KARST PROCESSES + LANDFORMS - created from Mind Map

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karstification process

essential independent variables that guide the development of the karst landscape = 'coordinate features'

Karst hydrologic cycle

CHEMICAL DRIVING FORCE

PHYSICAL DRIVING FORCE

HYDROGEOLOGIC SETTING

PRECIPITATION

chemical reactions drive towards equilibrium (saturation and cessation of dissolution)WITHOUT PRECIPITATION

PRECIPITATION

RELIEF

removes saturated solutions and replaces with fresh solutions

TECTONIC SETTING

THIICKNESS OF SOLUABLE ROCK

STRATIGRAPHIC AND LITHOLOGICAL SETTING

places limits on the extent of karst development

e.g. western Pennsylvania only 10-12m

E.G. Adriatic coast 9000m

Karst aquifer and associated groundwater basins assumed to be embedded in larger basins containing non-karstic rocks (ALLOGENIC)

Runoff originates from allogenic streams together with internal runoff (Qi = IN DIAGRAM )

Overland flow disappears into sinkholes -> internal runoff (Qa )

Karst aquifer assumed to have a single output = base level stream with flow QB from springs

Porosity and permeability OF AQUIFERS - ranges

KARST RECHARGE

Primary porosity

fracture porosity

conduit porosity

INFLUENCE ON

Karstic aquifers contain integrated systems of pipe-like conduits that act as drains for the highly localised transport of watery

Properties of carbonate aquifers vary widely

Water infiltrating percolates downwards under gravity until it reaches level where all pores are water-filled.

Phreatic zone

Vadose zone

water-saturated zone below the water table

unsaturated zone above the water table

OCCURS IN = Catchments of mixed geology

solution attributable both:

Denudation varies as geochemical evolution of the water different

1) precipitation (autogenic recharge)

AND 2) water that flows on to the karst from elsewhere (allogenic recharge)

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

VADOSE THEORY

DEEP PHREATIC THEORY

Caves formed by action of underground streams flowing at or above the water table

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

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

results in

IN = humid regions.

surface and subsurface features

Caocite Dissolution

changing water table levels

subsidence

Distinctive landforms

Saturated and undersaturated solutions

sinkholes

vertical shafts

disappearing streams

springs

underground drainage systems

caves

1) In the presence of water, calcite dissociates to yield a positively charged calcium cation and a negatively charged carbonate anion

2) Carbon dioxide gas dissolves in water to produce carbonic acid

3) The carbonic acid dissociates into a hydrogen cation (protom) and a bicarbonate ion.

4) Free protons H+ combine with carbonate anions HCO3 -

5) Continuing production of bicarbonate releases calcium ions Ca2+ into the boundary layer of the water

Both limestone and marble = composed of the mineral calcite= chemically is calcium carbonate: CaCO3

SUMMARY EQUATION = CaCO3 + H20 + CO2 ->

CaCO3 ->

Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface

CO2 + H20 ->

Hydration of aqueous CO2 to form neutral carbonic acid

H2CO3 ->

Ionization of the carbonic acid to form a bicarbonate ion and a proton

HCO3 - = which are more soluble in water than carbonate

H+ + CO3 2- ->

Reaction of the proton with a carbonate ion on the crystal surface to form a second bicarbonate ion

Kinematics

The ions are carried away in solution

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

MARBLE = metamorphosed limestone

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

Ca(HCO3 )2 = calcium bicarbonate

HOWEVER reactions are fully reversible = when precipitates are exposed to un-saturated ground water = re-dissolve

SATURATED = dynamic equilibrium with no more solution: the solution

UNSATURATED = there is net dissolution of the mineral

either

can lead to groundwater becomes SUPERSATURATED with dissolved minerals

supply of CO2 going into solution being exhausted

where it is balanced by outgassing of CO2

further dissolution = not possible, =carbonate salts of calcium and magnesium may precipitate from the water

features E.G. stalactites.

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

wet regions have much higher denudations rates than drier regions

EXAMPLE FLORIDA

LANDFORM

Distinctive landforms due to high rock solubility, which causes secondary porosity and subsidence.

FORMATION

the carbon dioxide cascade

Environmental variables affecting karst formation

1. As rain falls through the atmosphere, it picks up CO2 which dissolves in the droplets

2. Once rain hits the ground, it percolates through the soil and picks up more CO2

3. a weak solution of carbonic acid is formed

4. infiltrating water naturally exploits any cracks or crevices in the rock.

5.. Over long periods, with a continuous supply of CO2 -enriched water, carbonate bedrock begins to dissolve.

6. Openings in the bedrock increase in size

7. underground drainage system begins to develop, = allows more water to pass = accelerating the formation of karst

8. leads to the development of subsurface caves

EQUATION = H2O + CO2 ->

Hydration

CHEMISTY = carbonic acid reacts readily with limestone CaCO3 and/or dolomite (CaMgCO3)2

CHEMISTRY = Cavities and voids develop a= limestone or dolomite dissolved into component ions of calcium (Ca++), magnesium (Mg++) and bicarbonate (HCO3 - ).

PRECIPITATION= DOMINANT

TEMPERATURE

ORGANIC MATTER

TURBULENCE

wet regions have much higher denudations rates than drier regions

CHEMISTRY

Salts

Ph

Soils

Temperature

Pressure

Mixing

Speed of flow

Solubility:

Calcite and dolomite are ionic salts. In pure water they dissociate into their constituent ions

1) CaCO3 ->

2) CaMg(CO3 )2 ->

Calcite: pH ~ 8.4 CO2 in air dissolved in cloud droplets Falls as lower pH precipitation with very slow dissolution

Water, and Humic Acid; pH ~ 4-7 Precipitation soaks though O and A horizons and adds plant acids

CaCO3 (s) + 2H+ (aq) ->

Cold water contains more CO2

CO2 + H2O ->

deeper H2O , more CO2 absorbed -> more ‘aggressive’ water

can result in undersaturated H2O

runoff can move a long distance in a few days

runoff = time taken for a solution to approach saturation)

calcite in pure water = very small (less than that of quartz)

Add CO2 to water = carbonic acid = calcite begins to dissolve.

Evolution

: Karst aquifers form by flowing water containing carbon dioxide (CO 2) which dissolves carbonate rocks.

KARST PROCESSES + LANDFORMS