physical geology exam 4
Terms
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- mass wasting
- horizontal and vertical movement, any material can move, provides the bulk of the material for streams to carry.
- direct cause: gravity
- low slope - friction exceeds shear, high slope - shear exceeds friction
- fall
- freefall of any size fragments, cliffs, roadcuts
- slide
- coherent movement along a well-defined surface, faults, joints, bedding plains, may create its own surface.
- flow
- non-coherent movement, like a fluid. fragments move independently of each other. often produces lobe or tongue-like edges.
- slump
- combination of slide and flow. slide at head, flow at toe.
- creep
- slowest, most widespread form
- precipitating problems
- too much rainfall, earthquakes, fires, fluid soils like quick clays, planes of downslope weakness in the rock
- anthropogenic causation
- undercutting/overweighting slopes, creating oversteepened slopes, reactivating ancient landslides
- mitigating mass wasting
- easy to study and easy to ignore
- hazard mapping
-
-topography
-bedrock & soil nature
-bedding planes, joints, foliations
-prior movement - modifying construction
- removing or terracing overhanging slopes, improving water drainage, rock bolts, retainer walls, vegetation/erosion control, shelters
- subsidence
- purely vertical motion, mostly caused anthropogenically, few fatalities, but large property losses
- mitigating subsidence
- by the time it starts sinking, it's too late. adding a fill is an expensive band-aid. prevention is the key: surface mapping bedrock and soils, subsurface testing
- river
- stream - any channelized surface flow - collect, transport, dispense water and sediments.
- drainage networks
- an array of interconnecting streams, developing channels caused by water moving overland in the shortest/steepest route.
- basin
- separated by divides
- stream piracy
- (capture) - when the headward erosion of a stream crosses a divide, one stream steals the upper network of another
- drainage patterns and topography
- dendritic- network of branches (tree), rectangular- joints, radial- volcano, trellis- resistant ridge
- antecedent stream
- existed before typography
- superposed stream
- superimposed on older rocks
- high gradient streams
- far from base level, narrow, V-shaped valleys, features like rapids and waterfalls, stream cuts downward
- low gradient streams
- close to base level, wide,flat bottomed valleys, features like meanders and floodplains, stream cuts sideways
- curves
- (meanders) - grow, get cutoff, grow again. oxbow lakes and meander scars. over time, plain develops. covers with water during floods, floodplain.
- natural levees
- forms during floods. velocity slows when it leaves channel. lower velocity causes deposition.
- incised meanders
- when a low gradient river is rejuvenated, meanders stop cutting sideways, and start cutting down.
- braided streams
- form when stream has more sediment than it can carry. water flows through multiple channels.
- deltas
- form when sediment-laden water enters a body of quiet water.
- alluvial fans
- form when sediment-laden water enters a dry valley.
- discharge
- volume of water per unit time in a stream
- flooding
- most destructive natural hazard. normal, natural and cyclical.
- upstream
- (flash) - small streams, begin and end quickly
- downstream
- (riverine) - large streams, begin and end slowly
- hydrographs
- plots of stream discharge as a function of time for a given station. usage dependent on time period chosen. short-period (hours to days) used for flooding, long-period (months to years) used for water resources.
- 100-year flood
- a probability, not a deterministic idea. a 100-year flood = 1% chance per year.
- levees
- control structures: walls parallel to channel on floodplain. intended to keep floodwater off floodplain
- results of levees
-
armored riverbanks - river completely walled off from its floodplain, levee failure results in catastrophic flood.
sediment stored in channel. river bottom raised, so river levels higher. - flood mitigation
-
retention structures - collect and hold impervious surface runoff, allows discharge downstream slowly.
zoning, flood insurance, buy outs - oceanic sedimentation near continents
- fine-grained detrital rocks - siltstones, mudstones, shales.
- oceanic sedimentation not near continents
- shallow water - limestones. deep water - cherts. carbonates become soluble under pressure.
- spring tides
- when the pull is in the same direction
- neap tides
- when they pull at 90 degrees to each other.
- emergent coastlines
- formerly submerged features now exposed on land (ocean terraces).
- submergent coastlines
- formerly exposed features now submerged (estuaries, fjords)
- ocean terraces
- wave-cut benches that have been uplifted
- estuaries
- drowned river valleys
- fjords
- drowned glacial valleys
- west coast processes
- steep slopes: quick sand loss, small beach zones, littoral cells. california.
- east coast processes
- gentle slopes: slow sand loss. large beach zones, barrier islands. north carolina.
- barrier islands
- large sand bars that stand above sea level.
- groins
- intercepting longshore drift, walls perpendicular to beach trap sand on one side. current picks up new sand on the other.
- jetties
- maintaining inlets. wall perpendicular to beach block longshore current. blocked current drops sand on one side, current picks up new sand on the other.
- breakwaters
- block waves. walls parallel to beach stop waves. longshore current dies, sand dumped behind wall.
- seawalls
- armoring the dune line.
- mitigating coastal hazards
-
planning: geologic and emergency.
zoning: limit modifications in vulnerable areas. - porosity
- the percentage of a material that is open space
- primary porosity
- forms during deposition and lithification
- secondary porosity
- develops after rock forms
- permeability
- how easily a fluid passes through a material. permeable materials have connected pores, impermeable materials have isolated pores.
- aquifer
- rock/sediment layer that allows fluid to pass (highly permeable)
- aquitard
-
rock/sediment layer that prevents fluids from passing (highly permeable)
also called aquiclude. - unconfined aquifers
- not restricted by aquitards, local recharge, water level dependent on local conditions
- confined aquifers
- restricted by aquitards, remote recharge, water level independent of local conditions.
- pressurized confined aquifers
- artesian aquifers. recharge at a higher elevation, pressure surface above aquifer elevation. effect like a water main.
- karts
- filled with caves, sinkholes, sinking creeks. only forms in humid regions with carbonate bedrock.
- glaciers
- systems of flowing ice. relatively little impact currently.
- alpine (valley) glaciers
- found in mountainous regions worldwide.
- continental glaciers
- greenland and antarctica
- glacier formation
- snow, compact, repeat
- glaciers move by two means
- by flow and slip
- glacial erosion by plucking
- loosening and removing blocks from the bedrock
- glacial erosion by abrasion
- filing and grinding rock
- cirque
- bowl-shaped depression at the head of a glacier, u-shaped valleys
- finger lakes
- u-shaped valleys
- glacial deposition
- occurs at the margins of the ice, sediments can be deposited directly by ice
- outwash
- sediments carried from glacier by meltwater. may be sorted, rounded and stratified.
- erratics
- out of place rocks deposited by glaciers. igneous/metamorphic boulders on sedimentary layers.
- varves
- seasonal layers deposited in meltwater lakes. silt in spring; clay during winter.
- moraines
- till at the margins of the ice. names based on location: end, terminal, lateral, medial, ground.
- eskers
- sub-ice stream channels. meltwater deposits, so may be sorted and stratified.
- glacial period hypotheses
- changes in the sun, changes in the relationship of the Earth to the sun, changes in heat budget due to land arrangement, changes in heat budget due to atmospheric transparency