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Weathering

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.

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Weathering

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  1. 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

  2. Weathering 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.

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

  5. Weathering • II. Mechanical Weathering: breaks a mineral or rock into smaller pieces without changing their chemical makeup • Creates more surface area. • Frost Wedging: one of the most effective types of weathering in places where surface water is abundant • Temperatures fluctuate around freezing (0C)

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

  7. 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 amounts Helps weaken mineral bonds. Poor conductors of heat, so only outer portion expands and flakes break off the surface

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

  9. 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

  10. 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

  11. Weathering II. Mechanical Weathering F. Abrasion 1. Desert wind sand-blasting Wind blown particles collide into each other and rocks on ground, grind away & break off pieces 2. Glaciers scraping Rocks in base of glaciers scrap against underlying rock, grind away bedrock and themselves 3. Streams scraping Sediment transported by streams scrapes against the bottom of the stream bed

  12. Weathering II. Mechanical Weathering Angular Boulders decomposes and rounds

  13. 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! • 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

  14. 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! • Dissolution: dissolving of ions • 1. water dissolves ions: water is polar so it can dissolve ions (NaCl) and carry them away Weathered and unweathered limestone

  15. Weathering • 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

  16. 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)

  17. 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 solution Silicic 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

  18. Weathering IV. Factors affecting weathering A.Climate: water drives all chemical weathering 1. wet more chemical weathering 2. 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

  19. Weathering 1. What are the two types of weathering?

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