Chp 5: Weathering and Erosion

1. I. Define Weathering and Erosion - Weathering - “The decomposition and disintegration of rocks and minerals at the Earth’s surface by mechanical and physical processes. Weathering processes involve very little or no movement (or removal of) decomposed earth material. Erosion – “The removal of weathered rocks and minerals from the place where they formed. Forces or transporting agents involved in moving disintegrated earth materials are: Water – running water such as streams, rivers, etc. Wind– prevailing winds, tornadic storms, sea breezes, etc. Gravity – The influence of gravity causing landslides, avalanches, etc. Ice – in the form of glaciers II. Modification of the Earth’s Surface - Weathering, erosion, and transportation of earth materials Surface processes continually wear away rocks and landforms In geologic time they combine to wear away entire mountain ranges, reducing them to flat, low-lying plains. Chp 5: Weathering and Erosion

2. Weathering Effects quite apparent…. p.133

3. Wind and water leave curious rock formations behind due to erosion- Arches National Park, UT Fig. 5-CO, p.120

4. Dust storms in Central Plains during 1930’s Fig. 5-21, p.141

5. Gravity effects- loose rocks slide down hill… Heavily weathered granitic rock Fig. 5-1a, p.122

6. Breakdown into smaller particles… Fig. 5-1b, p.122

7. III. Types of General Weathering - Mechanical (or Physical) Weathering – The physical disintegration of rock into smaller and smaller pieces. The chemical composition of the rocks and minerals are not altered. The particles formed are called clastics (meaning “broken”) Chemical Weathering – occurs when air and water react chemically with rocks to alter their composition and mineral content. The final products not only differ physically from the parent material, but they are different chemical substances. (i.e. Limestone dissolving by acid rain releasing its calcite content as ions.) Differential weathering - Rocks weather by both mechanical and chemical processes occurring together. Since rocks are not homogenous in composition, usually parts weather at different rates. Called differential weathering resulting in an uneven surface. Spheriodal weathering – Because of differential weathering, the surface of rocks is many times sharp and angular, or cuboidal. These corners formed on the rocks are “attacked” from all three sides resulting in a “rounding” of the angular piece. This is spheriodal weathering. Chp 5: Weathering and Erosion

8. Differential weathering of limestone in Spain Fig. 5-2b, p.123

9. Honeycomb weathering – coastal CA Fig. 5-2c, p.123

10. Exfoliation dome in central CA Fig. 5-4b, p.125

11. IV. Types of Mechanical (Physical) Weathering - Frost wedging – Water expands upon freezing. If water seeps into cracks in the rock and freezes, the ice formed exerts pressure along the crack, expanding the crack, or breaking off a piece of rock. Many times the broken piece remains in place until the spring thaw, resulting in areas (such as mountain passes) of rock fall hazard. The loose angular rock debris at the base of mountains and cliffs is termed talus. Salt Cracking – Whenever salt water evaporates, the salts reform crystals. If water containing dissolved salts enters a crack in the rock and then evaporates, the pressure created by the newly forming salt crystals can break the rock. This is salt cracking and is common in deserts and shorelines. This is why it is not a good idea to salt driveways or sidewalks to rid them of ice. The concrete will eventually break apart. Abrasion – This is the mechanical wearing and grinding on rock surfaces by friction and impact with other rock materials. This gives the rocks a rounded appearance. This occurs in flowing water, wind actions (i.e. natural sand blasting), and glaciers Chp 5: Weathering and Erosion

12. Water enters cracks in rock, freeze and thaw breaks rock apart… Fig. 5-3, p.124

13. Fluids seep along fractures and aid in breakdown of rock into particles Fig. 5-10, p.132

14. Organic Activity – Plant roots can crack rock material by the hydraulic pressures associated with root growth. Burrowing animals can contribute to rock disintegration. Pressure Release Fracturing – Rocks buried deep within the earth are under the pressure of the overburden (country rock). As the overburden is eroded away, internal pressures of a granite pluton cause it to expand. This causes the surface of the granite to split and crack forming sheets and blocks of rock at the surface in a process known as exfoliation. This may also occur in rocks that are porous such as feldspar rich granites. Water may be “absorbed” by the feldspars causing them to swell and crack. This process of swelling by the addition of water is called hydration. This is one of the processes that can turn feldspars into kaolinite, a major clay-forming mineral. Thermal Expansion and Contraction - Heat causes matter to expand and cold causes matter to contract. Surface rocks exposed to the intense heat of the daytime sun heat up and expand. At night when it is cooler, the rocks contract. This constant expansion and contraction over many years causes the rocks to break apart. Enchanted Rock in central Texas was named so because of the cracking sounds it is supposed to make during this process. Chp 5: Weathering and Erosion

15. Tree roots grow into crack in rock and contribute to erosion of that rock- Organic erosion Fig. 5-6a, p.127

16. Sheet joint formed by expansion in granite..hammer is 30cm long Fig. 5-5, p.126

17. Stone Mountain is a large exfoliation dome in Georgia Fig. 5-4c, p.125

18. Slabs of granite bounded by sheet joints-note slabs are inclined Down Fig. 5-4a, p.125

19. V. Types of Chemical Weathering - 1. Oxidation – reactions with oxygen – rusting: 4 Fe + 3 O2  2 Fe2O3 iron + oxygen = iron oxide Oxidization reactions are common in nature and usually turns useful material into wastes. This is most common in iron bearing mafic minerals such as olivine, amphibole, and biotite. 2. Corrosion– reactions involving oxygen, water, and CO2 found in the air and water. Combinations of these can cause corrosive chemical conditions that can chemically weather rocks. 3. Weathering by Solution – dissolution whereby ions disperse into water. I.e. rivers flowing across limestone can dissolve Ca+ and CO3- and carry these ions away. 4. Acids and Bases – Acids are solutions with an abundance of free hydrogen ions (H+), while bases are solutions that have an abundance of free hydroxyl ions (OH-). Acids and bases dissolve minerals by pulling atoms out of crystals. Carbonic Acid (H2CO3) is formed in abundance in nature whenever CO2 dissolves in some rivers and streams Chp 5: Weathering and Erosion

20. Lichens growing on rocks derive nutrients from the rock and contribute to weathering Fig. 5-6b, p.127

21. Weathering effects enhanced by increasing surface area of any particle: Basic premise here is that the more surface area that is available for these processes to act upon, the faster the process will act and the more likely the rock will be broken apart…(weathered)… surface area increases, but the Volume stays the same!! Fig. 5-11, p.132

22. Chp 5: Weathering and Erosion 5. Acid Rain – During storms, Nitric Acid, (H2NO3) is formed by lightning breaking apart N2 in the atmosphere into N + N. This combines with water to form nitric acid causing rainwater to naturally become slightly acidic with a pH of 5.5 – 6.5. Pollutants in the atmosphere such as sulfur dioxide gasses can also contribute to acid rain. Soil formation is a prime end product of weathering and erosion

23. SOILS Fig. 5-15, p.136

24. Soil Formation- function of climate and vegetation Fig. 5-16, p.137

25. Brown- • Expansive soils • b. Expansive • soil in Dallas Fig. 5-20, p.140

26. laterite: deep red soil found in tropics • Slash and burn agriculture Fig. 5-18, p.138

27. Table 5-2, p.143

28. Contour plowing Fig. 5-23, p.143

29. pH of soil Areas where acid rain is a problem p.131

30. Underside of rock showing scaly white material known as ‘caliche’ Fig. 5-17a, p.137

31. Alkali soil in California-white material is NaCO3-very few plants live Fig. 5-17b, p.137

32. Weathering - “The decomposition and disintegration of rocks and minerals at the Earth’s surface by mechanical and physical processes. Erosion – “The removal of weathered rocks and minerals from the place where they formed. By these agents: water wind gravity ice Types of Weathering: 1. mechanical-physical disintegration into smaller pieces; no chemical change 2. chemical- air and water react to change chemical composition and size 3. differential- both chemical and physical, creates uneven surface 4. spheroidal- processes attack exposed surfaces, create a rounded surface Chp 5: Weathering and Erosion: Summary

33. Types of mechanical weathering: 1. frost wedging-water enters crack, freezes, expands, pushes crack open 2. salt cracking-salt left behind when water evaporates expands, breaks open rocks…occurs in deserts and shorelines. 3. abrasion-friction and impact between grains, rivers, glaciers and wind 4. organic activity- plant roots and burrowing organisms break rocks 5. pressure release- as buried rocks are exposed due to erosion, their overburden is removed leading to release of pressure and they expand or crack. Exfoliation and hydration are 2 examples of this… 6. thermal expansion and contraction: heat and cold-expand, contract Types of Chemical Weathering: 1. oxidation- reaction with oxygen…basically is ‘rusting’ 2. corrosion- reaction with oxygen, water and CO2 3. solution- dissolution of ions into water; rivers dissolve carbonates… 4. acids and bases- carbonic acid formed in rivers; dissolves minerals 5. acid rain-naturally occuring-lightning and Nitrogen; pollution contributes Chp 5: Weathering and Erosion

34. Primary end products formed by weathering and erosion are- -rock particles which turn into sedimentary rocks after transportation -soils which have important economic implications for human survival (i.e. growing crops, mining certain minerals (bauxite, kaolinite, etc) Human impact- -soil management schemes -exacerbate acid rain into a problem in some areas -mining and other activities make erosion worse than would normally occur… Chp 5: Weathering and Erosion

35. Fig. 5-2a, p.123

36. Fig. 5-7, p.128

37. Fig. 5-8, p.129

38. Fig. 5-9, p.129

39. Fig. 5-12, p.133

40. Fig. 5-13, p.134

41. Fig. 5-14a, p.135

42. Fig. 5-14b, p.135

43. Fig. 5-19, p.139

44. Fig. 5-22a, p.142

45. Fig. 5-22b, p.142

46. Fig. 5-22c, p.142

47. Table 5-1, p.134