Karst Geomorphology Flashcards
Importance of karst
- regions provide aquifers that are capable of providing large supplies of water (springs)
- > 25% of the worlds population either lives on or obtains its water from karst aquifers
- vital to understand its structure due to potential harzards:
- sinkhole, ground subsidence, groundwater contamination, variable water supple.
Karst defined
a landscape formed mainly by rock being dissolved by surface or groundwater
- rock (typically limestone or salt)
- acidic surface water
Karst distribution
- world distribution of surficial carbonate rocks
- formation (lakes/shallow marine systems)
- deposition (algae, shells, skeletal material, coral)
- precipitation of dissolved CaCO3
- chalk is very weak = no underground drainage
what is karst
- terrain with distinctive geomorphology and hydrology arising form a combination of high rock solubility and well developed secondary porosity
- the key to karst is the unusual subsurface hydrology
- also pseudokarst, thermokarst and vulcanokarst
Dissolution
process of rock dissolving when it comes into contact with water
karst process
- dissolution
- rock/mineral is dissolved by surface or groundwater and transported in solution
- karst mostly occurs in humid regions where carbonate rock is present
- (karst does occur in every region; temperate, tropical, polar)
Karst hydorlogy and geomorphology main processes
- solution
- precipitation
- subsidence
- collapse
Karst processes
- true karst
- holokarst
- fluviokarst
- Glaciokarst
- thermokarst
Karst drainage
- surface/near surface (epikarst)
- subsurface (endokarst)
- water table follows topography
- changes depth with changes in precipitation
Vadose
aerated zone above water table
Phreatic
saturated ground water zone
solution in Karst
- carbonate rocks dissolve in weakly acidic water
- rainwater becomes acidic by dissolving CO2 = carbonic acid
- soil water are often acidic through the release of organic acids from rotting vegetation
- weathered minerals may also release acids
carbonic acid formation
H2O + CO2 = H2CO3
- rainwater picks up carbon dioxide
H2CO3 = H+ + HCO3-
- carboic acid dissociates to produce hydrogen ions = acid (lowers pH)
- solution processes are more effective in warm/wet tropical climates
solution of silicate rocks
- crystalline quartzite has low solubility (10%)
- dissolution of weak amorphous (non-crystalline) silica = increasing porosity = weakening (glass)
- less common, slower process
cycle of development
- W.M.Davis, 1893
- Grund, 1903
- Cvijic, 1918
factors effecting Karst processes
- solubility of bedrock, % calcite
- climate, temp and moisture
- structure of limestone, joints, fracture, porosity
- vegetation/non carbonate geology, pH of groundwater
- atmospheric CO2, affects solubility of carbonates
surface Karst features (The Dinaric region, Slovenia)
- doline (depression in the land surface)
- ponor
- polje
- disappearing stream
- uvala
- limestone pavement
- fluting and grooving
- karst towers
- karst cones
- cave in karst tower
- resurgent stream with tufa bed
- cavern with stalagmites and stalactites
- subterranean channels
Karren
- bare forms and covered forms
- small scale solution/sculpturing of ground surface or caves
- grikes, gutter, decantation flutings
clinks and grikes
can develop into bogaz, corridors and streets (gorges)
- solution rates: 1mm/10-20 years
covered karren
develop where you have full or partial soil cover
- acts like an acidic sponge
limestone pavements
- juxtaposition of glacial and chemical weathering processes
- polygentic karst
closed depressions formation process
- surface solution, concentration around a joint = positive feedback
- cave collapse: underground solution = collapse
- piping, joint widening
- subsidence, dissolution of salts
Polje
- enclosed, flat bottomed, steep-sided depressions
- can extend over 100km
- based in bedrock or unconsolidated sediment
3 types: - border, (Dominated by allogenic sediment)
- Structural (geologically controlled graben)
- Base level, (controlled by water level)
Polje can also be classified by hydrology characteristics related to their drainage pathways
- closed polje (subterranean drainage)
- upstream open polje (subterranean drainage)
- downstream open polje (surface and subsurface drainage)
- up and downstream open polje (surface and subsurface drainage)
Polje uses
- characterised by seasonal flooding
- wetland areas favoured location for palaeolithic humans availablity resources)
- good source of palaeo-environmental and archaeological information
Cone karst
- dominant in tropical environments
- dolines interact under intense chemical weathering = cone karst
- high run-off rates and thick soil = >soil CO2 = more acid
fluvial karst
- karst landscapes dominated by subsurface water courses
- sinks through to the bed
- where water present, it is more effective agent than slope processes
- dry valleys, peak district Dorset
Dry valley formation
- postglacial meltwater and permafrost thawing
- large amounts of fluvial action
- high volumes of water, unable to sink as ground saturated, channels blocked or flow rate too high
- depending on context, seasonal flooding and drying can take plate today
cave formation
- kinetic: capillary flow = solution
- inheritance: pre-existing cavities are exploited
- hypergene: chemically enriched hydrothermal waters create cavities
Speleogenesis (cave formation)
- chemical solution
- fluvial erosion
- tectonics
- microorganisms
- pressure
- atmospheric influences
- human
cave types
classified based on their position relative to the water table:
- vadose
- deep phreatic
- water table or epiphreatic
speleogen type
- potholes & current markings (erosion flowing water)
- rock pendants & scallops (solution features)
- larger = slower flow
- smaller = faster flow
- directional indicators
speleothems
depositional features
- dripstone, straws
- flowstone (sheet-like, walls/floors)
- stalagmites/stalactites/columns
formation Lechuguilla
- tectonic uplift = ground expansion/cracking
- known for oil reserves (release H2S)
- H2S combines with groundwater = sulphuric acid
- acid dissolved limestone = cave system
- unusually created bottom up rather than top down
Karst landscapes are characterised by:
- Limited surface drainage
- Can form and be influenced by a multitude of contexts (Fluvial, Solution, Glacial, Thermokarst…)
- Extensive surficial and subterranean solution and erosion systems (evaporites, carbonates and occasionally silicate rocks)
- Range of scales of erosional and depositional features from microscopic karren to gorges and polje, straws and columns
- Karst landscapes are important as they act as sub-surface reservoirs of water
- They are also potentially hazardous if the subsurface systems are not well-mapped or understood.
