Weathering, Erosion, Soils, and Mass Wasting

1. Weathering, Erosion, Soils, and Mass Wasting

3. Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) of rock at or near Earth’s surface • Mass wasting – the transfer of rock and soil downslope under the influence of gravity • Erosion – the physical removal of material by mobile agents such as water, wind, ice, or gravity

4. Weathering • Two types of weathering • Mechanical weathering – breaking of rocks into smaller pieces • Chemical Weathering • Breaks down rock components and the internal structures of minerals • Most important agent involved in chemical weathering is water (responsible for transport of ions and molecules involved in chemical processes)

5. Mechanical Weathering • Four types of mechanical weathering • Frost wedging – alternate freezing and thawing of water in fractures and cracks promotes the disintegration of rocks • Ex. Pot holes in roads

6. Frost wedging

7. Weathering • Mechanical Weathering continued • Unloading – exfoliation of igneous and metamorphic rocks at the Earth’s surface due to a reduction in confining pressure • Thermal expansion – alternate expansion and contraction due to heating and cooling • Biological activity – disintegration resulting from plants and animals

8. Exfoliation of igneous rocks

12. Weathering • Chemical Weathering • Breaks down rock components and the internal structures of minerals • Most important agent involved in chemical weathering is water (responsible for transport of ions and molecules involved in chemical processes)

13. Weathering • Major processes of chemical weathering • Oxidation • Any chemical reaction in which a compound or radical loses electrons • 4 Fe + 3O2 2 Fe2O3 • Important in decomposing ferromagnesian minerals – rust • Hematite, limonite

15. Weathering • Major processes of chemical weathering • Dissolution • Aided by small amounts of acid in the water • Soluble ions are retained in the underground water supply • Hydrolysis • The reaction of any substance with water • Hydrogen ion attacks and replaces other positive ions

18. Weathering • Alterations caused by chemical weathering • Decomposition of unstable minerals • Generation or retention of materials that are stable • Physical changes such as the rounding of corners or edges

21. Effects of Weathering • Differential weathering • Masses of rock do not weather uniformly due to regional and local factors • Results in many unusual and spectacular rock formations and landforms

22. Joint-controlled weathering in igneous rocks

25. Weathering • Rates of weathering • Advanced mechanical weathering aids chemical weathering by increasing the surface area • Others factors affecting weathering • Rock characteristics • Rocks containing calcite (marble and limestone) readily dissolve in weakly acidic solutions

26. Weathering • Others factors affecting weathering • Rock characteristics continued • Silicate minerals weather in the same order as their order of crystallization • Climate • Temperature and moisture are the most crucial factors • Chemical weathering is most effective in areas of warm, moist climates

27. Rock Type and Climate • Warm, wet climates – carbonates (limestone, dolomite, marble, etc) weather faster (mostly chemical) and form our valleys • Sandstones, quartzites, metabasalts – ridge formers (subject to mechanical weathering • Arid climates (western US) – limestones the ridge-formers, sandstones valleys

28. Acid Rain • Environmental application of chemical weathering. • Rain water is normally slightly acidic, pH=5.6 as water mixes with CO2 in atmosphere. • Pollution from NOx and SOx mixes with H20 in air and forms H2SO4 and HNO3, making rain more acidic • Dissolves marble statues, destroys trees, fish

31. Soil • Soil is a combination of mineral and organic mater, water, and air • That portion of the regolith (loose surface sediment of rock and mineral fragments produced by weathering) that supports the growth of plants (Upper part of the regolith.)

32. Typical components in a soil that yield good plant growth

33. Soil • The soil profile • Soil forming processes operate from the surface downward • Vertical differences are called horizons – zones or layers of soil

36. Controls and triggers of mass wasting • Important triggers include • Removal of anchoring vegetation • Ground vibrations from earthquakes • May cause expensive property damage • Can cause liquefaction – water saturated surface materials behave as fluid-like masses that flow

37. Controls and triggers of mass wasting • Landslides without triggers • Slope materials weaken over time • Random events that are unpredictable

42. A lahar from the 1980 Mt. St. Helens eruption

43. An earthflow on a newly formed slope

44. Some visible effects of creep

46. Forms of flow • Slow movements • Solifluction • Promoted by a dense clay hardpan or impermeable bedrock layer • Common in regions underlain by permafrost • Can occur on gentle slopes

47. Permafrost regions in the Northern Hemisphere

49. Ground subsidence in Alaska due to permafrost