Limestone Differential Weathering EarthCache

Limestone is a pale grey rock that covers a large area of the Niagara Escarpment.

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.

Diagram showing bedding planes and joints

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 (gaps), separating raised sections called clints. You are here to look at the differential effect of layers (strata) of rock that are of a difference hardness or resistance to the elements of weather: heat, cold and wet being three of the main ones.

What effects weathering?

A rock's composition has a great effect on its rate of weathering. Rocks that are softer and less weather-resistant tend to wear away quicker than those that are crystalline (igneous rock) or have been subjected to more heat and pressure (metamorphic rock). When the more weather-resistant rock is left behind, this process is called differential weathering.

A rock's exposure to the weathering elements and its surface area can affect its rate of weathering. Rocks that are constantly bombarded by running water, wind, and other erosion agents, will weather more quickly. Rocks that have a large surface area exposed to these agents will also weather more quickly. As a rock goes through chemical and mechanical weathering, it is broken into smaller rocks. As you can imagine, every time the rock breaks into smaller pieces its surface area or part exposed to weathering is increased.

Graph showing differential weathering rates

The above graph, a Peltier diagram, shows how weathering is affected by temperature and rainfall (precipitation). Loius Peltier, an American physicist in 1950 and climatologist in 1950, predicted the rate and type of weathering that would occur from mean annual temperatures and mean annual rainfall.

There are 3 major types of weathering: physical, chemical, and biological.

1. Physical weathering is the breakdown of minerals, which remain the same; there are 5 major subtypes:
a. Block or Granular disintegration results in blocks of the parent material falling from the cliff and often increases the surface area susceptible to attack.
b. Freeze-thaw action can effect water in the cracks in rocks, freezing and expanding in volume by around 10% (in fact 9.05%) where diurnal temperature variations include crossing freezing point.
c. Exfoliation happens when there are changes in temperatures with a diurnal range of 50 to 70 °C which is very possible in deserts. Because rocks are relatively bad conductors of heat, only the outside few millimetres are affected. Also known as ‘onion skin’ weathering or differential expansion as different colour minerals expand and contract at different rates.
d. Dilation is the expansion of rocks (doming) by removal of overburden such as happens when ice sheets melt.
e. Salt weathering happens in the presence of saline solutions and causes crystals to grow, particularly in shady conditions. This causes flaking of surface or in small weathering pits and is faster in marine locations.

2. Chemical weathering is most intense in warm, wet climates. High temperatures promote chemical reactions and heavy rainfall provides the necessary moisture. This results in the alteration of the chemical composition of the weathered material due to a reaction which alters:

a. the composition of rock minerals
b. the volume of the rock
c. the strength and coherence of the rock

It 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.

There are 3 major products if chemical weathering:

a. secondary minerals
b. resistant minerals e.g quartz
c. soluble products

The result of weathering is often known as regolith and is often composed mainly of unweathered and/or insoluble residues such as quartz sand and pebbles.

Chemical weathering is capable of penetrating more deeply into the rock than physical weathering and is particularly effective where rock is alternately wetted then dried. An example of this can be where there is seasonal fluctuation in the water table. Another example is Carbonation. This particularly affects limestone as it is altered to calcium bicarbonate which is taken into solution and re-precipitated as tufa, or calcite to form stalactites, stalagmites and helictites.

A key element in weathering is the presence of water. Solution/dissolution obviously occur, 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. This can cause surface flaking, similar to salt weathering.

Hydrolysis is a complex reaction affecting minerals in igneous or metamorphic rocks e.g feldspar in granite – known as rotting – and produces potassium hydroxide and alumio-silicic acid. The former is carbonated and removed in solution. The alumio-silicic acid breaks down into clay minerals, notably kaolinite (china-clay) and is removed in solution. This is also known as spheroidal weathering, as it rounds off corners and affects statues, gargoyles etc.

Chemist Jacobus Henricus van 't Hoff said that the speed of a reaction increases by 2½ times when the temperature rises by 10°C, so chemical weathering is greater in humid tropical climates in the main.

3. Biological weathering is the effect of living things such as tree roots: as a tree grows, its roots extend into the ground. As they grow and thicken, rocks are prised apart. Ivy growing on a building can loosen the bricks. It also occurs on a slower, smaller scale where mosses and lichens overlay rock.

a. Decomposition produces humic acid and this can result in the process of Chelation, the break down of rock minerals by organic acids. Humic acid can also be produced by excreta, especially where large areas rock are occupied by large colonies of seabirds such as gannets
b. 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.
c. Burrowing animals such moles break up the rock and bring material to the surface where it is exposed to chemical weathering. Crustaceans on rocks at the coast bore holes in rocks and secretions of shellfish increase rate of weathering
d. Trampling. Elephants trampling vegetation in game reserves in Zambia and Namibia have led to soil erosion which exposes bedrock which is then affected by weathering.

In order to log this earthcache, please email me the answers to the following questions using the information that you find on the cache page, and at the physical site. As always, pictures are more than welcome, however I asked that they do not container any spoilers.