1. Grey in colour contains fossils
Recap
Rock:
Grey in colour contains fossils
Fizzes with HCl
By which method of weathering is this most likely to have formed and why?

2. Investigating Weathering
Key Question: Why are some rocks affected by weathering more than others?
HW:
Complete rock description cards for igneous, metamorphic and sedimentary?
Key Words:
Lithology
Relief
Rock Structure

3. Process? Frost Shattering The Screes: Wastwater, Cumbria.
An example of a Scree Slope

4. Study These Carefully
They are factors that
Affect weathering

5. What factors affect weathering?
Joints, faults and bedding
Lines of weakness
1. Lithology
2. Rock Structure
Limestones / Granite = Fast
Dolerite / Quartzite = Slow
3. Temperature
5. Relief
Warm =
Faster chemical
processes
Steeper =
faster
What factors affect weathering?
6. Man
4. Rainfall
NOW
Explain each with examples.
Which are the most to least important (traffic lights)?
High = faster
7. Time
8. Vegetation
Longer = more time
For breakdown
Speeds up
Roots / humic acid

6. What factors affect weathering?
1. Lithology
1. Lithology
2. Rock Structure
2. Rock Structure
3. Temperature
3. Temperature
5. Relief
5. Relief
What factors affect weathering?
6. Man
6. Man
4. Rainfall
4. Rainfall
7. Time
7. Time
8. Vegetation
8. Vegetation

7. Why do rocks and minerals weather?
Because they are out of equilibrium with the conditions under which they formed
Minerals in granite originally formed at high temperatures and at considerable depth, typically >700°C and 5-15km depth
All silicate minerals except quartz are unstable at the earth’s surface and are trying to re-adjust to the new conditions

8. Practical: Investigating Weathering
Look at the different photos of weathering.
Interpret the evidence for physical, chemical and biological weathering.
In your books:
Give each type of weathering a sub- heading
Write a short sentence to recap how that type of weathering breaks down rocks
List pieces of evidence to look for on exposed rock surfaces for that type of weathering.

9. Mechanical/Physical Weathering
Leads to disintegration of the bedrock into smaller, angular, but chemically identical fragments
Results in an increase in the surface area of rock exposed for chemical weathering to act upon

10. Freeze-Thaw activity often leads to the formation of Scree Slopes
Scree in profile
Wastwater Screes
Lake District
Scree shows crude grading finer at top, coarser at the base
Produces Angular Fragments

11. Olivine basalt dyke showing Exfoliation or Onion Weathering
Thin sheets of rock peeling off like the layers of an onion
Contact between dyke and country rock
30cm

12. The action of tree roots widening joints and bedding planes
Biological Activity
The action of tree roots widening joints and bedding planes
Root growth in confined spaces can exert immense stresses within rocks and widen any natural lines of weakness
Burrowing animals such as moles and rabbits create natural conduits for water to reach the bedrock

13. Biological Weathering – Tree Roots Widen Joints/Faults in Rocks

14. Chemical Weathering Leads to the decomposition of the bedrock
Only quartz is unreactive and not affected
Results in the formation of clay minerals from the breakdown of silicate minerals such as feldspars, mica, augite and olivine
Ions are also released into solution

15. Chemical Processes
Hydrolysis
Carbonation
Biological

16. Hydrolysis Silicate minerals react with water
Clay minerals and ions in solution are produced
Orthoclase feldspar decomposes to kaolinite (china clay) and releases ions of potassium and silicon into solution
Biotite mica decomposes to chlorite and releases ions of iron into solution

17. Hydrolysis - Kaolinised Granite
Iron oxide staining due to release of Fe ions from biotite mica
Biotite mica breaking down to form chlorite
Orthoclase feldspar altered to kaolinite by hydrolysis
Unaltered grey, glassy quartz
Granite is very crumbly and is described as Growan

18. Residual quartz grains following kaolinisation of granite on Carn Brea
Tee peg for scale
These grains represent the first stage in the formation of a new sedimentary rock, a sandstone
Loose, angular quartz grains mainly 1–5mm in diameter
Any clay minerals such as kaolinite have been washed or blown away

19. Clay deposits on the floor of Las Canadas Caldera, Tenerife.
Hydrolysis
The products of hydrolysis are clay minerals such as kaolinite, illite, montmorillianite and serecite.
Clay deposits on the floor of Las Canadas Caldera, Tenerife.
The clay has been derived from the breakdown of silicate minerals in igneous rocks such as feldspars, augite, olivine and micas

20. Chemical Weathering of Basalt by Hydrolysis and Oxidation
Feldspar and olivine weathered to a mixture of clay minerals and iron oxides
2cm
Roadside cutting, Masca, Tenerife
Augite phenocrysts up to 8mm in diameter relatively unweathered

21. Weak carbonic acid pH 5.5 is capable of dissolving carbonate minerals
Carbonation
Rainwater falling through the atmosphere picks up carbon dioxide to form a weak carbonic acid pH 6.0
Water infiltrating into the soil picks up more carbon dioxide from the soil air
Weak carbonic acid pH 5.5 is capable of dissolving carbonate minerals
Limestones, made of calcite (calcium carbonate) are most susceptible to this process

22. The Effects of Carbonation
Stalactites represent calcite being re-precipitated from solution as Tufa
Large cave systems are often produced by carbonation as here in the Kango Caves, South Africa

23. The Effects of Carbonation
20cm
St. Mary’s Church forms part of the rear of Truro Cathedral, much of the original carvings in the limestone are badly affected by carbonation and most of the detail has been lost in places.

24. Biological-Chelation
Rainfall percolating through humus
becomes an organic acid.(eg. Humic Acid)
Organic acids or chelating agents attack clay
minerals, releasing iron and aluminium into the soil
Chelation is Greek meaning ‘to claw’
The chelating agents combine with the metallic ions
(Fe, Al) to form organic-metal compounds called chelates.
Chelates are soluble and are washed
down the profile to accumulate at depth

25. Biological Weathering
Moisture is trapped between the moss/lichen and the granite leading to more rapid weathering by hydrolysis
Car keys for scale
A skeletal soil begins to develop in the joints etched by the moss/lichen
Lichen and moss have colonised the surface of the granite, particularly in the joints (Lithosere)

26. Biological Weathering
Plants and soil help trap moisture against the rock and they also contribute organic acids
Enlarged joints
Mosses and lichen are succeeded by grasses and heather as the organic content of the skeletal soil gradually increases
5cm