Weathering

1. Weathering Chapter-2 Sedimentary Geology

2. Weathering • Granites-granodiorides • San Lorenzo or Utuado- rock composed of: • feldspars (plagioclase 30-50%, K-feldspars 5-35%) • quartz 5-10% or 25-30% in quartz rich granites. • The sand in the rivers is composed mostly of quartz and feldspars are rare. • Most of the others minerals are weathered and their elements ends up forming clay minerals

3. Weathering Is the simple consequence of exposing pre-existing rocks to the conditions at the Earth’s surface: low temperature and pressure, organic activity, and chemically active substances such as water and atmospheric gases. Two types : physical and chemical weathering are the means by which pre-existing rocks and minerals change and come into equilibrium with the surface environment. Sedimentary rocks will not exist without weathering.

4. Physical weathering:disintegrating rocks into clasts • Four major mechanism of physical weathering: • Freeze-thaw • Insolation • Stress release (unloading) • Organic activity • Each process is slow and do the same, it turns solid, difficult to erode rocks, into smaller, movable, unconsolidated rocks and minerals.

5. Freeze-thaw (ice-wedging) • Active agent is water • The catalyst is temperature changes that occurs by hours, weeks, months, etc… • At 0° C (32 ° F) water freeze, a 9-10% volume expansion occurs. • Water freezing along cracks and fissures

6. Dilatación por congelación (frost wedging) • - el agua se expande cerca de 9% al congelarse. Cando agua entra en grietas en las rocas y luego baja la temperatura el agua se congela en la superficie de la grieta primero y luego en el resto de la grieta. Al congelarse el agua ejerce presión suficiente para fracturar aun mas la roca y hasta para separar fragmentos.

7. In Puerto Rico…. • A similar process occurs in PR when salts such as halite and gypsum crystallize in cracks and crevices. Evaporation of the water results in growth of the crystals in the walls of the outcrops.

8. Insolation • Refers to stresses generated when minerals are exposed to changing temperatures and undergo differential thermal expansion and contraction. • This process is common in zone of extreme temperature fluctuations such as deserts. • In wetter climates

9. Stress release • Occurs when rocks buried beneath overlying material experience high confining pressures. When eroded, overburden is removed, pressures drop, and the rock mass expands. • Expansion cracks or joints develops roughly parallel to the ground surface (onion skin-like).

10. Organic Activity • organism that live on or in weathering bedrock promote physical weathering. • plant roots seek out small pockets of soil in weathered rocks. • as plant grows, the roots lengthens and thickens. • gradually prying apart the cracks. • microscopic and megascopic organism living wthin soil fragment them further (ex. worms).

11. Otros tipos de Meteorización mecánica • El crecimiento de raíces de plantas en las fracturas ejerciendo presión en la roca. • El caminar de las animales en la superficie ayuda a romper algunos fragmentos.

12. Chemical weathering reactions • Some constituents dissolve complitely • ex. minerals such as calcite and halite they are carried away by ground water and runoff and can be precipitated elsewhere • 2) Constituents such as feldspars and micas are altered into new minerals (mostly clay minerals) • It involve several simultaneous chemical reactions • hydrolysis • hydratation • simple solution • oxidation-reduction • these reactions proceed better in the presence of air and water.

13. Simple solution • solid mineral + acid/water = ions in solution • bonds between ions in rigid crystalline lattice are broken and ions are disseminated in solution • ex. SiO2 + 2H2O> H4 SiO4 • quarts + water > hydrosylicic acid

14. Ex. • H2O + CO2 > H2CO3 • Water + carbon dioxide > carbonic acid • carbonic acid contain abundant hydrogen ions. because their small size they have a strong affinity for anions and displace other cations in minerals structures • CaCO3 + H2CO3 > Ca2+ + 2HCO3- • lmst.+ rainfall>dissolved Ca+d. bicarbonate • Lmst dissolve as the hydrogen ions displace calcium ions

15. Halite (NaCl) is extremely soluble. When sodium and chlorine become dispersed the water develops a salty taste. • NaCl + H2O > Na+ + Cl- • halite + water > ions dissolved in water • the solubility of halite is enourmous compared with the solubility of quartz • quartz 6ppm, halite thosands ppm

16. Hydratation and dehydratation(Solid mineral + water= New hydrated mineral; dehydratation is the reverse) • Some weathering processes involve the chemical combination of pre-existing minerals with water (hydratation); or, the removal of water from pre-existing minerals (dehydratation)

17. CaSO4 •2H2O > CaSO4 + 2H2O • gypsum > anhydrate + water • Fe2O3 + 3H2O > 2Fe(OH) 3 • hematite + water> limonite (oxide in soil)

18. Hydrolisis: • hidrogen ion + mineral with mobile cations = entirely dissolved minerals or partially altered minerals in which hydrogen ions replace mobile ions that are put into solution. • The replacement of cations in a mineral structure by hydrogen ions in the water or in acid.

19. Mg2SiO4 + 4H+ > 2Mg2+ + H4SiO4 • Olivine+from water/acid>ions in solution+dissolved silica • 2CaMgSiO6+16H+>2Ca2++2Mg2++4H4SiO4 • Pyroxene+from w/a > ions in solution+dissolved silica

20. Oxidation-Reduction • Atmospheric Oxygen gains electrons and is reduced as mineral constituents Lose electrons and are oxidized, producing new “rusted’ minerals. • Oxidation is the process by which an atom loses electrons • Reduction is the process by which an atom or ion gains electrons

21. (Fe 2+)SiO6 + O2 + H2O> 4Fe3 +(OH)3 +H4SiO4 • Pyroxene+atmosphericW/A>limonite+dissolved silica

22. The Controls • climate • High temperatures-chemical weathering is more effective in warmer climates. • Rainfall is important because chemical weathering requires water. • Hydrolysis and simple solution depend on the availability of the ion hydrogen= pH

23. pH • pH- the concentration of H+ ions in solution. • pH lower than 7 is acid • pH higher than 7 is alkaline • Natural water have pH between 4 and 9; rain and streams pH of 4 - 6.5 • Soils pH of 4 - 5 • Sea-water pH higher than 8

24. Eh-redox potential • Eh - Express the potential for either oxidation or reduction • Redox potential- measurement of the state of oxidation of the system. The value includes magnitude and sign (+/-) the higher the magnitude the more likely that the particular ion or atom will be either oxidized or reduced.