Physical Geology Notes Chapter Seven 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: * time * climate- rain & temperature * composition of parent material * vegetation cover * soil organisms * topography 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 * low. T. & low. R.- millions of years * paleosols are ancient soils recorded in rock as unconformities Soil Erosion * 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 Chapter Eight 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) Slope Failure * slump- down/out rotational movement on tilted concave up surface -can be annual event, associated w/ heavy rains or shock (earthquakes) * falls -rockfall- sudden free falling of detached bedrock from a steep slope -debris fall- accompanying regolith and vegetation * slides -rockslide- sudden movement of detached masses across bedding plane -debris slide- accompanying movement across plane of reg. and veg.
* talus-body of debris beneath a cliff; settle @ angle of repose Sediment Flows (controlled by) -relative proportion of solids, water, and air -physical and chemical properties of the sediment -steepness of slope -can start and stop * slurry flows- water saturated moving mass (soil & regolith ) (20-40% water) -solifluction- very slow (cm/yr) -debris flow- coarse * sand, poorly sorted, moves along alluvial fan (m/hr) -mudflow- highly fluid, moves along valley floor (km/hr) * granular flows- not water saturated (20%-0% water) -creep- loose deposits that move = colluvium, very slow (cm/yr) -earthflow- narrow shape, bulging front (m/hr) -abrupt shock to porous, wet sediment leads to liquefaction -grainflow- sand grains, etc: angle of repose is exceeded (km/hr) -debris avalanche- rare, spectacular event (100km/hr) -huge masses, pulverize and continue moving (stratovolcanoes) * *40% water constitutes a stream Mass Wasting in Cold Climates (especially active) * frost heaving- lifting of regolith by the freezing of ground-contained water * gelifluction- thin surface layer thaws in summer and refreezes in winter (unstable) -cm/yr * rock glaciers- ice cemented debris which move similar to glaciers -m/yr (Alps, Andes, Rockies) Subaqueous Mass Wasting -submarine slope failures cause turbidity currents * deltas- region of slumping, central channel to transport sediment, zone where sediment is deposited * continental slope- disturbed and contorted layering caused by earthquakes, slumping, and glacial cycles (ice ages) has built a thick wedge of sediment along base * oceanic island submarine flanks- lava flows and unstable rubble placed by massive volcanic landslides Triggering of Mass Wasting Events * shocks- earthquakes * slope modification- human construction * undercutting- stream or surf action * exceptional precipitation- ground becomes saturated and unstable * volcanic eruptions- slope failure and large volumes of released water * submarine slope failures- high internal pressure of water trapped under sediment bed, methane gas of organic matter, angle of repose issues Mass Wasting Hazards * assessment of hazards -maps of geology surrounding the site * mitigation of hazards -drain/pump soil to de-saturate -empty dams -minimize activity in danger zones Chapter Nine Streams: A body of water that flows down slope along a clearly defined natural passageway. Part of the hydrologic cycle. * streams as geologic agents: -carry water to the ocean -carry soluable salts to the ocean -shape the surface of the Earth -transport sediment to the ocean Factors in Streamflow Stream Channels * the passageway is called a channel * the load is the sediment and dissolved matter the stream transports (kg/m3) * the discharge is the volume of water passing a given point at a given time (m3/s) * gradient- the vertical distance a stream cannel falls between two points (m/km) * long profile- long curve of gradient from stream’s mouth to delta (m2) Dynamics of Streamflow * runoff- the portion of precipitation that flows on land surface * overland flow- broad sheets of runoff moving down slope * streamflow- the runoff and overland flow that flows in defined channels Changes Downstream * discharge increases * channel cross-sectional area increases * velocity increases slightly * gradient decreases * grain size decreases * mineralogy may change Floods (when the stream’s discharge exceeds the capacity of the channel) * First, discharge increases (shown by hydrograph) -channel is scoured -cross-sectional area increases -velocity increases * Then, discharge decreases -sediment is deposited -cross-sectional area decreases -velocity decreases * Floods are predicted using frequency curves. * Catastrophic floods are rare (Columbia River) Base Level (level below which a stream cannot erode the land) * types of base levels: -sea level (base level for most streams) -lakes -natural dams (landslides & lava) -artificial dams Channel Patterns factors * stream gradient * discharge * sediment load Types of Channel Patterns * straight channels- rare, sinuous max. depth, sandbar on opposite side * meandering channels- switchback bends in stream, gentle gradients -max velocity @ outer curve, pointbars on inner curve -oxbow lakes result from two meanders intersecting * braided stream- water divides and reunites through interconnected channels -separated by bars or islands -occurs most in streams w/ highly variable discharge and erodible banks -only 10% active at a given time Erosion by Running Water before streams form * sheet erosion- due to rain drops and overland flow -vegetation reduces runoff and roots percolate water after streams form * laminar flow- particles travel in parallel layers, slow and smooth velocity * turbulent flow- swirls and eddies, high and complex velocity -bed and channel walls maintain laminar flow (frictional drag) Transport by Running Water Types of Stream Load * bed load- coarse particles that move along stream bed (up to 50% load) via -rolling or sliding -saltation-short intermittent jumps * suspended load- fine particles that move along surface -silt and clay * dissolved load- dissolved substances (underground seepage) (product of chemical weathering) * sediment size correllated to velocity of stream (greatest w/ steep slopes or basins) * alluvium- name for solid particles wherever they are dropped downstream changes: * grain size decreases (opposite than expected) * composition/mineralogy of sediments (new rocks) * placer deposit- concentration of heavy minerals * sediment yield- amount of sediment eroded and transported Deposits by Streams (alluvium) * point bars (see: meandering channel pattern) * floodplains (part of natural valley) and natural levees (low ridge of alluvium along side of channel) (see: frictional drag and turbulent flow) * terraces- remnant of abandoned floodplain * alluvial fans- fan-shaped alluvium deposits where a channel leaves a valley * deltas- stream flows into standing water (sharp velocity drop) -fan vs. braid deltas (shape determined by sedimentation/erosion balance) -foreset layer =* topset layers of deposition -topset layers between channels (distributaries) Drainage Systems terminology * drainage basin- total area that contributes water * divide- line that separates adjacent basins * continental divide- basins drain to different oceans (follow techtonic allignment) -velocity- steep Andes, gradual Atlantic * stream order- system of numbering streams (first- order) drainage pattern development * rock type * rock structure * stream history (stream capture) Types of Drainage Patterns * dendritic * parallel * radial (center =* out) * rectangular (90 degrees) * trellised (criss-crossed) * annular (concentric radial) * centripetal (out =* center) * deranged (sand bars, topset layers) Stream Histories produce Drainage Patterns * consequent stream pattern- formed by uniform rock units and general slope of region (dendritic or parallel) * subsequent stream pattern- follow folds or rock units, formed in folded or faulted regions (trellissed) * antecedent stream- cuts across folds due to incision during uplift * superposed- cut across different rock units due to stream cutting and angular unconformity Chapter Ten Groundwater: All the water contained in spaces within bedrock and regolith.-less than 1% of all water, up to 750m deep -rainfall is source Zones Near the Surface * zone of aeration- water is present, but ground is not saturated * saturated zone- all openings are filled with water * water table- top of saturated zone Porosity- % of total volume of bedrock/regolith with pores -determins quantity of water a sediment or rock can contain -affected by sorting, arrangement, and cementation Permeability- how easily fluids pass through -molecular attraction affects flow -sediment larger than sand is very permeable Recharge Area- area of water input Discharge Area- area of water output water table shape * humid regions -river is discharge -river is low on water table * dry regions -river is recharge -river is high on water table movement of groundwater 1.
rain goes in (recharge area) * moves downhill thru zone of aeration (percolation-chief movement) 1. soil is less permeable than underground 2. some water held by molecular attraction * movement in zone of saturation * direction is generally downhill (hydraulic gradient) -Darcy’s Law- discharge=flow, permeability, and gradient * velocity relates to water table slope * moves from high points to low points * moves from high pressure to low pressure * subsurface water comes out (discharge area) Springs (groundwater emerging at the surface) * caused by intersection of water table with the surface (edge of 2 layers) * most major springs emerge from lava, limestone, or gravel * appear via -aquicludes- impermeable unit -aquifers- permeable unit in saturated zone permeable sediments and rocks * sand and course sediments * sandstone * limestone * jointed lava flow * jointed granite impermeable rocks * clay * mudstone * unjointed granite * bedrock Wells (drilled to intersect water table) (zone of saturation) * cone of depression- conical depression of water table surrounding well * perched water body- different confined aquifers at different depths Aquifers (body of highly permeable rock or regolith w/in the zone of saturation) types of aquifers * unconfined aquifer- its upper surface coincides with the water table -30% of water from high plains aquifer * confined aquifer- aquifer bounded by aquicludes -Dakota aquifer * artesian aquifer- confined aquifer under hydrostatic pressure -water can rise in artesian well to height of recharge area * complex regional aquifer- system of unconfined/confined aquifers connected by atrtesian aaquifers -Floridian aquifer Environmental Stuff Mining Groundwater (major source of freshwater, non-renewable resource) lowering of the water table * due to excessive consumption * causes land subsidence (artesian pressure supports rock weight above) * artificial recharge (spraying of biodegradable liquid waste) Water Quality chemistry of groundwater (dissolved components) * chlorites * sulfates * bicarbonates of Ca, Mg, Na, K, Fe -soft water vs. hard water: Ca, Mg * hydrogen sulfide * salts (NaCl) Groundwater Contamination * sewage * seawater * toxic wastes * agricultural poisons * underground storage of hazardous wastes Geologic Activity of Groundwater * dissolution- chemical weathering in soil (carbonate rocks) (carbonic acid) -limestone (completely away), dolostone, and marble -exceeds avg.erosion of mass wasting, sheet erosion, and streams -10mm/1000yr * chemical cementation- converts sediment into sedimentary rock -calcite, quartz, and iron are chief cementing substances * chemical replacement- dissolves one substance and replaces it with another -mineral and organic substances can be replaced (petrified wood) * carbonate caves and cave formation (cavern =* system of caves) -rate of formation = rate of dissolution (acidic water +) 1. shallow saturated zone * initial dissolution along joints by percolating groundwater -carbonic acid & sulfuric acid * deposition of carbonate formations on cave walls while a stream occupies the cave floor * continued deposition after the stream has stopped flowing * cave deposits -insoluable clay and silt -deposits chemically precipitated from dripping water: dripstone, stalactites -deposits chemically precipitated from flowing water: flowstone, stalagmites -columns- stalactites and stalagmites grown together -dripstone and flowstone can only be deposited if cave is partially filled with air (lie at or above water table) -crystals and cave form when in zone of saturation, mites/tites when in zone of aeration Karst Topography Factors of Karst Topography * warm temperature promotes dissolution * adequate rain =* groundwater, soil & plants -CO2 Features * sinkhole- dissolution cavity open to the sky (caused by falling water table) * solution valleys- channels of sinkholes * cones and towers- steep topography Karst Topography (small, closed basins, disrupted drainage (springs)) -steep hydraulic gradient -limestone or dolomite -usually in tropical areas * sinkhole karst- landscape dotted with sinkholes * cone karst/tower karst- thick, well-jointed limestone that separates into isolated blocks as it weathers -cone karst- closely spaced conical hills separated by deep sinkholes -tower karst- isolated limestone hills separated by expanses of alluvium * pavement karst- broad areas of bare limestone in which joints and bedding planes have been etched and widened by dissolution (glaciation areas) Chapter Eleven Glaciers: -A permanent body of ice that shows evidence of down slope or outward movement due to the pull of gravity. -recrystallized permanent snow types of glaciers * classified according to form and size * mountain glaciers and ice caps -cirque- occupies enclosed by headwall -a growing cirque glacier (spreading down and out) becomes a valley glacier -valley glaciers that extend to the sea are called fjord glaciers -when it spreads beyond the mountain front it becomes a piedmont glacier -an ice cap covers top of mountain, spreads outward * ice sheets and ice shelves -ice sheets are the largest glaciers on earth (Greenland and Antarctica=95%) -Antarctica’s Transantarctic Mountains, East sheet larger, + altitude, – T -ice shelves are thick, nearly flat sheets of floating ice -fed by glaciers, terminate w/ steep ice cliffs -Antarctic, Canadian Arctic Islands * classified according to internal temperature * temperate glacier- ice at pre …