Weathering and Mass Movement

1. Weathering and Mass Movement

2. Landform Development Dynamic Equilibrium Model Uplift creates potential energy of position (disequilibrium) Sun provides heat energy Hydrologic cycle provides kinetic energy Atmosphere and crustal reactions provide chemical energy

4. Landforms constantly adjusted toward equilibrium • Equilibrium Stability • Destabilizing Event (‘geomorphic threshold’ met) • (eg. lava flow, tectonics, heavy rainfall, forest fire, • deforestation, climate change) • 3. Period of Readjustment • 4. New Condition of Equilibrium Stability

5. Hillslopes Material loosened by weathering may be eroded and transported but the agents of erosion must overcome the forces of friction before downslope movement occurs Slopes are often convexo-concave Convex at the top (waxing slope and free face) Concave at the bottom (debris slope and waning slope in the depositional zone)

7. Weathering Processes Weathering processes disintegrate rock into mineral particles or dissolve them into water Two forms: 1. Physical weathering 2. Chemical weathering

8. Parent Material • Bedrock • Regolith • Sediments • Soil Thickness • Rate of organic and • mineral soil production • Rate of weathering • and erosion • Rate of organic soil • decomposition • 4. Time

10. Factors Affecting Weathering Rates • 1. Rock Composition and Structure • Jointing increases surface area exposed to weathering • Some rocks more soluble (eg. limestone) than • others (eg. granite) • Wetness and Precipitation • Promotes chemical and physical weathering • Temperature • Promotes chemical weathering • 4. Freeze-thaw cycles • Volume increase of H2O upon freezing mechanically • splits rock, especially in humid continental, subarctic, • polar and alpine environments

11. Joints and fractures enhance rates of weathering Smaller fractures throughout (large) Limestone bedrock, Kansas, USA Photo: J.S. Aber, 1977

12. 5. Hydrology (Soil water and Groundwater) Promotes chemical weathering within the parent material 6. Geographic Slope Orientation Affects exposure to sun, wind and precipitation Important worldwide, but especially at higher latitudes 7. Vegetation Acids from organic decay add to chemical weathering; shields rock and soil; roots hold soil together on steep slopes but split jointed bedrock 8. Time Effect of the above processes increases with time

13. Physical Weathering Processes • Rock is broken and disintegrated without chemical alteration • Surface area susceptible to chemical weathering increases • Freeze-thaw weathering • H2O increases in volume by 9% upon freezing • Repeated freezing and thawing breaks rocks apart • Humid continental, subarctic, polar and alpine environments • Frost wedging pushes portions of rock apart. • The loosened, angular rock falls from cliffs in steep areas • and accumulates downslope, forming talus slopes

14. Talus slope Glacier National Park, USA – formed due to freeze-thaw weathering)

15. Crystallization Dry weather: moisture drawn upward to rock surfaces Dissolved minerals crystallize. Crystals spread mineral grains apart (especially sandstone) Opened spaces are then open to water and/or wind erosion. Hydration Minerals absorb water and expand Stresses rock – grains forced apart Granular disintegration enhances chemical weathering due to large increase in exposed surface area

16. Pressure-release jointing Overburden removed through weathering Pressure released - heave for millions of years Layers of rock peel off in curved slabs “pressure-release jointing” Exfoliation (sheeting) leaves massive, arch and dome- shaped features on exposed landscapes

17. Exfoliation

18. Exfoliation Dome Half Dome, Yosemite National Park, USA

19. Chemical Weathering Processes Chemical weathering is the decomposition of rock minerals Minerals can: 1. Combine with oxygen or carbon dioxide in the air 2. Dissolve or combine with water Forms of Chemical Weathering: 1. Hydrolysis Minerals chemically combine with water in a reaction to the mild acids in precipitation water (eg. feldspar converted to clays and silica) Disintegration etches, erodes and softens rock

20. 2. Oxidation • Oxygen oxidizes metallic elements to form oxides • (eg. iron oxide, Fe2O3) • More susceptible to further chemical weathering • 3. Carbonation and Solution • Water can dissolve 57 natural elements and many of • their compounds – “universal solvent” • Carbonic acid (H2CO3) in precipitation • Reacts with rock minerals containing Ca,Mg, K and Na • Minerals dissolved into H2O (eg. CaCO3) • Washed away in rainwater • Cause of karst topography and landscapes such as • sinkholes, tower karst and stalagtites/stalagmites.

21. Florida Sinkhole

23. Stalactite and Stalagmite complex Photo: Vladimir Maltsen

24. Mass Movement Any unit movement of a body of material propelled and controlled by gravity. Slopes and gravitational stresses are always involved Physical and chemical weathering weaken rock near the surface, making it susceptible to mass movement Angle of repose: Slope achieved at equilibrium as grains flow downslope Driving force: Gravitational forces. The greater the slope angle, the greater the likelihood of mass movement. Resisting force: Cohesiveness and internal friction

25. Types of Mass Movements • Rockfall • - rock falls through air and hits a surface • - pile of irregular, broken rocks results • 2. Debris avalanche (faster than landslide since water or • ice fluidize the debris) • - rock, debris and soil • Landslides (translational or rotational) • - sudden movement of cohesive mass of bedrock/regolith

26. Flows (formed due to increased moisture content) • 5. Creep (persistent, gradual mass movement) • -very slow movement of individual soil particles due to • freezing and thawing, wetting and drying, temperature • changes and animal disturbance

28. Effects of Lahar Form of earthflow

30. Soil Creep

31. Debris avalanche