WEATHERING PROCESSES Physical and Chemical Weathering Weathering insitu

WEATHERING PROCESSES Physical and Chemical Weathering

Weathering: in-situ breakdown of material

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

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

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

Frost action (physical weathering) (Talus cones, Banff National Park, Alberta. Photo: Marli Miller, University of Oregon. http: //marlimillerphoto. com/talus. html)

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

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 -8 C

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

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, http: //www. nps. gov/goga/forteachers/ 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

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

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 000626. http: //libraryphoto. cr. usgs. gov/

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 000626. http: //libraryphoto. cr. usgs. gov/

Salt weathering • landforms • tafoni • honeycomb weathering granular disintegration • spalling Honeycomb weathering in block of Wingate sandstone Silver Falls canyon. Garfield County, Utah. 1921. Plate 9 -B in U. S. Geological Survey. Professional paper 164. 1931. ID. Moore, R. C. 98 mrc 00098

Salt weathering • landforms • tafoni Image file: /htmllib/batch 68 j/batch 68 z/mfe 01153. jpg • 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. 1935. Photo: Marli Miller, University of Oregion. Earth Science World Image Bank, photo hhrhuz, http: //www. earthscienceworld. org/

Salt weathering • landforms • tafoni • honeycomb weathering • granular disintegration • spalling J. R. Stacey, USGS Photographic Library, Image hcb 00980. http: //libraryphoto. cr. usgs. gov/

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

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

Pressure release (physical weathering) • sheet joints and exfoliation • rock bursts in deep mines Unloading (sheeting) joints in granodiorite. Yosemite National Park, California. Sr. F-52. jpg Photo showing sheet jointing: F. E. Matthes, USGS Photographic Library, Image mfe 00007. http: //libraryphoto. cr. usgs. gov/ Unloading joints in granitic rock, Yosemite National Park, CA. Note how joints on either side of the creek dip towards the creek. Sr. F-49. jpg M Miller

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

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

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

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

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 (Na. Cl) and calcite (Ca. CO 3)

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 1971. Figure 7, U. S. Geological Survey Professional paper 754 -A. Image hnkb 0004. http: //libraryphoto. cr. usgs. gov/ 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. 1897. Figure 15 in U. S. Geological Survey. Folio 151. 1907. Image file: /htmllib/btch 325 j/btch 325 z/kei 00117. jpg

Hydrolosis (Chemical Weathering) • water dissociates into H+ (hydrogen cation) and OH(hydroxyl anion) • H+ displaces other cations in mineral structure – K+, Na+, Ca 2+, Mg 2+ – may combine with hydroxly anion or be carried away in solution

Hydrolosis (Chemical Weathering) • hydrolysis promoted by: – decreasing p. H (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 – 4 Na. Al. Si 3 O 8 + 6 H 2 O = Al 4 Si 4 O 10(OH)8 + 8 Si. O 2 + 4 Na+ + 4 OH

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 http: //pubs. usgs. gov/of/2002/of 02 -437/gallery. htm)

Oxidation/reduction (Chemical • 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: Weathering)

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

Oxidation/reduction (Chemical Weathering) • reduction: opposite reaction – occurs below water table – reduced form of elements are mobile than oxidized because they're more soluble • result: weathering rinds 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 78 ACh 32, Kurupa anticline; magnification, 6. 3 x 10, photomicrograph. Central North Slope, Alaska. Published as Figure 45 in U. S. Geological Survey. Bulletin 1614, 1985. Image file: /htmllib/btch 434 j/btch 434 z/bws 00009. jpg 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. 1903. Image file: /htmllib/btch 163 j/btch 163 z/ggk 02031. jpg …. NOTE: different meaning of “reduction” in the literature.

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

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 (NH 4+, K+, Ca 2+, Fe 2+, etc. . . ) Anion-a negatively charged ion (NO 3 -, PO 42 -, SO 42 -, 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)