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WEATHERING PROCESSES Physical and Chemical Weathering.

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Presentation on theme: "WEATHERING PROCESSES Physical and Chemical Weathering."— Presentation transcript:

1 WEATHERING PROCESSES Physical and Chemical Weathering

2 Weathering: in-situ breakdown of material

3 Weathering: in-situ breakdown of material physical (or mechanical) weathering chemical weathering resistance to weathering function of: –internal resistance of material –magnitude of external forces

4 Weathering Process weaken resisting forces produce unique landforms produce regolith (weathering mantle), which may evolve to soil

5 This week’s agenda: explore physical and chemical weathering processes and landforms

6 Frost action (physical weathering) (Talus cones, Banff National Park, Alberta. Photo: Marli Miller, University of Oregon.

7 Frost action 9% volumetric expansion upon freezing –effective only in closed voids that are almost entirely saturated –effective when temperatures oscillate above and below freezing point

8 Frost action ice segregation –frozen water in voids generates a suction force pulling liquid water toward the ice –migrating water generates pressure forces sufficient to enlarge cracks –primary cause of frost heave in soils –only more recently accepted as mechanism working in solid rock –most effective in temperature range -3 to -8C

9 Frost action Landforms: –talus, talus cones, scree slopes

10 Salt weathering (physical weathering) salt crystallization: occurs as saline solutions evaporate salt crystal expansion: occurs when salt crystals get wet Honeycomb weathering in graywacke sandstone, Golden Gate National Recreation Area. Photo: National Park Service, graywacke-sandstone-faq.htm graywacke: a coarse usually dark gray sandstone or fine-grained conglomerate composed of firmly cemented fragments (as of quartz or feldspar) also called dirty sandstone

11 Salt weathering occurrence: hot and cold arid and semi-arid environments capillary rise brings saline water toward surface limited liquid water (either due to supply or phase) incapable of washing salts away hot arid regions: large diurnal changes in temperature and relative humidity promote repeated wetting and drying cold regions: cold temperatures encourage salt precipitation from solutions rocky coastal areas

12 Salt weathering rock susceptibility to salt weathering … is a function of: proportion of micro pores water absorption capacity surface texture presence of clay minerals Photo: K. Segerstrom, USGS Photographic Library image sk

13 Salt weathering landforms tafoni Cavernous weathering of granite ("tafoni") near Caldera. Chile. No date. honeycomb weathering granular disintegration spalling Photo: K. Segerstrom, USGS Photographic Library image Tafoni sk

14 Salt weathering landforms tafoni honeycomb weathering granular disintegration spalling Honeycomb weathering in block of Wingate sandstone Silver Falls canyon. Garfield County, Utah Plate 9-B in U.S. Geological Survey. Professional paper ID. Moore, R.C. 98 mrc00098

15 Salt weathering landforms tafoni honeycomb weathering granular disintegration spalling Sequoia National Park, California. Northern edge of the Siberian Outpost. The frost-split blocks of granite might readily be mistaken for glacial boulders, but, having the same composition as the underlying bedrock, they have obviously been formed in place. The granite here is sparsely jointed and breaks up into large blocks. These, in turn, are subject to granular disintegration which at this lower altitude is probably effected not only by frost action, but also by the solvent action of carbon dioxide. The latter, derived from decaying lichens and pine needles, is carried by water into the interstices between individual granules. Eventually the blocks lose their sharp angles and become rounded Image file: /htmllib/batch68/batch68j/batch68z/mfe01153.jpg/htmllib/batch68/batch68j/batch68z/mfe01153.jpg Photo: Marli Miller, University of Oregion. Earth Science World Image Bank, photo hhrhuz,

16 Salt weathering landforms tafoni honeycomb weathering granular disintegration spalling J.R. Stacey, USGS Photographic Library, Image hcb

17 Wetting & drying (physical weathering) susceptible soils & rocks: –soils with 2:1 layered clays (e.g. montmorillonite) –shale, clayey siltstones and sandstones, granite result: spalling, granular disintegration

18 Thermal expansion and contraction (physical weathering) different minerals have different coefficients of thermal expansion –e.g. quartz is about 3 times that of feldspar effectiveness of insolation debated result: spalling

19 Pressure release (physical weathering) sheet joints and exfoliation rock bursts in deep mines Photo showing sheet jointing: F.E. Matthes, USGS Photographic Library, Image mfe Unloading joints in granitic rock, Yosemite National Park, CA. Note how joints on either side of the creek dip towards the creek. SrF-49.jpg M Miller Unloading (sheeting) joints in granodiorite. Yosemite National Park, California. SrF-52.jpg

20 CHEMICAL WEATHERING progression from less stable minerals to more stable minerals –mineral stability

21 CHEMICAL WEATHERING progression from less stable minerals to more stable minerals –primary minerals - secondary minerals - new secondary minerals

22 CHEMICAL WEATHERING progression from less stable minerals to more stable minerals water is critical geochemical weathering: –driven by inorganic processes; –produces "rotten" rocks or saprolites pedochemical weathering: –controlled by biologic processes; –leads to formation of soil from saprolites

23 CHEMICAL WEATHERING TYPES Solution –virtually all chemical weathering involves some solution –the most common minerals are soluble to some degree in normal waters

24 CHEMICAL WEATHERING TYPES Solution –virtually all chemical weathering involves some solution –the most common minerals are soluble to some degree in normal except: silica when contained in quartz aluminum oxides - virtually insoluble under normal conditions ferric iron - requires very acidic fluids –solution of halite (NaCl) and calcite (CaCO3)

25 CHEMICAL WEATHERING result: –granular disintegration, –spheroidal weathering, –weathering pits, karst Photo right: N.K. Huber, USGS Photographic Library, Example of spheroidal weathering found in Isle Royale National Park, Michigan. Zone of spheroidal weathering in a basalt lava flow. This zone of partly decomposed rock escaped removal by glacial erosion because of its protected location. Circa Figure 7, U.S. Geological Survey Professional paper 754-A. Image hnkb Photo left: Characteristic outcrop of gabbro, one-half mile north of Toecane, looking northeast. Spheroidal weathering is characteristic of the gabbro in this region, and rounded boulders strew its surface. The rock is not metaphosed and presents a strong contrast to the Roan gneiss, which is similar composition. Mitchell County, North Carolina Figure 15 in U.S. Geological Survey. Folio Image file: /htmllib/btch325/btch325j/btch325z/kei00117.jpg/htmllib/btch325/btch325j/btch325z/kei00117.jpg

26 Hydrolosis (Chemical Weathering) water dissociates into H+ (hydrogen cation) and OH- (hydroxyl anion) H+ displaces other cations in mineral structure –K+, Na+, Ca2+, Mg2+ –may combine with hydroxly anion or be carried away in solution

27 Hydrolosis (Chemical Weathering) hydrolysis promoted by: –decreasing pH (increasing H+) –decomposition of organic matter (releases H+) –increased water temperatures (promotes dissociation) important mechanism for breaking apart primary minerals –example: albite weathers to kaolinite plus some residual silica –albite + water = kaolinite + silica + sodium ion + hydroxyl ion –4NaAlSi3O8 + 6H2O = Al4Si4O10(OH)8 + 8SiO2 + 4Na+ + 4OH

28 Hydrolosis (Chemical Weathering) Result of hydrolosis: –spalling, –weathering pits, –spheroidal weathering, –weathering rinds, –production of clay mineral Weathering rind in granitic rock. M.B. Miller WE-29.jpg fine grained mafic igneous rock with orange-brown, iron rich weathering rind (Photo: USGS

29 Oxidation/reduction (Chemical Weathering) oxidation –element in a mineral structure loses electrons increasing their charge –reaction between ions and oxygen results in formation of oxides and hydroxide –occurs above water table –examples:

30 Oxidation/reduction (Chemical Weathering) Oxidation examples: –ferrous iron (Fe+2) oxidizes to ferric iron (Fe+3) 4Fe+2 + 3O2 = 2Fe2O3 iron + oxygen = iron oxide (hematite) –olivine weathers through combination of hydrolysis and oxidation to form hematite olivine + water + oxygen = hematite + silicic acid 2Fe2SiO4 + 4H2O + O2 = 2Fe2O3 + 2H4SiO4 Most elements at earth's surface exist in an oxidized state

31 Oxidation/reduction (Chemical Weathering) reduction: opposite reaction –occurs below water table –reduced form of elements are more mobile than oxidized because they're more soluble result: weathering rinds Granite bowlder undergoing reduction by the separation of large flakes; spur between forks of Little Kern River, California. The flaking is believed to be due to heat caused by forest fires. Tulare County Image file: /htmllib/btch163/btch163j/btch163z/ggk02031.jpg …. NOTE: different meaning of “reduction” in the literature. /htmllib/btch163/btch163j/btch163z/ggk02031.jpg Original porosity is still retained in this sandstone sample from the Nanushuk Group, but some reduction in porosity probably took place during compaction. Quartzose grains were dissolved at grain contacts and welded. At some later stage, minor secondary porosity was created by dissolution of quartzose grains (arrow). Growth of chlorite in pore spaces completes diagenetic cycle. Sample 78ACh32, Kurupa anticline; magnification, 6.3 x 10, photomicrograph. Central North Slope, Alaska. Published as Figure 45 in U.S. Geological Survey. Bulletin 1614Bulletin 1614, Image file: /htmllib/btch434/btch434j/btch434z/bws00009.jpg/htmllib/btch434/btch434j/btch434z/bws00009.jpg

32 Ion Exchange (Chemical Weathering) Exchange of ions in minerals (usually cations) with ions in solution most common in clay minerals –cations held to clay surface by adsorption; not held too tightly –H+ and Ca+2 are most readily adsorbed; Na+2 is most readily released cation exchange capacity: propensity for adsorbing cations

33 Ion Exchange (Chemical Weathering) result: –production of new, more stable secondary (clay) minerals Cation-Exchange Capacity (CEC) Cation-exchange capacity is defined as the degree to which a soil can adsorb and exchange cations. Cation-a positively charged ion (NH4+, K+, Ca2+, Fe2+, etc...) Anion-a negatively charged ion (NO3-, PO42-, SO42-, etc...) Soil particles and organic matter have negative charges on their surfaces. Mineral cations can adsorb to the negative surface charges or the inorganic and organic soil particles. Once adsorbed, these minerals are not easily lost when the soil is leached by water and they also provide a nutrient reserve available to plant roots. These minerals can then be replaced or exchanged by other cations (i.e., cation exchange)


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