Weathering, Soil, & Mass Movements

1. Weathering, Soil, & Mass Movements Chapter 5

2. Weathering

3. Mechanical vs. Chemical • Breaks rocks down, but doesn’t change mineral composition • Increases surface area of rocks • Caused by 3 processes: • Frost wedging • Unloading • Biological Activity • Transforms rock into 1 or more new compounds

4. Mechanical • Frost wedging – mostly in the mountains • Water seeps into rock cracks, freezes, splits rocks • Piles of broken rocks are called talus • Unloading – uplift and erosion expose buried mass of igneous rock • Often produces large dome shaped formations • Biological Activity – plants or roots grow into rock cracks, which causes rocks to crack apart • Animals can also cause via burrowing or humans

5. Chemical • Water is the most important agent of chemical weathering • Water causes: • Rust • Carbonic acid • Clay is product of most chemical weathering • Spheroidal weathering is when rock changes into a more round shape b/c of chemical weathering

6. What Affects the Rate of Weathering • Rock characteristics • Cracks influence ability of water to get into rock • Mineral composition – marble faster than granite • Climate • Temperature & Moisture • Controls freeze-thaw cycles (frost wedging) • Vegetation present – more = more weathering • High temps & high moisture most favorable for chemical weathering

7. Differential Weathering – different parts of a rock weathers at different rates • Bryce Canyon

8. Soil

9. Soil • Weathering produces layer of rock and mineral fragments called regolith • Soil is the part of regolith that supports growth of plants

10. Characteristics of Soil • Composition (4 components, vary for different soils) • Mineral matter – broken down rocks • Organic matter – humus (decayed remains of organisms) • Water • Air ***good quality soil is made up ½ of mineral matter and organic matter and ½ of air and water to circulate***

11. Texture – proportions of different particle sizes • USDA categorizes soil based on percent clay, silt, or sand in the soil • Influences the ability to support plant life

12. Soil structure – determines how easily soil can be cultivated & how susceptible to erosion • Affects how easily water penetrates the soil

13. Soil Formation **5 factors interact during formation** • Parent Material – source of mineral matter in soil • Affect weathering and rate of soil formation • Time – longer a soil forms, the thicker it gets

14. Climate – variations in temp. & precipitation influences rate, type, and depth of weathering • Big effect on type of organisms living in soil • Organisms • Plants are main source of organic matter in soil • Organic matter releases nutrients as it decays • Microorganisms (fungi, bacteria, protozoans) are decomposers and convert nitrogen gas into compounds • Burrowing animals mix minerals and organic matter and helps water and air to penetrate soil

15. Slope • Steep slopes – erosion is accelerated thus soils are thin or nonexistent • Flat areas have little erosion • Dark color indicates lots of organic matter • Soils on south facing slopes are warmer and drier and influences the type of plants

16. Soil Profiles **varies by depth – divided into soil horizons • A Horizon – “topsoil,” mostly composed of leaves and plant structures (organic material) also has microorganisms and insects • Lower part is mixture of minerals and organic matter • B Horizon – “subsoil,” fine clay particles washed trough A horizon • Can form hardpan (compact, impermeable clay layer) • Lower limit of plant roots and animals • C Horizon – partly weathered parent material • b/w B horizon and unweathered parent material

17. Soil Types ***Depends on climate*** • Pedalfer – forms in temperate Eastern US (forests) • Pedocal – found in dry Western US • Laterite - forms in hot, wet tropical areas • Becomes very hard when dried • Used to build structures w/ dried bricks • Useless for crop growing

18. Soil Erosion • Sheet erosion, rills, trenches (gullies) • Rate of soil erosion depends on soil characteristics, climate, slope, and type of vegetation • Accelerated b/c of human activities

19. What happens if soil erodes faster than it is formed? • Lower productivity • Poor crop quality • Decreased food supply • Preserving the soil is the key to feeding the rapidly growing world

20. Mass Movements

21. Mass Movements – rock and soil move down slope b/c of gravity, landforms are product

22. 4 Factors that Trigger Mass Movements • Water – heavy rains and rapid snow melt saturates surface materials • Clay and sand become very slick when pores are full of water

23. Over steepened slopes – if steepness exceed stable angle (25-40), mass movements are likely • Creek can undercut a bank or humans excavate during construction of roads or building

24. Removal of vegetation – roots bind soil and regolith together • This makes more stable slopes • When removed, mass movements increase

25. Earthquakes - most dramatic • Can dislodge rocks and unconsolidated material

26. Types of Mass Movements Classified by material moved, how it moves, & speed of movement • Rockfalls – rocks or pieces of rock fall through the air • Common on over steepened slopes • Result of mechanical weathering: freeze-thaw or root-pry • Can trigger other mass movements

27. Slides – block of material moves suddenly along a flat, inclined surface • Rockslide – bedrock included (fastest mass movement) • Slumps – downward movement of block material along a curved surface • Not fast or far • Leaves a crescent shaped cliff • Common on over steepened slopes

28. Flows – material containing lots of water, thick fluid • Mudflows – quick moving • Arid regions • Takes boulders and trees with it • Earth flows – slower moving • Wet regions • Continuous over years

29. Creep – slowest mass movement • Contributing factor is freeze-thaw • Freeze expands soil particles, lifts • Thaw causes contraction, lower level • Each cycle moves particles slowly downhill • Cause of tilt of gravestones