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

Landforms constantly adjusted toward equilibrium 1.Equilibrium Stability 2.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

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)

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

Parent Material 1.Bedrock 2.Regolith 3.Sediments Soil Thickness 1.Rate of organic and mineral soil production 2.Rate of weathering and erosion 3.Rate of organic soil decomposition 4.Time

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) 2.Wetness and Precipitation Promotes chemical and physical weathering 3.Temperature Promotes chemical weathering 4.Freeze-thaw cycles Volume increase of H 2 O upon freezing mechanically splits rock, especially in humid continental, subarctic, polar and alpine environments

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

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

Physical Weathering Processes Rock is broken and disintegrated without chemical alteration Surface area susceptible to chemical weathering increases Freeze-thaw weathering H 2 O 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

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

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

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

Exfoliation

Dome Half Dome, Yosemite National Park, USA

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

2.Oxidation Oxygen oxidizes metallic elements to form oxides (eg. iron oxide, Fe 2 O 3 ) 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 (H 2 CO 3 ) in precipitation Reacts with rock minerals containing Ca,Mg, K and Na Minerals dissolved into H 2 O (eg. CaCO 3 ) Washed away in rainwater Cause of karst topography and landscapes such as sinkholes, tower karst and stalagtites/stalagmites.

Florida Sinkhole

Stalactite and Stalagmite complex Photo: Vladimir Maltsen

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

Types of Mass Movements 1.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 3.Landslides (translational or rotational) - sudden movement of cohesive mass of bedrock/regolith

4.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

Effects of Lahar Form of earthflow

Soil Creep

Debris avalanche