Drainage System

1. Drainage System Weathering Subsystem

2. Denudation • The physical landscape is the combined result of plate movements, the composition of the rocks, and the processes of denudation. • Rocks in or on the crust is attacked by the following processes: - weathering - mass wasting / movement - erosion - transportation

3. The process of Weathering • Weathering is the disintegration and decomposition of rocks at or near the Earth’s surface caused by physical and chemical changes. • It results in the formation of pieces of rock called regolith (風化層). • Weathering processes do not involve any movement. So the regolith is produced in-situ (原位).

4. The process of Denudation • The agents of erosion use regolith to break up surface rocks still further to form a layer of rock particles called rock debris (岩屑).

5. The process of Denudation • Mass wasting / movement is the movement of regolith and rock debris down the slopes under the pull of gravity. • Erosion refers to the wearing away of rocks by agents of erosion, e.g. rivers, waves, ice and wind. • Except for processes of hydraulic action and solution, all erosion results from agents of erosion dragging or blowing regolith and rock debris over the Earth’s surface.

6. The process of Denudation • Transportation refers to the removal of regolith and rock debris from where it is produced, by weathering and erosion, to the place of deposition.

7. Weathering • It is the first process to operate in shaping the landscape. • Rocks on the Earth’s surface are exposed to temperatures that range from –30oC to 35oC, and in some regions to rain and frost as well.

8. Weathering • Water and temperature changes cause rocks to break up by processes called physical disintegration (物理崩解) and chemical decomposition (化學分解).

9. The process of Weathering • Weathering is confined to a relatively narrow zone of the Earth’s surface. • It extends from the soil surface to the max. depth of penetration of mobile water, gases and energy derived from the atmosphere. • These processes turn exposed rock surfaces into a layer of rock fragments (regolith) that extends from the surface to the bedrock • (基岩).

10. The reasons for Weathering • It is a response to a disequilibrium. • What are the possible reasons?

11. Reasons for disequilibrium • 1. Exposure by uplifting e.g. tectonic processes like folding and faulting • 2. Exposure by vulcanicity / volcanic activities • 3. Erosion • 4. A fall in sea level • 5. Fluctuation in climate

12. Reasons for disequilibrium • 6. Changes in vegetation • 7. Impacts by human activities • e.g. pollution

13. The process of Weathering • The base of the active zone is generally marked by the water table, beneath which ground remains permanently saturated. • Beneath that level, weathering may operate, but a a much reduced rate.

14. How the landscape is shaped by external processes?

15. Sea or lake What are external processes? Denudation: the external processes which wear down rocks on the earth’s surface. External processes include denudation and deposition. Rocks are broken into fragments by moisture and temperature changes Weathering Mass wasting Fragments move down the slope under gravity Debris is removed by moving agents, e.g. river Erosion Load is carried by movingagents Transportation

16. Sea or lake What are external processes? When river and wave lose their energy, deposition occurs. Load is settled when moving agents lose their energy Deposition

17. How do rocks become weathered? Weathering: rocks disintegrate or decompose in situdue to changing temperature and pressure, moisture and action of living organisms. Weathering Mechanical Chemical These two types of weathering usually work together, not alone, on rocks.

18. How do rocks become weathered? 1 Mechanical weathering It is the breaking of rocks into pieceswithout changing their chemical composition. Mechanical weathering Pressure release of rock Thermal expansion and contraction Biological activity

19. 1 Mechanical weathering A Pressure release of rocks Rocks deep in the crust (plutonic rocks) are under great pressure exerted from the overlying rocks. Pressure from overlying rocks Deep plutonic rock

20. 1 Mechanical weathering A Pressure release of rocks The expansion of the plutonic rocks causes the outer layers of rocks to break away, andpeel off like onions (sheeting). Removal of overlying materials reduces the pressure on the rock below Crack The exposed rock is further subject to weathering. Pressure release causes expansion and sheeting

21. 1 Mechanical weathering A Pressure release of rocks Unloading is a form of physical weathering that occurs when the removal of overlying layers causes the rock to expand, cracking in layers parallel to the surface that break away from the rock in sheets.

22. B Thermal expansion and contraction 1 Block disintegration (on rocks with cracks) Rock releases heat and contracts Repeated expansion and contraction causes rock to break into blocks Rock is heated and expands Cracks enlarged Night-time Daytime

23. Features in Hong Kong Describe the appearance of the rock and identify the associated weathering process. A From weathering The rock breaks like slices of bread. Block disintegration (mechanical weathering) A rock structure at Tai Lang Shui, Tuen Mun (Credit: Association for Geoconservation, Hong Kong)

24. B Thermal expansion and contraction 2 Granular disintegration (on rocks with different minerals) Mineral A expands faster than adjacent ones (Mineral B) and pushes them apart Mineral A contracts faster than adjacent ones (Mineral B) and becomes detached Repeated expansion and contraction causes all mineral grains to loosen and detach Rocks break off into grains Night-time Daytime

25. B Thermal expansion and contraction 3 Exfoliation (on a uniform mass of rock) Repeated expansion and contraction causes the outer layer to peel off. A new surface is exposed to heating and cooling The outer layer absorbs more heat and expands more than the inner layer The outer layer loses heat faster and contracts more than the inner layer Concentric cracks develop Radial cracks develop Rock breaks into slabs Night-time Daytime

26. 1 Mechanical weathering C Biological activity Astree rootsgrow, they can forceopen the joints in rocks. The large physical pressure helps break the rock along the joints. Plant roots enlarge the joints in rocks (Credit: Tom Bean/DRK Photo)

27. How do rocks become weathered? 2 Chemical weathering • It is caused by thereaction between the rocks and the gases in the air, waterorother solutions(for example, acids) with which they come into contact. • Chemical reaction causes certain minerals tochange into more vulnerable ones. • Four processesof chemical weathering may work together, and result intwo special forms.

28. 2 Chemical weathering Four processes and two forms of chemical weathering Chemical weathering Hydrolysis Solution and carbonation Oxidation Hydration Honeycomb weathering Spheroidal weathering

29. A Four processes of chemical weathering 1 Oxidation (Credit: Richard Cummins/Getty Images) The process by whichminerals react and combine with oxygen. It is also called metal rusting. Example Iron oxidizing into reddish-brown iron oxide Oxidation occurs on the rocks used in this construction

30. Features in Hong Kong Describe the appearance of the rock and identify the associated weathering process. A From weathering Cracks occur on the rock. Reddish brown colour is found at the two sides of the cracks. Oxidation (chemical weathering) Mudstone on Tai Tsan Chau, eastern New Territories (Acknowledgement: The Head of the Geotechnical Engineering Office and the Director of Civil Engineering and Development, the Hong Kong SAR Government, for the permission to publish the photograph)

31. A Four processes of chemical weathering 2 Hydration • The absorption of water into the mineral structure, turning it into new but more vulnerable minerals. Example Calcium sulphate Water Gypsum becomes +

32. A Four processes of chemical weathering 3 Hydrolysis The process by whichminerals react with hydrogen in waterand break down the original structure. Example Silicates in feldspar Hydrogen Clay turns to +

33. A Four processes of chemical weathering 4 Solution and carbonation • The dissolving of soluble mineralsin pure water or weak acid. • Example:carbonic acid(rainwater + carbon dioxide) andacid rain(rainwater + sulphur dioxide) • Calcium carbonate in limestone can be dissolved and removed easily from the rocks in solution. Acid rain has caused chemical weathering on stone statues (Credit: Ray Roberts/Topfoto)

34. A Four processes of chemical weathering 4 Solution and carbonation

35. A Four processes of chemical weathering 4 Solution and carbonation Growing of plant roots Decomposition of plant releases releases Organic acid Humic acid • Both acids can attack the underlying rocks and weaken their structure.

36. B Two forms of chemical weathering 1 Honeycomb weathering • The removal of minerals in solutionproduces a honeycombed effect on the rock face. Honeycomb weathering on the rock face in Ping Chau

37. Features in Hong Kong A From weathering Describe the appearance of the rock and identify the associated weathering process. Deep holes like honeycombs are found on the rock surface. Honeycomb weathering(chemical weathering) A rock at Cheung Chau

38. B Two forms of chemical weathering 2 Spheroidal weathering As water seeps through joints,weathering works from the joints towards the interior of the rock. Original joint planes Water seeps through joints More water favours faster weathering at the intersection of joint planes Unweathered corestones Weathered material which will be removed eventually Unweathered bedrock

39. Percolation of water along previous joints 2 Deep weathering profiles Granite is vulnerable tochemical weathering. It will develop into a deep weathering profile beneath the earth’s surface. Soil surface Thick profile (may reach up to 60 metres) Bedrock The cross-section of the weathering profile of granite

40. Percolation of water along previous joints 2 Deep weathering profiles In a mature profile, spheroidal weathering turns all the rock materials intosoil at the top layer. Soil surface Fine regolith Thick profile (may reach up to 60 metres) Bedrock The cross-section of the weathering profile of granite

41. Percolation of water along previous joints 2 Deep weathering profiles • __________ weathering forms rounded corestones. Spheroidal • More and bigger corestones are found nearer the bedrock. Soil surface Fine regolith Thick profile (may reach up to 60 metres) Angularity and size of corestones increase with soil depth Bedrock The cross-section of the weathering profile of granite

42. B Two forms of chemical weathering 2 Spheroidal weathering • The corestones getsmallerandroundedas time goes by. (Credit: Chlaus Lotscher/Peter Arnold Inc.) Unweathered corestones

43. 3 Tor formation When the overlying materials above the granite corestones are removed, the corestones are left astors. Amah Rock is an example of tors (Credit: Association for Geoconservation, Hong Kong)

44. Features in Hong Kong Describe the appearance of the rock and identify the associated weathering process. A From weathering Outer layers of rock have peeled off like onions. Reddish brown patches are found on the rock. Exfoliation (mechanical weathering) Oxidation (chemical weathering) A rock at Ninepin Group

45. Comparison – weathered vs unweathered Chemical weathering Which of them is under chemical weathering? Which one is fresh? Limestone

46. Other forms of physical weathering A Salt-crystal weathering • It operatesextensively in dry climates and is the result of the growth of salt crystals in rock pores. Groundwater moves to the surface through capillary action and evaporates, leaving the salts behind producing grain by grain breakup of sandstone. Salt weathering of building stone on the island of Gozo, Malta

47. Other forms of physical weathering A Salt-crystal weathering Crystallized salt on the rock surface and in rock fissures in granite

48. Other forms of physical weathering B Frost action • It is one of the most important physical weathering processes in cold climates. It occurs when water freezes in joints in the rock, and the expansion of the water during repeated freezing and thawing to cause the joints to be enlarged.

49. Other forms of physical weathering B Frost action

50. How does rock materials help weathering processes? • 1. Structure - the pathways of percolating water • 2. Texture / particle size - the structure of rock - the size and surface area - e.g. fine grains are more easily weathered due to the presence of more porous space (i.e. porous rock / high porosity)