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Weathering of Rocks. Why we see weathering 1.Most minerals are not stable at the Earth’s surface...

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Weathering of Rocks
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Page 1: 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

Page 2: 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.

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

Page 3: 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.

Igneous Sedimentary

Metamorphic

Melting

Erosion + Deposition

Melting

Ero

sion

+ D

epos

itionB

urial + Heating

Buri

al +

Hea

ting

Weathering in the Rock Cycle

Page 4: 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.

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

Page 5: 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.

Types of Weathering

Mechanical – physical breakdown of rocks.

Chemical – decomposition of rocks by chemical reactions.

Page 6: 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.

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\

Page 7: 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.

Mechanical Weathering - making smaller pieces

C:\Documents and Settings\Administrator\My Documents\Courses\physical breakup.MOV
Page 8: 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.

Mechanical Weathering

1. Frost Wedging

2. Salt Wedging

3. Biological Wedging

4. Unloading

5. Thermal Expansion

Page 9: 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.

Types of Mechanical Weathering

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

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

Page 10: 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.

Frost Wedging

C:\Documents and Settings\Administrator\My Documents\Courses\frost wedging.MOV
Page 11: 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.

Types of Mechanical Weathering

Salt wedging – growth of minerals in cracks

• Desert environments• Water evaporates, ions in solution

combine to form minerals

Page 12: 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.

Salt Wedging

Page 13: 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.

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.

Page 14: 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.

Biological Wedging

Page 15: 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.

Removal of overlying material allows rock to expands

Top layers expand more than deeper layers

Causes sheeting or exfoliation

Exfoliation Domes

Unloading

Page 16: 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.

Sheeting Sheeting

Unloading

Page 17: 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.

Thermal Expansion

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

2. heat causes expansion; cooling causes contraction.

Page 18: 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.

Chemical Weathering

• Chemical alteration of minerals.

• Results in new minerals and ions in solution.

• Water and acid are essential.

Page 19: 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.

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).

Page 20: 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.

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-

Page 21: 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.

Carbon Dioxide + Rain

Becomes Acid

Dissolves Minerals (i.e. silicates)

Leaves ----Clay

Carries away---Ions

Silica

Acid Hydrolysis & Secondary Minerals

Page 22: 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.

Dissolution

Minerals dissolve in water

For example: salts

C:\Documents and Settings\Administrator\My Documents\Courses\dissolution.MOV
Page 23: 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.

Oxidation1. Iron silicate

dissolves

2. Iron oxidizes

3. Oxidized iron combines with water

4. Leaves iron oxide

Page 24: 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 Products

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

Sediments

Dissolved Ions

Page 25: 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.

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

Page 26: 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.

Factors influencing Weathering Rates

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

2. Topography

3. Climate

4. Vegetation5. Time

Page 27: 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.

Climate and Weathering

Page 28: 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.
Page 29: 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.

Benefits of Weathering

• Creates soil

• Produces clay, sand, and gravel

• Produces minerals

Page 30: 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.

Soils

Page 31: 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.

Terminology

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

Soil – uppermost part of regolith.

Rock particles, new minerals, organics

Page 32: 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.

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.

Page 33: 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.

More Terminology

Soil profile – sequence of soil horizons.

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

Composition, color, texture

Page 34: 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.

Soil Horizons

Page 35: 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.
Page 36: 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.

A horizon

• Zone of leaching (lots of chemical

weathering)

• Organic rich, often dark in color

• Decaying organic matter releases

nutrients

Page 37: 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.

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

Page 38: 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.

C horizon

Parent material – grading from weathered to unweathered.

Parent material can be:

1. Bedrock

2. Stream sediments

3. Volcanic ash

Page 39: 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.

Controls of Soil Formation

1. Parent Material

2. Time

3. Climate

4. Plants and Animals

5. Slope


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