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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