planet earth weathering_powerpoint_presentation

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  • 1. Weathering and Soil

2. Summary of Important Concepts

  • Soilis a layer of weathered rock, minerals, and organic matter at the earths surface that supports plant life.
  • The main factors that determine the characteristics of a soil areclimate(mainly temperature and rainfall),topography , type ofparent rock material ,organic activity , and theamount of timethatthe soil has been forming.
  • Soil forms fromregolith- broken up rock material at the earths surface. Soil forms as regolith undergoesweathering-the physical and chemical breakdown of rock .
  • - Physical weatheringis breakdown of rock by physical forces.Example:rock wedged apart by freezing water or by plant roots.
  • - Chemical weatheringis breakdown of rock by chemical reactions.Examples:dissolution, oxidation, and hydrolysis reactions.

3. What is SOIL? It has been said that soil representsa few inches between humanity and starvation.This phrase puts the importance of soil in perspective.Soilis alayer of weathered rock, minerals, and organic matter at the earths surface that supports plant life . Without soil, human life would not be possible.This figure illustrates the composition of a typical soil. Soil is composed ofmineral matterfrom weathered rock;water ,gases , andorganic matter(the remains of plant and animal material and bacteria). 4. Soils main important uses for humanity are summarized here. 5. Weathering and Soil Formation Soil forms fromregolith- the term used forbroken up rock material at the earths surface .Rock (regolith) exposed at the earths surface undergoesweathering-the physical and chemical breakdown of rock at the earths surface . Two main types of weathering occur:physicalweathering andchemicalweathering.Physical weatheringis breakdown of rock by physical forces.Chemical weatheringis breakdown of rock by chemical reactions that occur when rock and mineral matter interact with water and air. 6. Physical Weathering and Chemical Weathering work hand in hand : Physical disintegration creates more surface area for chemical processes to act upon minerals.Smaller pieces have a greater surface area: volume so they are easily attacked by chemical weathering. 7. Water is a Polar Molecule:Although it is neutral, the charges build up on either side of the molecule.It is more positive on one side (H) and more negative on the other (O).Hydrogen bonding between water molecules gives it surface tension . 8. Role of Water. 1.Universal Solvent:Right shows a water molecule.Note the slightly positive charge on one end and slightly negative charge on the other.Although it is electrically neutral, the slight difference in charge on either side makes the molecule easily dissolve weakly (ionic) bound minerals. 2.Hydrolysis:Water can split to form H+ and OH- ions. The number of H+ ions in a solution determines the pH of the solution where pH=1 is acidic with many H+ ions, 7 is neutral and pH=14 Basic with few H+ ions. 3.Expands When Frozen; densest at 4 o C. 9. Physical Weathering: Physically disintegrating but not changing chemically. 10. 11. Frost Wedging , very effective at breaking up rocks.Only in climates with frequent freeze/thaw cycles.Water seeps into cracks in rock.Temperature drops resulting in the ice at the surface to freeze (thus forming a CAP).The remaining water freezes and expands exerting a great force against the rock causing it to crack.This process occurs in temperate climates that has significant precipitation (to supply the water to freeze and thaw). 12. Thermal Contraction and Expansion

  • Thermal contraction and expansion is also related to temperate climates, especially those with high daily temperatures and extremely low night time temperatures (like the desert)that is a WIDE daily range in temperature.
  • During the day time, hot high temperatures cause atoms to spread further apart (expand) and at night, cold drops in temperatures cause atoms to contract.
  • Repeated on a daily basis, thermal contraction and expansion due to wide daily ranges in temperatures effectively render rocks weak and they break.

13. Abrasion : The longer sediments or rocks or pebbles are in transit, the more bumping and abrading it undergoes.Right: Well rounded sediments indicate LONG TRANSPORTATION TIME. Left: The shorter the transportation time, the more angular the clasts (sediments/rocks).Not enough time for abrasion. 14. The shorter the transportation time, the more angular the clasts (sediments/rocks).Not enough time for abrasion. 15. Pressure Release/Unloading: Intrusive rocks form within the earths crust under severe confining pressures (and so are stable under pressure).Once the pressure (layers of rock above are removed) is relieved, the rock expands and cracks. 16. Biological Weathering : plant/tree roots can be very powerful.Root systems can squeeze through cracks, and as the plant grows, enlarge the cracks. 17. Biological weathering by lichens growing on rocks!Metabolic byproducts of the lichen can chemically weather the minerals. 18. Chemical Weathering: breakdown of rock by chemical reactions that occur when rock and mineral matter interact with water and air. 19. The main types of chemical weathering are:Dissolution by Water:Dissolving minerals into their constituent ions or molecules into solution. . Carbonation: Dissolution of carbonate minerals (limestones and marble).Requires the solution to be acidic.Hydrolysis:Decomposes silicates; this reaction produces clay minerals - the most important minerals in soil.Oxidation:Free oxygen atoms dissolved in water combine with iron in the rock, producing iron oxide minerals. Important: nearly all chemical weathering proceeds faster under wetter conditions. 20. Dissolution by Water Dissolving minerals into their constituent ions or molecules into solution. Water being a polar molecule can gang up on charged ions (i.e. sodium and chloride).Because of the polar nature of the water molecules, they are able to pull Na from Clonce this is done, water molecules surround the ions preventing them from forming ionic bonds.Straight dissolution by water dissolves ionic bonds easily. Dissolution can also take place a little quicker if the water is acidic (has many H+ ions dissolved in it)see carbonation and hydrolysis. 21. 22. Dissolution by Acid(Carbonation using carbonic acid example) Carbonation: Dissolution of carbonate minerals specifically (calcite in limestones and marble).Requires the solution to be acidic.Where we can get acid: H 2 O + CO 2-> H 2 CO 3-> H ++ HCO 3 - Water + Carbon Dioxide -> Carbonic Acid -> Hydrogen Ion + Bicarbonate Ion The acid splits to H+ ions and Bicarbonate ions.The more H+ ions in solution, the more acidic the solution and the lower the pH.Carbonation Reaction Equation: CaCO 3+ H ++ HCO 3 --> Ca +2+ 2(HCO 3 ) - Calcite + Hydrogen Ions + Bicarbonate Ion -> Calcium Ion + 2 Bicarbonate Ions. After dissolving rocks with calcite, the H+ ions are no longer dissolved in solution rendering the solution no longer ACIDIC. 23. Chemical Weathering of Limestone: easy to do so long as you have some ACID in the rain.Marble (limestone is the parent) headstones dont last as long as granite headstones (or buildings for that matter). 24. This limestone has been gradually dissolved by water from breaking waves, as shown by the pitted, honeycomb texture. 25. Karst Topography: Weathering of limestone (calcite) forms KARST (see little hills below).Well learn more about this during GROUNDWATER lecture but for now, groundwater essentially dissolves limestone.When rocks above these underground tunnels and caves are eroded away, the limestone is exposed.More weathering of the limestone at the surface will occur, and eventually these little hills will dissolve away. 26. Hydrolysis Decomposes silicates; this reaction produces clay minerals - the most important minerals in soil.4KAlSi 3 O 8+ 4H ++ 2H 2 O -> 4K ++ Al 4 Si 4 O 10 (OH) 8+ 8SiO 2 Orthoclase + Hydrogen Ion + Water -> Potassium Ion Kaolinite (clay min.) + Quartz Feldspar is a KAl silicate.If we take a weak acid and dissociate it into H+ + HCO3- in water, the H+ ions will act upon the feldspar.Note potassium feldspars chemical formula and then look at the chemical formula after hydrolysis.Where did the K+ ions go?Basically hydrolysis weakens the mineralH+ is the smallest ion (atomic number of 1).Hydrogen ions basically kick out the K+ ions (much larger in size) and shove themselves into the crystalline latticetremendously weakening the mineral.Imagine a cinder block wall.Wherever there would be a K ion (lets just say every tenth cinder block for example), remove every tenth cinder block and replace it with a lego.You can see how the integrity and strength of the cinder block wall is significantly weakened by sticking smaller ions in place of larger ones! 27. The importance ofwaterin chemical weathering is dramatically illustrated here. The granite obelisk at the left was carved about 3500 years ago in Egypt. In the dry climate of Egypt the rock surface remains fresh even after all those years.A twin obelisk carvedat the same timewas moved to New York City about 100 years ago. After surviving unscathed for 3400 years in arid Egypt, the wetter climate of New York quickly broke down the rock surface by chemical weathering! 28. Stone Mountain, Georgia: made of granite and granite is made of different minerals which have different silicate structure and thus different stabilities.Certain minerals are more susceptible to chemical weathering.Preferential weathering forms little divots in Stone Mountain. 29. Recall the igneous pluton on the left.The silicate minerals are more stable at the earths surface than surrounding (and once overlying sedimentary rocks)