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Decomposition of rocks & minerals by CHEMICAL & BIOCHEMICAL REACTIONS
Changes the chemical composition of minerals and rocks that are unstable at the Earth's surface (decomposes them), converting them into more stable compounds, or dissolving them away |
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collective term for the features created when groundwater dissolves soluble rocks underground or when surface water dissolves exposed soluble rocks
(Landforms formed by karstification - caves, sinkholes, towers, speleothems)
CaCO3 - calcite (in limestone, marble)
CaMg(CO3)2 - dolomite (sedimentary stone composed mainly of dolomite: dolostone, dolomitic marble) |
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| dissolution or chemical decay of rock that produces landforms by rock removal in solution |
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| when ionic bonds between ions are broken and now separated ions are carried away in water |
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| when the water carrying dissolved ions from dissolution evaporates the dissolved ions combine again into solid form as new minerals |
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the material from which nearly all caves form, most abundant soluble rock
major component: CALCITE, very soluble |
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- water gains CO2 as it passes through the atmosphere and soil
- water and CO2 combine to form CARBONIC ACID (H2CO3) which attacks and dissolves limestone by chemical weathering (dissolution)
- carbonic acid reacts with calcite (CaCO3) in limestone to produce calcium ions and bicarbonate ions
- ions get carried away in water, leaving voids in bedrock
- ions often carried out to sea, where marine animals use calcite to form shells |
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Carbonic acid - from ATMOSPHERE and PLANT DECAY
Humic & other organic acids
Sulfuric & hydrosulfuric acids
Weathering iron sulfide, hydrosulfuric acid & oxygen |
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| natural underground cavities, the most common geological products of limestone dissolution |
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STEPS
1. need preexisting fractures in bedrock - permeability
2. rapid penetration of acidic surface water - widens fractures (vados caves), reaches water table - saturated or understaturated(<- will make more caves) solutions
3. at water table - mixing of soil solution & groundwater (saturated - high CO2) + (saturated - low CO2) = UNDERSATURATED - get intermediate solution which allows for further cave formation (because line on diagram is CURVED, falls below line of saturation)
4. regional erosion -lowers water table, some caves drain and are ventilated
5. saturated or near saturated solutions (soil moisture) drips into caves, loses CO2 to air in cave, becomes super-saturated - SPELEOTHEMS |
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formations resulting from the deposited build up of of precipitate from dissolution on cave surfaces
- consist largely of TRAVERTINE (name applied to calcium carbonate when it forms cave deposits) |
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stony travertine structures resembling icicles that hang from cave CEILINGS
- forms by water high in dissolved solids dripping through cracks in cave ceiling, some water evaporates or loses CO2 as it hits air in cave, becomes supersaturated, and deposits calcium carbonate on cave ceiling - grows in a hollow tube, then tube gets clogged forcing water to find alternate route (usually side or top of stalactite) |
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travertine deposits that accumulate on floors of caves from water that drips from their ceilings
- columns form when stalagmites and stalactites join |
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| Surface expression of the geology of dissolved limestone and the work of near-surface groundwater |
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circular surface depressions that appear in most limestone terrains
solution sinkholes - form when groundwater rich in carbonic acid dissolves limestone at or just below the surface
collapse sinkholes - occur when the roofs of caves collapse under the weight of overlying rocks |
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| water soaks rapidly into an absorbent karst plain - many streams drain completely into sinkholes after flowing a short distance across the surface |
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| as a series of neighboring sinkholes expands and joins together the surface overlying broad sections of underground stream channels may collapse - bridges form where the segments remain that do not collapse over the newly exposed channel |
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| vertical, shaft like caves - high to low elevation, form in widening cracks |
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| Causes of Chemical Weathering |
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Definition
1. dissolution (mineral or rock in water begins to come apart, splits into ions, but water is not involved in actual reaction)
**2. CARBONATION - carbonic acid formed from water and carbon dioxide, then in presence of calcium carbonate (limestone) - IMPORTANT REACTION:
--CaCO3 + H2O + CO2 -> Ca ions + 2HCO3
3. hydrolysis - reaction WITH water (not just in presence) H2O -> H+ + OH- (for quartz, silica needed to get into solution, getting silica into solution not easy either)
**4. ACID HYDROLYSIS - faster than hydrolysis, extra H+ (comes from carbonic acid, sulfur acids, organic acids, anything contributing H+ ions)
- aluminum-silicate + H2O + H+ -> clay mineral + cations + OH + silica
5. hydration - addition of H2O (becomes part of reaction)
**6. OXIDATION - 4Fe+3 + 3O2 -> 2Fe2O3 (hematite - forces iron to give up an extra electron) |
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