KARST PROCESSES + LANDFORMS

karstification process
1. essential independent variables that guide the development of the karst landscape = 'coordinate features'
  1. CHEMICAL DRIVING FORCE
    1. PRECIPITATION
      1. chemical reactions drive towards equilibrium (saturation and cessation of dissolution)WITHOUT PRECIPITATION
  2. PHYSICAL DRIVING FORCE
    1. PRECIPITATION
      1. removes saturated solutions and replaces with fresh solutions
    2. RELIEF
  3. HYDROGEOLOGIC SETTING
    1. TECTONIC SETTING
    2. THIICKNESS OF SOLUABLE ROCK
      1. places limits on the extent of karst development
        e.g. western Pennsylvania only 10-12m
        E.G. Adriatic coast 9000m
    3. STRATIGRAPHIC AND LITHOLOGICAL SETTING
2. Karst hydrologic cycle
  1. Karst aquifer and associated groundwater basins assumed to be embedded in larger basins containing non-karstic rocks (ALLOGENIC)
  2. Runoff originates from allogenic streams together with internal runoff (Qi = IN DIAGRAM )
  3. Overland flow disappears into sinkholes -> internal runoff (Qa )
  4. Karst aquifer assumed to have a single output = base level stream with flow QB from springs
  5. Porosity and permeability OF AQUIFERS - ranges
    1. Primary porosity
    2. fracture porosity
    3. conduit porosity
    4. INFLUENCE ON
      1. Water infiltrating percolates downwards under gravity until it reaches level where all pores are water-filled.
      2. Phreatic zone
        water-saturated zone below the water table
      3. Vadose zone
        unsaturated zone above the water table
    5. Karstic aquifers contain integrated systems of pipe-like conduits that act as drains for the highly localised transport of watery
    6. Properties of carbonate aquifers vary widely
  6. KARST RECHARGE
    1. OCCURS IN = Catchments of mixed geology
    2. solution attributable both:
      1. 1) precipitation (autogenic recharge)
      2. AND 2) water that flows on to the karst from elsewhere (allogenic recharge)
    3. Denudation varies as geochemical evolution of the water different
      1. BECAUSE
        1) soil and boil vary from point to point
        recharge is organised differently
        AUTOGENIC CASE = diffuse and relatively even
        ALLOGENIC CASE = ocused + high volume + solution restricted to line of watercourse
Theoretical Cave Development Models in Carbonates
1. VADOSE THEORY
  1. Caves formed by action of underground streams flowing at or above the water table
  2. solution takes place most actively at the top of the saturated zone BECAUSE aggressiveness is lost once water descends through sinkholes and enters the water table
2. DEEP PHREATIC THEORY
  1. origin = Davis's work on Peneplains
KARST = A distinctive topography in which the landscape is largely shaped by the dissolving action of water on carbonate bedrock
1. results in
  1. surface and subsurface features
    1. sinkholes
    2. vertical shafts
    3. disappearing streams
    4. springs
    5. underground drainage systems
    6. caves
  2. Caocite Dissolution
    1. 1) In the presence of water, calcite dissociates to yield a positively charged calcium cation and a negatively charged carbonate anion
      1. CaCO3 -><- Ca++ + CO3 2-
      2. Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface
    2. 2) Carbon dioxide gas dissolves in water to produce carbonic acid
      1. CO2 + H20 -><- H2CO3
        Hydration of aqueous CO2 to form neutral carbonic acid
    3. 3) The carbonic acid dissociates into a hydrogen cation (protom) and a bicarbonate ion.
      1. H2CO3 -><- H+ + HCO3
        Ionization of the carbonic acid to form a bicarbonate ion and a proton
    4. 4) Free protons H+ combine with carbonate anions HCO3 -
      1. HCO3 - = which are more soluble in water than carbonate
      2. H+ + CO3 2- -> <- HCO3
        Reaction of the proton with a carbonate ion on the crystal surface to form a second bicarbonate ion
    5. 5) Continuing production of bicarbonate releases calcium ions Ca2+ into the boundary layer of the water
      1. Kinematics
        1. Desorption of the second bicarbonate ion and its diffusion across the boundary layer into the bulk solution
        2. Release of the calcium ion from the crystal
        3. Diffusion of the calcium ion across the boundary layer into the bulk solution
      2. The ions are carried away in solution
    6. Both limestone and marble = composed of the mineral calcite= chemically is calcium carbonate: CaCO3
      1. MARBLE = metamorphosed limestone
      2. Dolomite is a double carbonate of calcium and magnesium: CaMg(CO3 )2 = less soluble than calcite = karst on dolomite = less well developed than on limestone or marble
    7. SUMMARY EQUATION = CaCO3 + H20 + CO2 -><- Ca(HCO3 )2
      1. Ca(HCO3 )2 = calcium bicarbonate
      2. HOWEVER reactions are fully reversible = when precipitates are exposed to un-saturated ground water = re-dissolve
  3. changing water table levels
  4. subsidence
  5. Distinctive landforms
  6. Saturated and undersaturated solutions
    1. SATURATED = dynamic equilibrium with no more solution: the solution
      1. either
        supply of CO2 going into solution being exhausted
        where it is balanced by outgassing of CO2
      2. can lead to groundwater becomes SUPERSATURATED with dissolved minerals
        further dissolution = not possible, =carbonate salts of calcium and magnesium may precipitate from the water
        features E.G. stalactites.
    2. UNSATURATED = there is net dissolution of the mineral
      1. SOMETIMES refferred to as AGGRESSIVE
        Aggressive dissolution = flow is focussed in pre-existing openings in the rock OR in the zone of water-table fluctuation where ground water is in contact with the atmosphere
2. IN = humid regions.
  1. wet regions have much higher denudations rates than drier regions
  2. EXAMPLE FLORIDA
LANDFORM
1. Distinctive landforms due to high rock solubility, which causes secondary porosity and subsidence.
FORMATION
1. the carbon dioxide cascade
  1. 1. As rain falls through the atmosphere, it picks up CO2 which dissolves in the droplets
  2. 2. Once rain hits the ground, it percolates through the soil and picks up more CO2
  3. 3. a weak solution of carbonic acid is formed
    1. EQUATION = H2O + CO2 -> <-H2CO3
      1. Hydration
  4. 4. infiltrating water naturally exploits any cracks or crevices in the rock.
    1. CHEMISTY = carbonic acid reacts readily with limestone CaCO3 and/or dolomite (CaMgCO3)2
  5. 5.. Over long periods, with a continuous supply of CO2 -enriched water, carbonate bedrock begins to dissolve.
    1. CHEMISTRY = Cavities and voids develop a= limestone or dolomite dissolved into component ions of calcium (Ca++), magnesium (Mg++) and bicarbonate (HCO3 - ).
  6. 6. Openings in the bedrock increase in size
  7. 7. underground drainage system begins to develop, = allows more water to pass = accelerating the formation of karst
  8. 8. leads to the development of subsurface caves
2. Environmental variables affecting karst formation
  1. PRECIPITATION= DOMINANT
    1. wet regions have much higher denudations rates than drier regions
  2. TEMPERATURE
  3. ORGANIC MATTER
  4. TURBULENCE
CHEMISTRY
1. Salts
  1. Calcite and dolomite are ionic salts. In pure water they dissociate into their constituent ions
    1. 1) CaCO3 -><- Ca++ + CO
    2. 2) CaMg(CO3 )2 -> <- Ca++ + Mg++ + 2CO
2. Ph
  1. Calcite: pH ~ 8.4 CO2 in air dissolved in cloud droplets Falls as lower pH precipitation with very slow dissolution
3. Soils
  1. Water, and Humic Acid; pH ~ 4-7 Precipitation soaks though O and A horizons and adds plant acids
    1. CaCO3 (s) + 2H+ (aq) -><- Ca++ (aq) + CO2 (g) + H2O
4. Temperature
  1. Cold water contains more CO2
    1. CO2 + H2O -><- H2CO3 -><- H+ + HCO3 -
5. Pressure
  1. deeper H2O , more CO2 absorbed -> more ‘aggressive’ water
6. Mixing
  1. can result in undersaturated H2O
7. Speed of flow
  1. runoff can move a long distance in a few days
    1. runoff = time taken for a solution to approach saturation)
8. Solubility:
  1. calcite in pure water = very small (less than that of quartz)
    1. Add CO2 to water = carbonic acid = calcite begins to dissolve.
Evolution
1. : Karst aquifers form by flowing water containing carbon dioxide (CO 2) which dissolves carbonate rocks.

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KARST PROCESSES + LANDFORMS

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