Weathering

1. Weathering • Geomorphology is the science of the origin, evolution, form and spatial distribution of landforms. • “Geo” = “earth”; “morphos= “shape”; “ology” = science; hence, “science of the shape of the earth.” • Landscapes continually adjust toward an equilibrium state. Weathering and erosion continually try to adjust the landscape toward an equilibrium state, an ultimate base level (sea level?), while uplifting processes continually attempt to keep the landscape from reaching that base level. This constant, fluctuating balance between weathering/erosion and uplift is called Dynamic Equilibrium.

2. Weathering • Factors influencing the equilibrium state: • rock materials • rock structure • climate • topographic relief • Equilibrium is approached episodically through geomorphic thresholds – points at which destabilizing energy overcomes resistance against movement or change. • Landscapes have inertia – the tendency of matter to remain at rest unless affected by an outside force. When there is a change in rock materials, structure, climate, or relief, the landscape reaches an unstable condition wherein there may be a sudden change.

3. Weathering • Most rocks and minerals are formed deep within the Earth's crust where temperatures and pressures differ greatly from the surface. • The physical and chemical nature of materials formed in the Earth’s interior are characteristically in disequilibrium with conditions occurring on the surface.

4. Weathering • Because of this disequilibrium, these materials are easily attacked, decomposed, and eroded by various chemical and physical surface processes. • Weathering - breakdown and alteration of rocks and minerals at or near the Earth's surface into products that are more in equilibrium with the conditions found in the surface environment.

5. Weathering:Regolith, Soil, and Parent Materials

6. Weathering • Weathering products are a major source of sediments for erosion and deposition. • Many types of sedimentary rocks are composed of particles that have been weathered, eroded, transported, and finally deposited in basins. • Weathering also contributes to the formation of soil by providing mineral particles like sand,silt, andclay.

7. Weathering • Elements and compounds extracted from rocks and minerals by weathering processes supply nutrients for plant uptake. • The oceans are saline as a result of the release of ion salts from rocks and minerals on the continents. Leaching and runoff transport these ions from land to the ocean basins where they accumulate in seawater. So we see, weathering is a process that is fundamental to many other aspects of the hydrosphere, lithosphere, and biosphere.

8. Weathering • There are three broad categories of weathering mechanisms: • Chemical • Physical • Biological

9. Weathering • The process of weathering can result in the following three outcomes on rocks and minerals: • The complete loss of particular atoms or compounds from the weathered surface. • The addition of specific atoms or compounds to the weathered surface. • A breakdown of one mass into two or more masses, with no chemical change in the mineral or rock.

10. Weathering • The residue of weathering consists of chemically altered and unaltered materials. • The most common unaltered residue is quartz. • Many of the chemically altered products of weathering become very simple small compounds or nutrient ions that can be dissolved or transported by water, released to the atmosphere as a gas, or taken up by plants for nutrition. • Some of the products of weathering, less resistant alumino-silicate minerals, become clay particles. • Other altered materials are reconstituted by sedimentary or metamorphic processes to become new rocks and minerals.

11. Physical Weathering • Physical weathering is the breakdown of mineral or rock material by entirely mechanical methods, without any chemical change in the rock material, brought about by a variety of causes. Some of the forces originate within the rock or mineral, while others are applied externally. Both of these stresses lead to strain and the rupture of the rock.

12. Physical Weathering Processes • Abrasion - mechanical wearing or grinding of a rock surface; normally occurs through the erosional transport of material by wind, water, or ice. • Frost action involves the 9% increase in volume of water as it freezes, overcoming the low tensile strength of rock.

13. Physical Weathering Processes • Boulders rounded as wave action rolled them against one another

14. Physical Weathering Processes • Delicate Arch, Arches National Park, Utah- differential weathering of resistant rock strata has preserved the arch beneath the structure as surrounding rock was eroded away. Note person in inset for scale.

15. Physical Weathering Processes • Crystallization – crystal growth - the change from liquid to solid crystalline form produces a volumetric change that overcomes the low tensile strength of rock. • Two primary types of crystal growth in void space of rocks: ice and salt. • Ice formation – water increases volume by 9% as it freezes, rupturing the rock. • Especially prominent in temperate and polar regions of the earth. • The threshold temperature for frost action is at least - 5° Celsius, and it is at this temperature that the most effective rupturing occurs. • Salt formation – salt increases volume from 1 to 5 percent depending on the temperature of the rock or mineral surface. • Most common in hot arid regions, but may occur in cold climates. • Greatly enhances potential for frost weathering.

16. Physical Weathering Processes • Insolation weathering - the physical breakdown of rock by expansion and contraction due to diurnal temperature changes and the physical inability of rocks to conduct heat well. • Results in differential rates of expansion and contraction - the surface of the rock expands more than its interior, eventually causing the rock to rupture. • May also be due to variance in colors of mineral grains in rock (dark colored grains absorb more heat than light colored grains leading to rupture at mineral boundaries.

17. Physical Weathering Processes • Slaking- alternate wetting and dryingleads to accumulation of successive layers of “ordered” water molecules between the mineral grains of a rock, increasing the thickness of the water such that it pulls the rock grains apart over time with great tensional stress. • This can happen in as little as twenty cycles of wetting and drying. • Pressure release jointing due to unloading. • Erosional exposure of igneous rock masses that were created deep within the earth subjects them to low-pressure/low T conditions unlike those where they were formed. As they “decompress” in all directions, jointing occurs, exposing them to surface weathering and decomposition. • Horizontal Fractures occur due to unloading of pressure with exposure; • Vertical Fracturing(Spalling) occurs due to bending stress of sheets unloading three-dimensionally. • Leads to process called exfoliation.

18. Physical Weathering Processes Half Dome, Yosemite National Park, CA, a glacially eroded granite exfoliation dome (+/-5,000 ft. of relief from top to valley bottom) Robert W. Christopherson

19. Physical Weathering Processes Half Dome, Yosemite National Park, CA, east side view. The loosened slabs or rock are subject to further weathering and down-slope movement. Robert W. Christopherson

20. Physical Weathering Processes Great arches form in the White Mountains of New Hampshire Bobbe Christopherson

21. Physical Weathering Processes Rock sheeting along Beverly Sund, Nordaustlandet Island, Arctic Ocean Bobbe Christopherson

22. Chemical Weathering Processes • Chemical weathering involves the alteration of the chemical and mineralogical composition of the weathered material. • The most common chemical weathering processes are: • Hydrolysis • Oxidation • Reduction • Hydration • Carbonation • Solution

23. Chemical Weathering Processes • Hydrolysis - reaction between mineral ions and the ions of water (OH- and H+) • Results in decomposition of the rock surface by forming new compounds on and in rock surface • increases pH of the altering solution through release of hydroxide ions • Especially effective in weathering of silicate and alumino-silicate minerals because of their electrically charged crystal surfaces. Ex.: 2KAlSi3O8 + 2H+ + 2HCO3- + H2O-  Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4SiO2 orthoclase carbonic acid weathering kaolinite silica

24. Chemical Weathering Processes • Oxidation is the reaction and combination of metallic compounds and oxygen (for example, rusting) in which the metallic element loses an electron to an oxygen ion. • Results in the removal of one or more electrons from a compound, causing the structure to be less rigid and more unstable. • Forms a class of compounds called oxides – the most common are the oxides • Red and yellow staining of soils is common in tropics with Hi T and precip. • Ex.: 4FeS2 + 11H2O + 15O2  Fe2O3 + 2Fe(OH)3 + 8H2SO4 pyrite water weathering hematite limonite hydro-sulfuric acid

25. Chemical Weathering Processes • Oxidation of iron minerals produces brilliant red color in sandstone, Red Rock Canyon, Nevada Bobbe Christopherson

26. Chemical Weathering Processes • The brilliant reds of Fe and Al oxides in soils produced by a warm, moist climate in Sumter County, GA Bobbe Christopherson

27. Chemical Weathering Processes • Reduction - reverse of oxidation, caused by the addition of one or more electrons to a metallic element producing a more stable compound.

28. Chemical Weathering Processes • Hydration - rigid attachment of H+ and OH-ions to a reacted compound. • H+ and OH- ions may become a structural part of the crystal lattice of the mineral. • Promotes other decomposition reactions by expanding the crystal lattice offering more surface area for reaction. Ex.: CaSO4 + 2H2O  CaSO4•2H2O Anhydrite water rxn gypsum

29. Chemical Weathering Processes • Carbonation - the reaction of carbonate and bicarbonate ions with minerals, especially in a high-CO2 environment. • Forms carbonic acid - a product of carbon dioxide and water - is important in solution of carbonates and the decomposition of mineral surfaces because of its acidic nature. Ex.: H2O + CO2  H2CO3 H+ + HCO3- water carbon dioxide carbonic acid hydronium ion bicarbonate

30. Chemical Weathering Processes • Solution is the dissolving of a mineral by water and the ions it carries as it moves through and around rocks and minerals. • most effective in areas that have humid and hot climates. Examples of common, easily dissolved minerals: halite (NaCl - salt) calcite (CaCO3) from limestone

31. Chemical Weathering Processes • Climate is the most important factor affecting all of the above-mentioned chemical weathering processes: • Regulates moisture and temperature • Tropical weathering rates, where T and moisture are at max. = three and a half times higher than rates in temperate environments.

32. Chemical Weathering Processes • Chemical Weathering processes act on the joints in granite to dissolve weaker minerals, leading to a rounding of the edges of fractures Bobbe Christopherson

33. Chemical Weathering Processes • Rounded granite boulder showing spheroidal weathering and disintegration of rock Bobbe Christopherson

34. Chemical Weathering Processes • Spheroidal weathering in the rugged Alabama Hills, CA (note Mt. Whitney in background) Bobbe Christopherson

35. Biological Weathering Processes Biological Weathering – due to the activity of an organism. • Types range from bacteria to plants to animals. • Biological weathering can be either chemical or physical in character. • Some of the more important processes are: • Fracture by consumption, burrowing, or root growth; • Movement and mixing causing different physical and/or chemical conditions; • Respiration – can produce CO2  carbonic acid  Solution

36. Biological Weathering Processes • More biological weathering processes: • Chelation – organisms produce organic chelates that decompose minerals and rocks by the removing of metallic cations; • Organisms can influence the moisture regime in soils and enhance weathering. Shade, root masses, and humus all act to increase the availability of water in the soil profile. Water is a necessary component in several physical and chemical weathering processes. • Organisms can influence the pH of the soil leading to solution: • Roots can release CO2 leading to carbonic acid formation, lowering the pH; • Cation exchange reactions, in which plants exchange basic cations for hydrogen ions, can lower soil pH by increasing H+ concentration.

37. Biological Weathering Processes Tree roots growing in and on fractures in bedrock Robert W. Christopherson

38. Biological Weathering Processes Roots can exert tremendous force on sides of rock joints Robert W. Christopherson

39. Biological Weathering Processes Lichens helping to disintegrate ropy “pahoehoe” basalt lava in Iceland. Basaltic lava is less resistant to weathering than granite (inset shows surrounding, moist landscape) Robert W. Christopherson