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Learn about weathering, erosion, mass wasting, and the rock cycle in this lecture. Discover how rocks disintegrate and decompose near Earth's surface, factors affecting weathering, chemical processes like hydrolysis, oxidation, dissolution, and more.
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Lecture 20 Weathering
About Weathering, etc. • Weathering produces all the soils, clays, sediments, and dissolved substances. • Erosion is the removal of sediments by natural processes such as wind and rivers. • Mass wasting is the downslope movement of masses of Earth materials.
Weathering, Erosion, Mass Wasting, and the Rock Cycle ● Weathering is the process of converting rock into sediment and forming soils, a major process in the rock cycle. ● Erosion and mass wasting are the processes that loosen and transport soil and rock downhill.
Weathering • Igneous, metamorphic and sedimentary rocks form at depth or at high temperature, in an environment where they are stable. If these rocks are brought to the Earth’s surface and/or cooled, they will become mechanically and chemically unstable Weathering The disintegration and decomposition of rocks at or near the surface of the Earth Disintegration Physical weathering; to de-integrate the constituent parts (minerals) of a rock Decomposition Chemical weathering; to de-compose the chemical structure of the minerals themselves
Controls on Weathering ● Properties of the parent rock ● various minerals weather at different rates ● a rock’s structure affects its susceptibility to cracking and fragmentation
Controls on Weathering ● Other important factors ● climate (rainfall and temperature) ● soil (presence or absence) ●time (length of exposure)
Chemical Weathering ● Occurs when minerals react with air and water ● role of water (hydrolysis) ● carbon dioxide (carbonic acid) ●moist soils
Decomposition • Mostly effected by water • As water falls through air and flows through soil, carbon dioxide is dissolved within it: • Hydrogen (hydronium) and bicarbonate ions are effective at attacking minerals
Chemical Weathering: The Role of Increasing Surface Area 2 cm 2 cm
2 cm 1 cm 1 cm 2 cm
2 cm 1 cm 1 cm 2 cm Large rocks have less surface area for chemical weathering …
2 cm 1 cm 1 cm 2 cm Large rocks have less surface area for chemical weathering… … than small rocks do, so smaller rocks weather more quickly.
Hydrolysis Hydrolysis Replacement of ions by or ions from water • What do we note in a rock’s color? • Hydrolysis can change the chemical composition of a rock: • Potassium feldspar Hydronium Water Ionic K Clay Silica
Oxidation Oxidation Loss of an electron • Iron is perhaps the most readily oxidized of the common, rock-forming cations, i.e. • Magnetite Water Oxygen Goethite
Dehydration Dehydration Loss of water • commonly results in formation of hematite from goethite • Goethite = yellowish, hematite = reddish • (or silvery, as in rock shops!)
Dissolution Dissolution Breaking into constituent ions and holding in solution Limestone Carbonic acid Ionic Ca Bicarbonate • Typical reaction for limestone, which is common rock type in western New York • Can result in unusual topography called karst • There are some karstic features along Main Street, Amherst (near throughway)
Leaching Leaching The removal of soluble matter by aqueous solutions • Not so much a type of chemical reaction, but important to chemical weathering • There is a loss of mass with leaching • e.g., in the rock in the preceding example, many of the ions released by hydrolysis have subsequently been taken into solution and removed by leaching
Decomposition • Pit formation in Wheeler Crest granodiorite at June Lake, CA, caused by differential mineral decomposition
Decomposition • Differential decomposition of syenite, Princess Sodalite Mine, Bancroft, ONT
Decomposition • Differential decomposition of syenite, Princess Sodalite Mine, Bancroft, ONT. Close up view.
Physical Weathering ●What determines how rock breaks? ● natural zones of weakness ● activity of organisms ●frost wedging ●exfoliation
Physical Weathering ●Physical weathering and erosion ● duration of weathering ● bedrock type ●climate ● topography
Disintegration • Does not change the chemical composition of the rock • Caused by • freezing water • heating and cooling • unloading • abrasion • organic processes
Freezing • Most effective where there are many freeze-thaw cycles (like here) • Water expands 9% in volume when it freezes • Called frost wedging
Heating and Cooling • Fire probably responsible for most of the mass removed by heating and cooling in many areas (particularly semiarid Western U.S.) • Expansion during the day from insolation and heating also may contribute • Contraction at night with cooling • Diurnal cycle
Types • Each mineral has a different coefficient of thermal expansion • Granular disintegration • Heating on outside of the rock (differential heating) • Spalling or spallation
Granular disintegration and decomposition • Wheeler Crest granodiorite deeply weathering in pits due to granular disintegration following differential decomposition.
Spallation by fire • Fire spallation of a granitic boulder following the Old Fire, Oct, 2003
Spallation by fire • Fire spallation of a granitic boulder following the Old Fire, Oct, 2003. Note chips on ground ready for erosion.
Unloading Unloading Release of pressure as rocks near surface from depth • Expansion of rock in planes parallel to surface • Tensional stress perpendicular to surface • Brittle failure • Exfoliation or sheeting
Physical Weathering: Exfoliation Face of Half Dome, Yosemite Valley, CA, showing granitic sheeting
Exfoliation • Royal Arches, Yosemite Valley, showing sheeting of granite
Abrasion • Running water, ice and wind carry particles • These particles strike surfaces breaking off other particles • Sandblasting or abrasion
Glacial abrasion • Glacially abraded and striated granitic boulder near Bancroft, ONT
Ventifactionweathered by aeolian abrasion • Ventifacted Wheeler Crest granodiorite, Sierra Nevada, CA
Organic destruction • Roots pry rocks apart with growth Salt Weathering • Salt is carried in solution (salt spray) and crystallizes, expanding in cracks
Disintegration and decomposition working together • Decomposition and disintegration work together in Bandelier National Momument, NM, to form distinct weathering features
Rate of Weathering • Clearly, weathering procedes with time (as measurements suggest) • As it procedes, it is influenced by • Rock structure • degree of fracturing • Rock type • i.e., some minerals more unstable under surface conditions
Rate of Weathering (cont.) • Climate • wet fast decomposition; variable precipitation fast disintegration • warm fast decomposition; variable temperature high diurnal variation fast disintegration • Topography • steep little retention of water; flat great retention of water
Results • Broken down rock; rock available for erosion • Movement of ions • Soils and regolith
Soil: The Result of Weathering ● Soils as geosystems ● input material ● transformations and translocations ●output material
Soil: The Result of Weathering ● The basic soil-forming processes result in losses (transformations) and additions (translocations).
Losses Additions Organic material Airborne dust Water erosion Chemicals and minerals from bedrock Wind Leaching BEDROCK