Weathering: Chemical alteration and mechanical breakdown of rock and sediment.
* depth of air/water penetration
* as distance of formation from surface rises, so does susceptibility to weathering
* physical weathering: The physical breakup of rocks.
* chemical weathering: The decomposition of rocks and minerals caused by stability-increasing chemical reactions.
-rate of weathering positively linked to presence or absence of high mountains, and thus to plate tectonics
Physical Weathering-Mechanical Processes (water, fire, roots)
* development of joints- rock moved upward after erosion, causes fractures
-columnar joints in igneous rock
* crystal growth- salts precipitated from seeping groundwater
* frost wedging- frozen seeping water
* effects of heat- rocks don’t conduct, spall (outer shell) breaks away
* plant roots- seeds germinate in cracks
Chemical Weathering-Chemical Reactions (stabilize to surface temp)
* carbonic acid- rain + CO2 makes carbonic acid, frees H+ (comp. chg.
* hydrolysis- water ions replace mineral ions
-decomposed potassium feldspar, kaolinite replaced
* leaching- water removes soluble materials from bedrock/regolith
-seeps into water, bad taste
* oxidation- post-weathering iron release, goethite prod w/ hydration (O)
-hematite produced w/ dehydration
-intensity of color tells time/severity of weathering
* dissolution- carbonic acid dissolves calcite (previously unsoluble)
Results of Weathering
* common rocks- granite’s high silica hydrolysis=* clays & soluble min.
-basalt oxidizes, limestone dissolves
* conc of stable minerals- resistant to attack, @ streambeds/beaches b/c spec. grav. (cw)
* weathering rinds- light colored rind surrounding a darker core of rock
-composed of solid byproduct of chemical weathering
* exfoliation- spalling off of outer rock shells caused by stress during weathering
* spheroidal weathering- rock tendency to be rounded by weathering
-weathering most effective w/ *surface area, & subdivision
Factors of Weathering
* rock type/structure- minerals: quarts & granite resistant
-differential weathering based on composition & structure
* slope- steep =* rapid, gradual =* slow
* climate- moisture and heat promote reactions (chem. react. prev.
@ =* eq.)
-carbonate rocks (calcite diss.) in different regions
* time- hundreds/thousands years for a few mm
Soils: The part of the regolith that can support rooted plants. Formed by the weathering of bedrock: mineral component mixed with organic material.
* soil horizons: identifiable succession of subhorizontal weathered zones
* distinct physical, chemical, and biological characteristics
* collectively known as soil profile (parent material =* surface)
* O- surface organic debris
* A- humus, chemical leaching
* E- gray/white, present in evergreen forests (acidic soil)
* B- enriched in clay produced by weathering within the horizon
* K- only in arid zones (rich in calcium carbonate)
* C- deepest, slight weathered parent material, lacks distinct properties, yel/brn oxidation
* soil forming factors:
* climate- rain & temperature
* composition of parent material
* vegetation cover
* soil organisms
Soil Types (influence the process, classified by chem/phys properties)
* polar soils- well drained, lack well-developed horizons, weak oxidation
-wetter env’ts: tundra causes water-logged, organic rich soil, A not B
* temperature-latitude soils- well-developed horizons, unique to env’t
-deciduous Affisols, evergreen Spodosols, mountainous Entisols and Inceptisols, prairie Mollisols
* desert soils- no leaching produces solid, impervious layer of caliche
* tropical soils- heavy weathering, leaching produces iron-rich laterite
Environmental Aspects of Soils
* rate of soil production: dependent on climate (in regolith, not bedrock (cw)
* mod. T.
& high R.- rapid, within 100s years
* mod. T. & avg. R.- 100,000 years
T. & low. R.- millions of years
* paleosols are ancient soils recorded in rock as unconformities
* indirect effects- lake infilling behind dams
* rate of soil loss- 7% of soil lost each decade
* control of erosion- soil conserving farming practices, terracing, tree planting
* the world economy- soil is nonrenewable resource
Mass Wasting: The movement of regolith and masses of rock down slope under the pull of gravity.
-No Carrier: Gravity is the primary force for mass movement. Requires slope.
* role of gravity
* shear stress (as slope +, tc +)
-perpendicular component of gravity (perpendicular to inclined surface)
holds in place
-tangential component of gravity (parallel to inclined surface)
causes to move
* shear strength: internal resistance to movement
-particle cohesion and plant roots govern
* role of water
-capillary attraction is cohesive until saturated (failure)
Mass Wasting Processes
-landslides = down slope movement of rock and/or regolith.
* types of mast wasting
* slope failure- sudden down slope movement of coherent masses (rocks) (dry)
* sediment flows- down slope flow of mixtures (sediment, water, and air)
* slump- down/out rotational movement on tilted concave up surface
-can be annual event, associated w/ heavy rains or shock (earthquakes)
-rockfall- sudden free falling of detached bedrock from a steep slope
-debris fall- accompanying regolith and vegetation
-rockslide- sudden .