI site...)
Carbon Cycle
Weathering: Limestone
The Earth's Crust: thinner than you think
Earthquakes & Plate Tectonics
Formation of Sedimentary , Metamorphic and Igneous Rocks
Weathering of Gravestones: an Investigation (Excellent!)
and many more....
