Weathering & Erosion

1. Weathering & Erosion CE-301-Lecture 10 Lecturer Abdul jabbar Semester 2 15 Feb 10 – 18 Jun 10 Reference: CHAPTER 3 SURFACE PROCESSES Geology for Engineers 7th Edition by F.G.H. Blyth and M.H. de Freitasgoiit weterinh weathering

2. What is weathering? Weathering is a set of physical, chemical and biological processes that change the physical and chemical properties of rocks and soil at or near the earth's surface.

3. More about weathering • Definition – the breakdown of rock to form sediment [very small pieces of rock] Or The breakdown do the materials of Earth’s crust into smaller pieces. • Weathering happens to rocks that are NOT MOVING • Weathering is part of the Rock Cycle

4. There are three types of weathering • Mechanical [sometimes called physical] : disintegration/ disaggregation of rocks via mechanical processes. Temperature, abrasion in wind, rain drops • Chemical: Decomposition of rocks and minerals via chemical reactions at the Earth’s surface. Chemical agents: acids in rain air and rivers etc • Biological: Mechanical and chemical changes directly associated with animals and plants. Plant roots, burrowing animals, root gases increasing acidity

6. Mechanical Weathering • Mechanical Unloading. Vertical expansion, erosion reduces load opening fractures • Mechanical Loading. Impact and abrasion of wind borne particles in deserts and effect of intense rain drops • Thermal Loading. Expansion of freezing water, high and low temperatures • Wetting and drawing. Repeated loss & abrasion of water in certain clays • Crystallization. Formation of crystals in fissures and pores, originally in solution • Pneumatic Loading. Waves effect on trapped air in cliffs.

7. Mechanical weathering , a rock is broken down into smaller pieces without changing its mineral composition. Types of mechanical weathering include loading/unloading, frost heaving and wedging, Plants roots, Friction and impact, Burrowing of animals and thermal expansion.

8. Loading / Unloading • Joints parallel to ground open up forming ‘sheets’, 1m • Spallings, small platy fragments fall of • Many open up during quarrying with sound once relieved of stresses

9. Frost Wedging Repeated freezing breaks of flakes & angular fragments (Screes)

10. Frost Heaving • Freezing process increases volume in pores

11. Plant Roots

12. Friction and Repeated Impact • Temperature variation in hot climate, flakes of outer layer of rocks split off ,process is exfoliation • Cracks may form, salts and water cause decomposition and increase weathering called insolation

13. Burrowing of Animals

14. Thermal Expansion/Temperature Changes

15. Chemical Weathering Chemical weathering breaks rocks down chemically adding or removing chemical elements, and changes them into other materials. Chemical weathering consists of chemical reactions, most of which involve water.

16. Chemical Weathering • Some Processes • Solution. CO2 in soil profile making carbonic acid H2CO3 with percolating rain water. • Oxidation. Oxygen with mineral forms oxides • Reduction. Oxygen leaves mineral • Hydration. Absorption of water, expands clays, hastens the above processes

17. Chemical weathering happens quickly in warm, moist environments because water is needed for the chemical reactions. The warm weather speeds up the reactions. The agents of chemical weathering Water , Oxygen , Carbon dioxide, Living organisms & Acid rain

18. Water • Water weathers rock by dissolving it

19. Oxygen • Iron combines with oxygen in the presence of water in a processes called oxidation • The product of oxidation is rust

20. Carbon Dioxide • CO2 dissolves in rain water and creates carbonic acid • Carbonic acid easily weathers limestone and marble

21. Living Organisms • Lichens that grow on rocks produce weak acids that chemically weather rock

22. Acid Rain • Compounds from burning coal, oil and gas react chemically with water forming acids. • Acid rain causes very rapid chemical weathering

23. Wet Climate • More vegetation, more severe • More Co2 activity • More rains weathered surfaces flushed more quickly • More effect on Gypsum, lime stone and NaCl etc • Calcium bicarbonate is formed • More water through fissures, lead to shallow holes • In dense vegetation few hundred metres weathered rock • Sandstone & shale more resistant • Even granite is affected, closely spaced joints more weathering • Karst Topography?

24. Chemical Weathering • Karst Topography • A type of landscape in rainy regions where there is limestone near the surface, characterized by caves, sinkholes, and disappearing streams. • Created by chemical weathering of limestone

25. Chemical Weathering Sinkholes Features of Karst: Caves Disappearing streams

26. Not all minerals are prone to chemical weathering. For example, feldspar and quartz, are common minerals in the rock granite, have very different levels of resistance to chemical weathering. Quartz doesn’t weather very easily, but feldspar does. Over a long time, it chemically changes into clay minerals.

27. Biological weathering involves processes that can be either chemical or physical in nature.Biological weathering can be considered special types of mechanical or chemical weathering.

28. Biological weathering is the breakdown of rock caused by the action of living organisms, including plants, burrowing animals, and lichens. A lichen is a combination of fungus and algae, living together in a symbiotic relationship. Lichens can live on bare rock, and they break down rocks by secreting acids and other chemicals.

29. Some biological weathering processes are: • Rocks can break because of animal burrowing. • Rock surface kept damp by plants, increase in solvent action. • Tree roots grow into cracks and widen them, which helps physical weathering. • Bacteria, lichens and other organisms secrete acidic solutions, which helps chemical weathering.

30. Lichens on rocks cause biological weathering.

31. What is erosion? Erosion is defined as the removal and movement of earth materials by natural agents. Some of these agents include glaciers, wind, water, earthquakes, volcanoes, tornadoes, hurricanes, mud flows, and avalanches.

32. How are erosion & weathering different? Weathering involves two processes [mechanical, chemical] that often work together to break down rocks. Both processes occur in place. No movement is involved in weathering.

33. As soon as a rock particle (loosened by one of the two weathering processes) moves, we call it erosion or mass wasting. Mass wasting is simply movement down slope due to gravity. Rock falls, slumps, and debris flows are all examples of mass wasting. We call it erosion if the rock particle is moved by some flowing agent such as air, water or ice.

34. In a nutshell: if a particle is loosened, chemically or mechanically, but stays put, we call it weathering. Once the particle starts moving, we call it erosion.

35. What are the different types of erosion?

36. Erosion Agents • Rivers, winds, moving ice & waves • Loosening, dislodging and carrying particles of soils, sediments and rock pieces

37. Water is the most important erosion agent and erodes most often as running water in streams or rivers.

38. Water in all its forms is erosional. Raindrops create splasherosionthat moves tiny particles of soil. Water collecting on the surface of the soil collects as it moves towards tiny streams and creates sheet erosion.

39. The Work of Rivers Deposition more likely at lower velocities Erosion more likely at higher velocities

40. The Work of Sea WAVE EROSION breaking waves + silt, sand, gravel = erosion

41. Coastal Erosion • Arches and stacks • Sea exerts sawing effect on cliff base, cutting horizontal notch which weakens • Atmospheric denudation wears off upper part • Debris moved to ‘off shore’ zone by waves • Arch’s crown worn out stack is formed

42. Waves in oceans and other large bodies of water cause coastal erosion. The power of ocean waves is awesome; large storm waves can produce 2000 pounds of pressure per square foot. The pure energy of waves along with the chemical content of the water is what erodes the rock of the coastline.

43. Wave action in Calvert County Maryland

44. Erosion by wind is known as aeolian erosion (named after Aeolus, the Greek god of winds) and usually occurs in deserts. Aeolian erosion of sand in the desert is partially responsible for the formation of sand dunes.

45. Painted Desert National Monument

46. The Work of Wind What Controls Transport by Wind? • Wind strength • More wind, more material! • Amount of available surface material • No particles, no transport • Size of particles • The smaller the better! • i.e., dust is much easier to transport than sand

47. The Work of Wind Wind Transport • Suspension • Sliding • Rolling • Saltation

48. The Work of Wind Suspension of Dust • Only small particles (<20 m) can remain in suspension for long-time periods.

49. The Work of Ice What is a Glacier? ...definition • A Glacier is • part of hydrological cycle • thick ice mass • formed on land • made by snow: • accumulation • compaction • recrystallization • able to flow

50. The erosive power of moving ice is actually a greater than the power of water. however since water is much more common, it is responsible for a greater amount of erosion on the earth's surface.