3.2 Rocks and weathering: weathering

Question 1

Define weathering

Answer 1

Weathering is the gradual breakdown or decay or rocks in situ (in their original place) at or close to the ground surface.

Question 2

Define physical (mechanical) weathering

Answer 2

Physical (mechanical) weathering: is the mechanical breakdown of rocks largely du to temperature change. The chemical composition of the rocks remains unaltered.

Question 3

What is freeze-thaw weathering

Answer 3

When water freezes, it expands its volume by 9%. If it freezes within a rock, freeze-thaw weathering can take place. Freeze-thaw weathering occurs in rocks which have cracks in them, are jointed, or have bedding planes that allow water to enter the rock. The climate must allow temperatures to fluctuate below and above freezing.

A long series of cycles of water freezing and thawing in the cracks can lead to the separation of blocks of rock producing landscapes filled with loose blocks of rocks called block fields (sometimes called felsenmeer). It takes place when water gets into small spaces and cracks in a rock. If it then freezes and expands it can put enormous pressures of up to 2100kg/cm squared on the rock.

Most rocks can only take pressures of about 500kg/cm squared before they split apart. Where bare rock is exposed on a cliff or slope, fragments of rock may be forced away from the face. Freeze-thaw is most effective where temperatures frequently fall below freezing (ie daily/diurnally rather than seasonally) and where there is an availability of water, therefore temperatures should not generally be below -5 degrees to -15 degrees; it it does, there is unlikcley to be any water present as it will be frozen permanently. Therefore this process tends to be most active in polar periglacial areas and in mountainous alpine areas.

Frost shattering is another type of freeze-thaw weathering which takes place in porous rocks, such as chalk. These rocks absorb water, but can also break off in sheets. When the water freezes within these rocks the process is powerful that the whole rock can fall apart and shatter.

Question 4

What is heating/cooling

Answer 4

Heating/cooling weathering is most effective in dark crystalline rocks, which absorb heat. This process requires a wide diurnal temperature range, ideally from greater than 25 degrees down to 0 degrees. Therefore, the clear skies and high temperature range of tropical hot deserts are prime locations for this process.

As rocks are poor conductors of heat, this leads to the surface of the rock expanding, causing granular disintegration as the surface breaks up into small grains of rock or block disintegration as larger blocks or plates of rock are detached from the main body of rock.

Question 5

What is salt crystal growth

Answer 5

This process takes place in areas with temperatures of around 26-28 degrees, when two types of commonly occurring salt – sodium sulphate and sodium carbonate – expand in size by up to 300%. When salt is dissolved in water (brine), evaporation will leave behind salt crystals which attack and disintegrate the surface of the rock.

Question 6

What is pressure release (dilation)

Answer 6

This process takes place when heavy overlying rocks are removed by erosion. The release of this pressure causes the exposed rocks to expand which may cause them to split apart parallel to the rock surface. The removal of melting ice sheets ad glaciers from an area is a primary cause of pressure release.

Question 7

What is vegetation root action

Answer 7

Roots can grow into joints and bedding planes where there may be moisture and washed in soil. Seedlings may grow in such areas. In both cases extreme pressure can be exerted by the growth of roots leading to, or adding to, mechanical weathering break up.

Question 8

Define chemical weathering

Answer 8

Chemical weathering: is the breakdown of rocks through chemical changes to the composition of the rock.

Question 9

What is hydrolysis

Answer 9

Hydrolysis is a complex chemical process involving water that affects the minerals in igneous and metamorphic rocks. It is particularly active in weathering the feldspar found in granite whereby material is removed in solution leaving a clay, called kaolinite/kaolin or China clay.

Question 10

What is hydration

Answer 10

Hydration – affects minerals that have the capacity to take up water. As they absorb/soak up the water they expand and change their chemical composition which often makes them weaker and less resistant to erosion. For example, gypsum changing into anhydrite.

Question 11

What is carbonation

Answer 11

Carbonation – is particularly associated with limestone and chalk. Precipitation falling through the atmosphere absorbs carbon dioxide to form a weak carbonic acid. This acidulated rainwater will react with any rock containing calcium carbonate (mainly the many different types of limestone) converting it to calcium bicarbonate which then dissolves. Carbonation is more active in cold conditions, as cold water can hold more carbon dioxide.

Question 12

Why do different types of weathering occur in different climatic zones?

Answer 12

Physical Weathering
- Frost shattering is important in a climate where temperatures fluctuate at around 0 degrees Celsius but if a climate is too cold, or too warm, or too dry, or too wet (covered by vegetation) it will not operate.

Chemical Weathering
- This increases as temperature and rainfall totals increase. The rate of chemical weathering (around about) doubles with every 24C temperature increase.
- Recent theories suggest that in humid tropical areas, direct removal by solution may be a major factor in the lowering of landscape, due to the continuous flow of water through the soil.
- Chemical weathering is strong in warm moist climates e.g. rainforests.

Question 13

How does climate influence the type and rate of weathering

Answer 13

Climate – acts as the main overall influence on weathering, affecting the rate (speed) and the type of weathering. This will in turn influence the thickness of the weathered material found on a slope – the regolith.

It also affects slope transportational processes – the processes that move weathered material down a slope – including rainwash and a wide range of mass movements. It plays an important role in both physical and chemical weathering and it also supplies water for chemical processes and influences the speed of chemical reactions.

Temperature and precipitation control the type and speed of weathering processes. For example, in physical weathering, water is required for freeze-thaw as well as temperatures that move above and below freezing. In exfoliation, high temperatures are needed for the rock surfaces to expand and these need to fall for the surface to contract.

Chemical weathering is most effective in areas with both high precipitation and high temperatures which speed up most of the process. The active retreat or lowering of a slope will be more pronounced in areas where chemical weathering is active, such as the humid tropics. Areas with little precipitation, such as the arctic tundra and deserts will have chemical weathering processes that are slow and relatively ineffective.

Question 14

How does rock type influence the type and rate of weathering

Answer 14

Rock type – refers to the composition of the rock and it influences both the rate and type of weathering due to 3 factors:

• Differences in the chemical composition of rocks. For example, limestone is composed of calcium carbonate which makes it very susceptible to chemical weathering through solution and carbonation.
• Differences in the cements that hold together sedimentary rocks. For example, some cements are iron oxide-based and are very susceptible to oxidation, whereas silica-based cements made of quartz are very resistant to oxidation.
• The presence or absence of pores (holes) will affect its vulnerability to weathering processes. Porous rocks (eg chalk) have a high proportion of cracks, while non-porous rocks (such as slate) do not.

Question 15

How does rock structure influence the type and rate of weathering

Answer 15

Rock structure – Such as bedding planes, joints, fault lines, and cracks that are present in rocks will often have a major influence on the angle of a slope. Earth movements can tilt these structures. The angle of a tilted rock is called its dip. This can also affect the susceptibility of a slope to mass movement. Where the angle of the dip is inwards, towards the slope, the slope will tend to be stable; if the dip is down a slope, the slope may fail and rock slides can occur.

Question 16

How does vegetation influence the type and rate of weathering

Answer 16

Vegetation can help prevent weathering in a few ways. One of the primary ways is by stabilizing soil and preventing erosion. When soil is eroded, it can expose underlying rock to the elements, which can accelerate weathering. Vegetation, particularly deep-rooted plants such as trees, can help stabilize soil and prevent erosion by anchoring the soil in place and absorbing excess water. This can help protect the underlying rock from weathering.

Additionally, some plants release organic acids into the soil through their roots. These acids can react with minerals in the soil and break them down, a process known as chelation. While this can contribute to weathering over time, it can also help to create soil that is more nutrient-rich and better suited for plant growth.

However, vegetation can also contribute to weathering through physical and chemical processes. For example, the roots of some plants can exert pressure on rocks, causing them to crack or fracture. Additionally, some plants release acidic compounds into the soil that can accelerate chemical weathering by dissolving minerals in the rock.

Finally, vegetation can help prevent weathering by providing shade and reducing the amount of direct sunlight that reaches the ground. Sunlight can accelerate weathering by causing thermal expansion and contraction of rocks, which can lead to cracking and fracturing. By reducing the amount of sunlight that reaches the ground, vegetation can help slow down this process.

Question 17

How does relief influence the type and rate of weathering

Answer 17

Relief – has an indirect effect on the local climate, and therefore on local processes. For example, in mountainous areas in temperate climates, freeze-thaw action also plays an important role, besides the dominant weathering processes. In these areas, the height of the land may cause freezing temperatures and freeze-thaw action on exposed rocks.

In areas of steep relief, slope processes, such as slides and flows, can expose rock to various processes of weathering. In contrast, in lowland areas, rock may be protected by thick layers of soil and weathered material. The accumulation of water at the base of slopes may also provide more water for chemical processes to take place.

The aspect of a slope may also affect rates of weathering. In the Northern Hemisphere, rates of physical weathering are greater on north-facing slopes (sometimes called ubac slopes), which experience longer periods of freezing temperatures having longer periods of time in the shadow and a lack of direct sunlight compared to south-facing slopes (sometimes called adret slopes). The opposite is the case in the Southern Hemisphere.