Weathering

I. Definitions:the process by which rocks and

minerals break down at or near the Earth’s surface

Weathering:Produces soil, releases the

minerals and elements that we need in our food

Breaks down everything we build.

Erosion: the process by which moving water, wind, ice, or gravity carries pieces of weathered rock away from the bedrock and deposits them somewhere else

I. Definitions:Weathering: the process by

which rocks and minerals break down at or near the Earth’s surface

Work together to create sediment which is the raw material for sedimentary rocks.

Sediment: unconsolidated

particles created by the weathering and erosion of rock, precipitated from water, or secretions of organisms.

Weathering

Weathering

II. Mechanical Weathering: breaks a mineral or rock into smaller pieces without changing their chemical makeup

Creates more surface area.

Weathering

II. Mechanical Weathering: breaks a mineral or rock into smaller pieces without changing their chemical makeup

Creates more surface area.

A. Frost Wedging: one of the most effective types of weathering in places where surface water is abundant

Temperatures fluctuate around freezing (0C)

Weathering

II. Mechanical Weathering—breaks a mineral or rock into smaller pieces without changing their chemical makeup—creates more surface area.

A. Frost Wedging

Water expands when it freezes (up to 9%), e.g., water bottle example

When water gets into cracks and freezes, it exerts enough force on the rock to break it. Enlarges cracks, loosens rock and may dislodge it.

Top freezes first (in contact with air), so as water below freezes it can’t expand up, it expands to the sides

Weathering

II. Mechanical Weathering

B. Crystal Growth: salt Water may get into crack then

evaporate—growing crystals put pressure on rock, pushing them apart.

Important process on coastal cliffs

C. Thermal Expansion & Contraction: fires or in deserts

Heat causes a mineral’s structure to actually expand—different minerals expand by different amountsHelps weaken mineral bonds.

Poor conductors of heat, so only outer portion expands and flakes break off the surface

Weathering

II. Mechanical Weathering

D. Exfoliation

When land above large igneous intrusions is eroded away, the overlying pressure is reduced so the rock expands

Expansion is parallel to the exposed surface

Fractures into sheets parallel to exposed

Sheets may break free and fall from the sloping surface of the exposed rock

Weathering

In mines, slabs from walls of newly cut tunnels have been known to explode off the walls

II. Mechanical Weathering

D. Exfoliation

When land above large igneous intrusions is eroded away, the overlying pressure is reduced so the rock expands

Expansion is parallel to the exposed surface

Fractures into sheets parallel to exposed

Sheets may break free and fall from the sloping surface of the exposed rock

Weathering

II. Mechanical Weathering

E. Biological

Roots grow into fractures, as the plant grows, the roots expand, put force on the rock and expand the cracks

Weathering

II. Mechanical Weathering

F. Abrasion1. Desert wind sand-blastingWind blown particles collide into

each other and rocks on ground, grind away & break off pieces

2. Glaciers scrapingRocks in base of glaciers scrap

against underlying rock, grind away bedrock and themselves

3. Streams scrapingSediment transported by streams

scrapes against the bottom of the stream bed

Weathering

II. Mechanical Weathering

Angular Bouldersdecomposes and rounds

Weathering

III. Chemical Weathering: Alters the composition of rocks and minerals, usually through chemical reactions involving water

Water is the most important factor controlling the rate of chemical weathering!

A. Dissolution: dissolving of ions

1. water dissolves ions: water is polar so it can dissolve ions (NaCl) and carry them away

a. in solution, ions dissolved in water

b. precipitated, when the water leaves the ions behind to form solid compounds

Weathering

III. Chemical Weathering: Alters the composition of rocks and minerals, usually through chemical reactions involving water

Water is the most important factor controlling the rate of chemical weathering!

A. Dissolution: dissolving of ions

1. water dissolves ions: water is polar so it can dissolve ions (NaCl) and carry them away

Weathering

Weathered and unweathered limestone

III. Chemical Weathering: Alters the composition of rocks and minerals, usually through chemical reactions involving water

2. Water may react with other compounds in the environment to form acids that then dissolve rock

• H2O + CO2 H2CO3 carbonic acid

b. CaCO3 + H2CO3 Ca+2 + 2 HCO3

bicarbonate ions

c. ions then carried away in water and they leave holes in the rock

d. creates caves: water in cracks, cracks grow

Weathering

III. Chemical Weathering: Alters the composition of rocks and minerals, usually through chemical reactions involving water

B. Oxidation—forms rust

Positive ions in a mineral combine with oxygen to form an oxide

Example: iron in a mineral combines with oxygen in the atmosphere

4Fe+3+3O2 2Fe2O3 (hematite = rust)

Weathering

III. Chemical Weathering:

C. Hydrolysis: ions from water (OH- or H+) replace other ions in minerals to form new minerals

Feldspars (Na-plag, Ca-plag, K-spar) 60% of minerals in crust = clay

Example: 2 KAlSi3O8 + 2H+ 9H2O

Al2Si2O5(OH)4 + 4 H4SiO4 + 2K+K-spar + clay (kaolinite) + silicic acid

In solutionSilicic acid comes into play when

creating (cementing) sedimentary rocks

Potassium transported by water = nutrient for plants

Clays accumulate at surface of earth as soils or wash out to sea as mud

Weathering

IV. Factors affecting weathering

A. Climate: water drives all chemical weathering

1. wet more chemical weathering2. hot (dry) more mechanical

weathering (heat helps break bonds)

B. Organisms: burrow and churn up the surface exposing unweathered minerals to the atmosphere

C. Time: more time = more weathering

D. Composition of minerals: some minerals more resistant to weathering than others

Early formed minerals (high temp) weather faster

Later formed minerals (lower temps) are more stable near Earth’s surface

Weathering

Weathering

1. What are the two types of weathering?

Weathering

Weathering