Weathering of Rocks. Why we see weathering 1.Most minerals are not stable at the Earth’s surface 2.The Earth wants to be flat – lowest energy state Topography.

Weathering of Rocks

Why we see weathering

1. Most minerals are not stable at the Earth’s surface

2. The Earth wants to be flat – lowest energy state

• Topography provides energy

Igneous Sedimentary

Metamorphic

Melting

Erosion + Deposition

Melting

Ero

sion

+ D

epos

itionB

urial + Heating

Buri

al +

Hea

ting

Weathering in the Rock Cycle

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

Mechanical weathering breaks down rocks/minerals

Smaller particles increases the amount of exposed surface area

Does not change the chemical composition of the minerals\

Mechanical Weathering - making smaller pieces

Mechanical Weathering

1. Frost Wedging

2. Salt Wedging

3. Biological Wedging

4. Unloading

5. Thermal Expansion

Types of Mechanical Weathering

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

Must have:• Adequate moisture• Cracks in rocks• Freeze/thaw cycles

Frost Wedging

Types of Mechanical Weathering

Salt wedging – growth of minerals in cracks

• Desert environments• Water evaporates, ions in solution

combine to form minerals

Salt 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.

Biological Wedging

Removal of overlying material allows rock to expands

Top layers expand more than deeper layers

Causes sheeting or exfoliation

Exfoliation Domes

Unloading

Sheeting Sheeting

Unloading

Thermal Expansion

1. repeated daily heating and cooling of rock; ex. Desert environments

2. heat causes expansion; cooling causes contraction.

Chemical Weathering

• Chemical alteration of minerals.

• Results in new minerals and ions in solution.

• Water and acid are essential.

Types of Chemical Weathering

1. Hydrolysis - any reaction in which water participates.

a) Ion exchange – H+ replaces other cations.

b) Dissolution - mineral completely dissolves,

leaving only ions in solution.

c) Oxidation - reaction in which elements gain

or lose electrons (example: rust).

Carbon Dioxide + Rain

Becomes Acid

Dissolves Minerals (i.e. carbonates such as calcite)

Carries away---Ions

Acid Hydrolysis - Dissolution

H2O + CO2 H2CO3

CaCO3 + H2CO3 Ca2+ + 2HCO3-

Carbon Dioxide + Rain

Becomes Acid

Dissolves Minerals (i.e. silicates)

Leaves ----Clay

Carries away---Ions

Silica

Acid Hydrolysis & Secondary Minerals

Dissolution

Minerals dissolve in water

For example: salts

Oxidation1. Iron silicate

dissolves

2. Iron oxidizes

3. Oxidized iron combines with water

4. Leaves iron oxide

Weathering Products

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

Sediments

Dissolved Ions

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+

Products of Weathering

Factors influencing Weathering Rates

1. Rock structures – chemical/mineral composition, physical features

2. Topography

3. Climate

4. Vegetation5. Time

Climate and Weathering

Benefits of Weathering

• Creates soil

• Produces clay, sand, and gravel

• Produces minerals

Soils

Terminology

Regolith – blanket of loose, weathered rock debris covering unweathered bedrock.

Soil – uppermost part of regolith.

Rock particles, new minerals, organics

How soil differs from regolith:

1. More chemical weathering in soil.

2. Soil has structure (layers called

soil horizons).

3. Soil retains nutrients and

moisture, essential for abundant

plant growth.

More Terminology

Soil profile – sequence of soil horizons.

Soil horizon – layer within a soil profile that has distinct characteristics.

Composition, color, texture

Soil Horizons

A horizon

• Zone of leaching (lots of chemical

weathering)

• Organic rich, often dark in color

• Decaying organic matter releases

nutrients

B horizon

• Zone of accumulation – minerals (clay and iron oxide) are washed down from above.

• High clay content, reddish color from iron.

• Able to retain moisture because of clay content

C horizon

Parent material – grading from weathered to unweathered.

Parent material can be:

1. Bedrock

2. Stream sediments

3. Volcanic ash

Controls of Soil Formation

1. Parent Material

2. Time

3. Climate

4. Plants and Animals

5. Slope