Lecture notes and reading

1. Lecture notes and reading • Lecture notes • All OHPs available via the web. http://www.st-and.ac.uk/~www_sgg/personal/jw9link/jwteach/index.html • Key Reading; Weathering and Soils lectures • Briggs, D. et. al., 1997. Fundamentals of the Physical Environment, pp 211-233, 322-337 and 338-359. • Press, F. and Siever, R. 1997. Understanding Earth, pp 134-161. • Ritter, D.F., et al., 1995. Process Geomorphology, pp 49-84 and 85-97. • Strahler, A.N. and Strahler, A.H. 1992. Modern Physical Geography. pp 281-297, 445-457 and 458-483. ESP 1

2. Weathering & soil systems • Section structure • 4 lectures and one practical class. • Aims & Objectives • To explore the main processes in weathering and pedogenic systems. • To introduce the main controls upon those processes. • To identify possible connections between these and other geographical systems. ESP 1

3. Lecture 1 • Context • Weathering and soil environments as systems. • Why study weathering? • Applied reasons for understanding weathering and soil systems. • Why do things weather? • Classification of weathering processes • A 'process model' of weathering ESP 1

4. Context: weathering & soil system • Integrated system • Difficult to separate the weathering, detrital and soil systems as they clearly overlap and interact with each other. ESP 1

5. Why study these systems? • Academic worth • Study a subject for it's own sake and as an intellectual exercise. • Applied context • In the case of weathering and soils, it is also possible to think of a range of more practical reasons for developing our understanding. ESP 1

6. Weathering: applied context (1) • The rock cycle ESP 1

7. Weathering: applied context (2) • Environmental change • The type and degree of rock weathering can provide us with useful information on environmental change. ESP 1

8. Weathering: applied context (3) • The built environment ESP 1

9. Weathering: applied context (3) • Rock type and soil type • Rock type has a strong control upon soil development and therefore weathering is a key element to understanding soils. ESP 1

10. Soils: applied context • Soils and food production • Soil is a vital resource for food production. • Soils and human impact • Understanding soil formation processes helps us place rates of soil loss by natural or human induced erosion into context. • Soils and hydrology • Soils act as an important part of the hydrological cycle and therefore their type and formation have some implications for water resources. ESP 1

11. Why do rocks weather? • Some images to get you thinking….. ESP 1

12. Why do rocks weather? • Ice cubes…. ESP 1

13. Why do rocks weather? • The ice cubes a few minutes later…. ESP 1

14. Why do rocks weather? • …. And a few more minutes later…. • …. And so on….. ESP 1

15. Why do rocks weather? • Relative stability & environmental conditions • What environmental conditions did the rock/minerals form in? • What environmental conditions do the rock/minerals now find themselves in? • In which set of conditions are the rock/mineral most stable? • Environmental stress • e.g. temperature and pressure conditions at the Earth’s surface. • Rock/minerals therefore experience stress. • The response is to ‘weather’…. To new forms of rock/mineral that are stable in these new conditions. ESP 1

16. Weathering: a definition • Polynov, 1937 • Weathering is the change of rocks from the massive to the clastic state. • Reiche, 1950 • Weathering is the response of materials which were at equilibrium within the lithosphere to conditions at or near its contact with the atmosphere, hydrosphere and biosphere. • Ollier, 1984 • Weathering is the breakdown and alteration of materials near the earth's surface to products that are more in equilibrium with newly imposed physio-chemical conditions. ESP 1

17. Weathering: a working definition • A useful ‘working’ definition? • Weathering is the in situ decomposition and disintegration of rocks and minerals at and near the earth's surface by physical, chemical and biotic processes. • Key elements • 'at or near the earth's surface'.... • 'rocks and minerals'..... • (and artificial materials?) • 'in situ......' • (but also weathering in transit!) • 'decomposition and disintegration....' ESP 1

18. Weathering: complexities • All minerals are different...... • Every mineral species has, by definition, a unique chemical composition and arrangement. • Mineral combinations in different rock types • ... and every rock type is made up of some particular combination of these various mineral types..... and therefore responds to weathering processes differently. • Weathering therefore complex • Need generalizations or underlying concepts which allow us to get a broad understanding without necessarily studying how weathering occurs in every rock type under every different climatic regime! ESP 1

19. Weathering: some simplifications • Classification of weathering processes • Physical weathering processes. • Also known as ‘brittle fracture’. • Chemical weathering processes. • Also known as ‘crystal lattice breakdown’. • Biological weathering processes. ESP 1

20. Physical weathering • Physical weathering is…. • the breakdown of material by entirely mechanical methods, brought about by a variety of causes. • Some of the forces originate within the rock, while others are applied externally. • The stresses lead to strain, and eventually to rupture. • By inference, it does not result in any chemical, compositional change within the rock, but solely its disaggregation into smaller pieces. ESP 1

21. Chemical weathering • Chemical weathering is…. • the breakdown of material by entirely chemical methods. • Rocks formed in one set of physical and chemical conditions, when moved away from those conditions, are essentially, moving further away from their point of chemical equilibrium. • In this sense, they become less chemically stable, and prone to chemical alteration into different forms (new mineral types, solutions and gases) which are more stable in these new environmental conditions. ESP 1

22. Biological weathering • Biological weathering includes…. • both physical and chemical processes of weathering. • These processes are, in some way, initiated or enhanced by some biological activity. • Overlap between each type. • Therefore an obvious overlap between the biological weathering and the other two types. • e.g. burrowing animals or plant root growth are biological processes that produce a physical breakdown of rocks or minerals. • There are also processes which could be classified as both physical & chemical. • e.g. salt crystal growth. ESP 1

23. Weathering: a systems ‘model’ • A conceptual ‘model’ of the weathering system ESP 1

24. Force vs. resistance (1) • A contest? • Like many geomorphological systems, this model of the weathering system can also be considered as a contest between force and resistance. • The forces • The environment, through various activating agencies, applies forces to the rock which can cause the rock to be broken down and altered. ESP 1

25. Force vs. resistance (2) • The resistances • These forces are only half the equation however. • Rocks exhibit resistances to these applied forces. • The level of resistance depends upon the minerals from which the rock is composed, how tightly these are bonded together and the rocks larger scale mechanical properties (presence of structures or lies of weakness). • These resistance's will therefore vary both between rock types and even in different parts of the same rock type. • Why does the rate of weathering vary? • Because, in any given location, the exact nature or the applied force and the exact nature of the resistance to that force vary. ESP 1

26. Conclusions and Summary • What have we done today? • Identified our objectives for these four lectures. • Made a start towards these objectives by trying to do five things today........ • Context: Weathering and Soil environments as systems • Applied reasons for understand weathering and soil systems • Weathering: why do things weather? • Definition and classification of weathering processes • Process model of weathering and force vs. resistance. ESP 1

27. Next? • Weathering processes ESP 1