Mechanical Weathering - Arizona State University
Mechanical Weathering
1. Water in cracks freezes
2. Crystal growth
3. Rocks expand as exposed
4. Rocks shrink as cooled
5. Organic Activity
Chemical Weathering
1. Rocks dissolve
Certain rocks and minerals
Right chemical conditions (acidic)
Natural acids?
Evidence of solution
Hard water is where dissolved stuff (ions) goes
2. Oxidation
Oxidation is losing electrons
3. Minerals converted to clay
Hydrogen ions in water switch places with positive ions (K, Na, Ca, Mg, Fe) in minerals
Especially if acidic (acids, by definition, have lots of hydrogen ions)
Frees positive ions (like K, Na, Ca), which end up
in rivers, lakes, and groundwater
as nutrients for plants
as salts in soil (bad, bad, bad)
salt deposits (Na and K), limestone (Ca), and dolomite (Ca and Mg)
making sea water salty
Where does stuff go?
Iron + oxygen = rust (stays around)
Most silica doesn't dissolve
Ends up as sand (most sand is quartz = silica)
Si + Al + H + Water = clay
Some silica does dissolve
pore spaces of rocks
ocean water
Rounded Chunks
Controls of Weathering and Soil Formation
Soil = minerals + organic matter + water + air
1. Parent Material
Some rocks weather fast
Highly fractured rocks
Bedrock vs gravels
Some rocks are more soluble than others (marble and limestone dissolve easily)
Minerals that form (crystallize) at higher temperatures and pressures are less stable at the surface of the earth (they are out of equilibrium with the conditions under which they formed)
Rocks altered (feldspar turned to clay) by previous interactions with hot fluids are easily weathered
2. Time
3. Climate
Hot, wet climate = much chemical weathering
Cold or dry climate = mostly mechanical weathering
4. Type of animals and plants
5. Slope
Soils best developed on flat to rolling terrain (good drainage, little erosion, and water can soak in)
Soils poorly developed on steep slopes (water and weathered material washes off)
Soil Profile
Soil-forming processes operate from the surface downward
Upper part of soils
Lots of organic matter
Fine-grained material (clay) is washed downward
Soluble material is leached out and taken downward
Middle part of Soil (B Horizon)
Less organic matter and biologic activity
Accumulation of clay washed down from above
Deposition of soluble substances as water evaporates
Lower part of Soil (C Horizon)
Rotten rock debris
Little organic material
Soil Types
Clay (aluminum) and rust (iron-oxide) rich
Forms in temperate and humid climate (East U.S.)
Abundant rain means soluble stuff is leached and carried away by groundwater
Clays and iron oxides are less soluble, so stay in soil
Red color due to iron oxides
Calcite-rich
Dry climates
Accumulation of calcium carbonate (calcite) because water evaporates, leaving soluble material in soil
Caliche = heavy calcite accumulation
Calcite builds up with time
Can tell how old soil is by amount of caliche
Tropical Soils
Hot and very wet climates
Intense chemical weathering = thick soils (laterites)
Lots of organic matter and organic acids
Leaching of calcite, ions like K and Na, and some silica (poor for growing things, except rainforests)
Most Aluminum and much Nickel and Manganese is mined from these soils
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