Term
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Definition
| The mechanical and/or chemical breakdown of rocks exposed at the Earth's surface into smaller particles that may differ in composition from the original substance. |
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Term
| Weathering occurs "in situ". What does this mean? |
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Definition
| Weathering occurs with no movement. |
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Term
| What is mechanical weathering, and what kind is there? |
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Definition
Involves processes that fragment rocks into smaller particles by exerting forces that exceed the strength of the rock.
Includes:
-frost wedging
-salt wedging
-exfoliation
-root wedging |
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Term
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Definition
-water along fractures in rocks freezes, expands and exceeds the tensile strength of the rock, causing it to split
-volume of ice is up to 9% greater than water
-frequent freeze-thaw cycles
-produces large angular blocks of rock called talus |
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Term
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Definition
-similar to frost action; caused by pressure created by crystallization of salt particles in pore spaces and along fractures
-as water evaporates, minerals are left behind to grow and exert pressure on the surrounding rock
-common in: coastal areas, arid climate, polluted air |
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Term
| Pressure Release -> Exfoliation |
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Definition
-pressure release at depth, rocks are subjected to some degree of elastic compression from the weight of the overlying rocks
-removal of overburden releases this elastic component of deformation and the rock expands
-commonly this results in the formation of fractures parallel to the ground surface; this is called exfoliation (or sheeting) |
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Term
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Definition
-occurs when a plant, especially trees, sink root systems into existing joints and fractures. As the root grows it forces the fracture to expand
-relatively minor weathering force in rocks, but is very important for soil development |
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Term
| What is chemical weathering? |
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Definition
| Disintegration of rock-forming minerals due to chemical reactions with atmospheric gases and water. |
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Term
| What are some causes of chemical weathering? |
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Definition
Ingredients for weathering "stew"
-oxygen, carbon dioxide and other gases from the atm
-water from rain and snow
-source minerals (the rock to be weathered)
Many rocks that have formed underneath the earth in the absence of atm gases are unstable under atm conditions, they therefore disintegrate faster when exposed at or near the surface |
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Term
| Solution (or dissolution) |
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Definition
| "Soft rocks" (e.g. evaporites, carbonates) can be dissolved because their minerals react with water and acid |
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Term
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Definition
Over time, silicates such as feldspar react with the H+ ion in water and "breakdown".
They produce clay minerals such as kaolinite and release ions such as K and Si that are removed in solution
So... acid rain increases weathering |
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Term
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Definition
-involves the reaction of minerals with carbonic acid
-forms by combination of water and CO2 contained in atm |
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Term
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Definition
| Some minerals can absorb water or become new hydrated minerals |
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Term
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Definition
-reaction of free oxygen with metallic minerals
-most affected are rocks, which contain metals in reduced form, e.g. Fe-sulphides (pyrite) and Fe-silicates (biotite, amphibole, pyroxene) |
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Term
| What are the effects of oxidation? |
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Definition
Fe oxidation is one of the most common effects seen on the surface of the earth.
Oxidation often leads to coloration of rock: high iron content -> often reddish in color |
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Term
| What aids in mechanical and chemical weathering of rocks and minerals? |
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Definition
-Faults/fractures/conduits
-porosity of the rocks
-surface area increase of materials
-climate (temperature, moisture)
-composition of minerals |
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Term
| What is the role of faults, fractures and joints in weathering? |
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Definition
| Note that any faults or fractures provide conduits along which water and roots can penetrate more deeply into the rock mass and will promote further weathering. |
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Term
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Definition
-Many rocks have natural cavities and pore spaces due to their unique formation (e.g. basalt has gas cavities, sandstone has spaces between clasts)
-These spaces allow water and gas to penetrate the rock and chemically weather more readily |
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Term
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Definition
| Changes the surface to volume ratio. Therefore more surface area for the weathering to attack! |
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Term
| What is the effect of Temperature? |
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Definition
-mechanical as discussed: freeze-thaw effects
-Chemical: weathering rates INCREASE with temperature
-rule of thumb: increase 2x for every 10C rise |
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Term
| Effects of Composition: Goldich's Weathering Series |
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Definition
-Minerals that formed under high temperatures are the most unstable under atmospheric condition
-minerals that formed under lower temperatures less susceptible to weathering
-also, weathering products (clays, oxides, hydroxides) are least susceptible to additional weathering |
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Term
| Chemical weathering case study: Acid Rock Drainage |
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Definition
-mining processes lead to the exposure of material that has been buried deeply for millions of years or has never seen the surface before
-these materials may be "unstable" under atm conditions
-depending on the composition these materials may be "extremely prone to weathering" and lead to hazardous acid rock drainage |
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Term
| Acid Rock Drainage Problem Overview |
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Definition
-most of the material that is being mined is discarded as
-waste rock (overburden or low grade ore, coarse material)
-mine tailings (slurry of small particles and fluid, left over after extraction process)
-these rocks contain sulphide and metal rich minerals |
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Term
| How is acid rock drainage generated? |
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Definition
-Sulfide minerals undergo oxidation under atmosphere conditions
-this is problematic for the local ecosystem, water contamination, etc. |
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Term
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Definition
Manage inactive and abandoned mine sites to remediate and contain current contamination and to minimize future pollution
-cover of waste rock piles and tailings
* dry cover
* wet cover
* water cover
-treatment of seepage prior to discharge to streams or lakes
-minimize long-term treatment cost |
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Term
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Definition
Design mining operation and waste disposal to minimize generation and release of acid mine drainage
-put most reactive material on bottom of tailings pile far away from atm
-blend reactive rock with non-reactive rock
-sub-aqueous or sub-marine disposal of mine waste
-addition of alkaline materials such as lime to waste prior to deposition |
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