The weathering subsystem

1. The weathering subsystem

2. Introduction • Weathering is the breakdown of rock which is in contact with the atmosphere • It includes physical weathering and chemical weathering • Physical disintegration of rock masses into smaller particles • Chemical weathering alter the particular materials into dissolved ions or clay material • These sand and rock fragment form regolith

3. The depth of the regolith will depend on the nature of the rock,the intensity of atmospheric process and the rates of removal • Weathering is the process by which rocks adjusted to their environment • It prepare rocks for the process of slope movement

4. Causes of weathering • Erosion uncovers underlying rocks • Faulting and folding push rocks that were deep within the crust to the surface • Volcanic eruptions may cause magmas to be the subject to weathering • A fall in sea level may expose marine sediments • Changes in the climate • Man changes the weathering environment by putting pollutants into the atmosphere

5. Factors affecting weathering • The nature and structure of rock material Mechanical structure: • Many sedimentary rock posses bedding planes (層面).They are used by the weathering process • Joints are line of weakness. They are formed under cooling and contraction,compression,tension and dilation(膨脹) • Cleavage (裂痕)is the small parallel cracks formed in platy minerals such as mica • Some rocks such as sandstone or conglomerate (礫岩)may contain pores and allow water to enter

6. Chemical composition • As rock is a collection of various materials,each of which has a different chemical changes through weathering than others • For example : limestone composed of CaCO3(炭酸鈣) is slowly dissolved by rainwater • Quartz is very resistant

7. Climatic condition • The importance of water • Changes in the volume caused by the addition or removal of water produce mechanical stresses in a rock and may cause it to split apart or crumble(碎裂) • Most chemical changes requires the presence of water: solution , carbonation, hydration and hydrolysis, oxidation (溶解、炭化、水化、水解、氧化作用)

8. Temperature • a wide temperature range causes the surface layers of exposed rock to expand and contract. • The rate of chemical reaction tend to rise if the temperature increase • This is also same to biological activities

9. The vegetation cover,the topography and the length of time may all affect the weathering process

10. Mechanical weathering

11. Chemical Weathering

12. Biotic weathering

13. Weathering Environments

14. Weathering and Man • Weathering produced by weathering have economic value • Bauxite (鋁土礦) provides aluminium, kaolinite(高嶺石) is used in paper and ceramics industries • Weathering is a fundamental process in soil formation,thus influencing man’s use of land • Weathering alters the stability of materials,affecting construction work and road building • Man is reinforcing the weathering process.For example,salt is used to keep roads free of ice

15. Weathering in the tropical rain forest • Chemical weathering • The high rainfall and temperature favours chemical weathering • There is break down of silicates into regolith, consist of sand and clay • The rock which is highly permeable allow chemical weathering of fresh rock • Hydrolysis and oxidation results in igneous rocks being disintegrated to form laterite soil (磚紅壤) and bauxite,while sedimentary rocks are converted to soft clay

16. Biological Weathering in tropical rain forest • Plants root wedging cause the joint block separate • Fauna and burrowing animal help to disintegrate the rock Transport • The rocks are decomposed to great depths over 30 meter • That means the TRF has a thick regolith • The river also carry a lot of solute load too

17. Weathering in tropical desert • Mechanical weathering • It is important because of the diurnal range of temperature • Minerals expand when heated and contract when cooled • The continue expansion and contraction can break up the rock • This can be seen in granular disintegration(粒狀崩解)

18. Exfoliation • it occurs when there is release of the pressure on granite rocks when the overlying material is removed. The rock expands,producing sheeting layers • More resistant rock ,like gneiss, stand up as inselbergs(島山). This is produced by the process of parallel retreat.

19. Chemical weathering • no frost weathering • There is little chemical weathering as the supply of water is limited • The upward movement of water by capillary action may occur after rain • Honeycomb(蜂窩狀),deep tafoni(風化窗),niches(雪凹),shallow caves ,rock arches (拱頂)and pit (坑)are produced by chemical weathering

20. There is also hydrolysis of exposed granite ,creating egg-shaped boulders and pinnacles (尖石) • The end product is sand and the regolith is thin. Little clay and solute will result • Alluvial fan (冲積扇)will be form by this deposit

21. Weathering in tundra • Frost action is the most important • As water freeze, its volume will be 10% larger • The repeated growth and melting of ice crystal produce frost-shattering (凍裂作用) • Talus slope (岩屑堆)may accumulate along the cliff

22. Heaving (隆起) • The cold in winter will cause the permanently frozen subsoil to heave upwards • There is the growth of ice lenses/needles(冰晶狀體), water beneath the stones freeze faster than the surrounding silt

23. Water from the silt migrates to the crystals and begins to form an ice lens or perpendicular (垂直的) ice needles • Such growth can push the stone upwards,sorting soil material

24. Freeze and thaw action assists the flow of debris by solifuction (融凍泥流) • the meltwater cannot penetrate the permafrost,it mixes the semi-frozen mud and flow down the slope • The thawing of frozen ground can produce karst scenery(岩溶景色)

25. The intense contraction of the frozen ground produce a serious of cracks.These cracks may fill with water which freezes at depth to form an ice-wedge. Continue contraction and expansion may produce hexagonal pattern – stone polygon (石多邊形)