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Weathering of Rocks

Hoodoos

Weathering in the Rock Cycle

SedimentaryRocks

IgneousRocks

MetamorphicRocks

Magma

Sediment

Pressure And Cementation

Weathering/Erosion

Weathering/Erosion

Heat and Pressure

CoolingHeat

Heat

Pressure E

rosi

on

Wea

ther

ing

Three Dynamic Processes of Breaking and Removing rock

1. Weathering - the disintegration

and decomposition of rock at or

near the surface

2. Erosion

3. Mass Wasting

Types of Weathering

Mechanical – physical breakdown of rocks.

Chemical – decomposition of rocks by chemical reactions.

Mechanical Weathering makes smaller pieces

Mechanical Weathering

1. Frost Wedging

2. Salt Wedging

3. Biological Wedging

4. Unloading

5. Thermal Expansion

Frost Wedging

Types of Mechanical Weathering

Frost wedging – water penetrates into cracks, expands when it freezes.

Must have:

• Adequate moisture

• Cracks in rocks

• Freeze/thaw cycles

Salt Wedging

Types of Mechanical Weathering

Salt wedging – growth of minerals in cracks

• Desert environments

• Water evaporates, ions in solution combine to form minerals

Biological Wedging

Types of Mechanical Weathering

Biological wedging – plant roots penetrate into cracks causing cracks to widen.

Must have:

• Climate hospitable for plants

• Adequate moisture and temperature

Unloading

removal of pressure of deep burial

Sheeting

Unloading

•repeated daily heating and cooling of rock; •heat causes expansion; cooling causes contraction.

Thermal expansion

Chemical Weathering

Results in new minerals and ions in solution.

Chemical alteration of minerals.

Water and acid are essential.

Types of Chemical Weathering

1. Ion Exchange – H+ replaces other

cations.

2. Dissolution - mineral completely

dissolves, leaving only ions in solution.

3. Oxidation - reaction in which elements

gain or lose electrons (example: rust).

Hydrolysis - any reaction in which water participates.

Carbon dioxide reacts with water to form carbonic acid (Equation 1). Carbonic acid then dissociates to give the hydrogen ion (H+) and the hydrogen carbonate ion (HCO3-) (Equation 2). The ability of H2CO3

to deliver H+ is what classifies this molecule as an acid, thus lowering the pH of a solution.

Ion Exchange

Carbon Dioxide + Rain

Becomes Acid

Dissolves Minerals

Leaves ----Clay

Carries away--- Ions

Silica

Dissolution

Certain minerals dissolve in water

Oxidation

Iron Silicate dissolves

Iron Oxidizes

Oxidized Iron combines with Water

Leaves Iron Oxide

Relative susceptibilityTo weathering

Mineral Residual Products Material in Solution

Quartz quartz grains silica

Feldspar clay minerals silica, K+, Na+, Ca2+

Amphibole (hornblende)clay minerals, limonite,

hematite silica, Mg2+, Ca2+

Olivine limonite, hematite silica, Mg2+

Factors influencing Weathering Rates

1. Rock Structures – chemical/mineral composition, Physical features

2. Topography

3. Climate

Spheroidal Weathering

Elephant RockState Park, MO

Results of Weathering

Result of Weathering

Regolith – a loose layer of broken rock and mineral fragments.

Dissolved ions

Benefits of Weathering

• Creates soil

• Produces clay, sand, and gravel

• Produces minerals

Erosion and Transport

Modes of Transport

1. Water

2. Wind

3. Ice

4. Gravity

Erosion – transport of unconsolidated Earth material from one place to another

Erosion by Water

Removal of regolithLoose material is easily picked up by

flowing water.

Downcutting of stream channel

Sediments abrade stream bottom, dislodging pieces of bedrock

HeadwardErosion

HeadwardErosion

Transport by WaterSaltation – particles move downstream in

short jumps.

Bed load – material transported by saltation

Suspended load – material carried in water for long distances.

Transport by Water

Erosion by Wind

Deflation - Loose material can be picked up by wind

Abrasion - Windblown sediments can "sandblast" rocks.

Abrasion – airborne particles chip off small fragments of other rocks.

Erosion by Wind

Transport by Wind

Same processes as water – saltation, bed load and suspended load.

Deflation – removal of small, loose particles. Can form desert pavement.

Transport by Wind

Erosion by Ice

Plowing – loose material is "bulldozed"

Plucking – pieces of bedrock are pried loose.

Abrasion – pieces of rock in the ice grind against bedrock below.

Abrasionand

Plucking

Transport by Ice

Particle size and method of Transport

Water – smallest particles to small boulders.

Wind – smallest particles to sand size.

Ice – smallest particles to boulders as big as a house.

Mass WastingTransport by gravity

The downslope movement of unconsolidated Earth material due to gravity.

•What is mass wasting?

•What causes mass wasting?

•Why is it important to know about

mass wasting?

•What can be done to control

mass wasting?

Watch for fallen Rocks

Slope Failures - Slump

Creep

Creep

Stability reduced by:

1. Adding water to the material of a slope

2. Increasing the steepness of the slope

3. Removing material from the lower

part of the slope

4. Earthquakes

5. Removal of vegetation

•What is mass wasting?

•What causes mass wasting?

•Why is it important to know about

mass wasting?

•What can be done to control

mass wasting?

Engineering Solutions

Rock Nets

Soil Nailing

Avalanche BarriersSwitzerland