LECTURE 12. WEATHERING & MASS WASTING Introduction

1. LECTURE 12. WEATHERING & MASS WASTING Introduction Rock exposed at the Earth's surface is under almost constant attack by processes which break it down into fragments and transport it away; 3 stages are involved: Weathering - the initial breakdown of solid rock; mass wasting - downslope movement of the fragments; erosion - long distance transportation of the fragments. The combined effect is the overall lowering of the earth's surface, termed DENUDATION. Harry Williams, Earth Science

2. IN SITU DOWN SLOPE LONG DISTANCE TRANSPORT Harry Williams, Earth Science

3. WEATHERING: includes both MECHANICAL processes, which exert force and physically break a rock apart, and CHEMICAL processes, which decompose a rock by altering its chemical composition. Mechanical weathering: 1. Frost wedging: 9% expansion Bare cliff faces are vulnerable to frost wedging. Harry Williams, Earth Science

4. Resulting fragments accumulate as a TALUS SLOPE. Frost wedging is most effective where temperature oscillates above and below freezing. 2&3. SALT WEDGING & HYDRATION: the first is similar to frost wedging, except it involves the growth of salt crystals in cracks from salts dissolved in water; most effective in semi-arid and coastal areas. The second refers to the absorption of water by some minerals, making them swell and exert force, breaking the rock. Frost and salt wedging are aided by natural joints (cracks) in rock. These result from contraction of cooling magma or tectonic stresses. Harry Williams, Earth Science

5. 4. EXFOLIATION. This applies to large bodies of igneous rocks, particularly granite batholiths, that are exposed at the surface of the Earth when the overlying rock is eroded away. The reduction of overburden pressure allows the igneous rock to expand upwards, which causes it to crack, usually parallel to the surface, producing an EXFOLIATION DOME. The sheets of rock produced are often peeled off in onion-like layers by other weathering processes, such as frost wedging. Harry Williams, Earth Science

6. 5. THERMAL EXPANSION: Thermal expansion weathering is caused by large daily temperature changes, which are especially pronounced in hot dry areas such as deserts. Different minerals expand and contract at different rates; this sets up DIFFERENTIAL STRESSES within a rock, which eventually can cause the rock to shatter, producing angular fragments. Harry Williams, Earth Science

7. 6. ORGANIC ACTIVITY: The growth of plant roots has a similar effect to the growth of ice - the growing root wedges a rock apart, breaking it up into fragments. The effect of tree roots on city sidewalks clearly demonstrates the effectiveness of this process. Harry Williams, Earth Science

8. CHEMICAL WEATHERING: Refers to the decomposition of rock by various chemical reactions, which involve either adding or removing elements from minerals in the rock. Water is nearly always required for chemical weathering and so this form of weathering is not very effective in dry areas, such as deserts. Most chemical reactions are more effective at higher temperatures and so chemical weathering is most pronounced in hot, wet areas, such as the Tropics. Chemical weathering “rounds off” angular edges of boulders. Harry Williams, Earth Science

9. 1. SOLUTION: certain minerals (but not many) simply dissolve in water. Atoms of minerals forming the rock are simply pulled out and dispersed into the water surrounding the mineral; if the water then flows away, the atoms are carried away with it. A good example is HALITE (rock salt). 2. OXIDATION: oxygen from the atmosphere combines with certain minerals - especially iron-bearing ones - to form new softer minerals. This is essentially the same as rust and produces colorful “desert varnish” on rocks. Harry Williams, Earth Science

10. 3. HYDROLYSIS: This is the commonest form of chemical weathering and is particularly effective against feldspar minerals - the commonest mineral group. Hydrolysis refers to the chemical combination of minerals with ACIDIC WATER, resulting in the formation of new minerals. Most natural SOIL WATER is slightly acidic - the acid comes from 2 main sources: 1. water dissolves a certain amount of carbon dioxide as it falls through the atmosphere, forming weak carbonic acid. 2. decaying organic matter at the ground surface releases weak organic acids. Harry Williams, Earth Science

11. E.g. Granite: feldspars -> clays; quartz particles drop out -> sand. This chemically weathered granite boulder has been “rounded off” (spheroidal weathering). The feldspars have been chemically altered to clay minerals; the quartz is resistant to chemical weathering - quartz particles simply drop out and become sand grains. The clays also become detached - many end up as a component of soil. Harry Williams, Earth Science

12. 4. Carbonation: involves carbon dioxide from the air dissolved in rain water to form weak carbonic acid; this dissolves carbonate rocks, especially limestones, often leaving no residue (this is why caves are common in limestone areas). Chemical weathering depends on the presence of water and most reactions are more intense under higher temperatures; for these reasons, chemical weathering is most pronounced in the hot wet Tropics; mechanical weathering is most pronounced in cold wet climates, or at high altitudes. Harry Williams, Earth Science

13. MASS WASTING (DOWNSLOPE movement of sediment by gravity): Controls On Mass Wasting: Mass wasting occurs on slopes (even very gentle slopes) because weathering attacks the surface rock allowing the resulting weak soil and regolith to be pulled downslope by gravity. Since weathering is a continuous process, many slopes become weaker and weaker until eventually they can no longer withstand the pull of gravity and move downslope. Rapid mass wasting events can be triggered by earthquakes, heavy rain or snow, erosion, mining, construction, etc. These movements are considered “natural hazards”. Harry Williams, Earth Science

14. Types of Mass Wasting: Based on: 1. MATERIAL TYPE: Rock, Soil (debris=coarse sediment; earth & mud =fine sediment). Note: the deposits of mass wasting are collectively known as COLLUVIUM. 2. TYPE OF MOTION: A. Falls: Free-fall of material under gravity - forms cliffs and talus slopes. Rockfall from a steep cliff; talus slope at base of cliff. Harry Williams, Earth Science

15. Slides: movement of slope material over a well-defined planar surface. These commonly occur where there are joints, bedding planes or fractures parallel to the surface of a slope. 1925 Gros Ventre rockslide, Wyoming. Sandstone slid over a saturated clay layer. Harry Williams, Earth Science

16. Sometimes the slide is rotational - producing a slump. scarp Note “stepped” appearance of slope Harry Williams, Earth Science

17. Harry Williams, Earth Science

18. Flows: High fluid content; behaves like a viscous liquid flow. Coarse material mixed with mud = DEBRIS FLOW. Fine material alone referred to as an EARTHFLOW (unconfined) or MUDFLOW if confined to a channel. When mudflows emerge from channels they build ALLUVIAL FANS. Harry Williams, Earth Science

19. Earthflow (saturated soil, unconfined to channel). Harry Williams, Earth Science

20. Mudflow: stream choked with mud/clay. Common in flash floods in arid regions. Where mudflows emerge from channels onto flat valley floors, alluvial fans are built. Harry Williams, Earth Science

21. Alluvial fans on Mars. Harry Williams, Earth Science

22. Creep: Slow movement of near-surface material (e.g. cms/year). Common on most slopes above a few degrees. Caused by gravity, freeze/thaw, wetting/drying, rainsplash, burrowing. Shows up as curved tree trunks, bent walls, "wrinkled" slope surface (terracettes). Harry Williams, Earth Science