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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