WEATHERING Two categories of weathering: 1. Mechanical weathering (or Physical weathering) • Produces particles (grains) 2. Chemical weathering • Commonly produces clays, oxides that are insoluble minerals, and ions in solution MECHANICAL WEATHERING = mechanical breakdown of rock and minerals into smaller particles, which become available for transport (EROSION) 2x2 m cube to 8 cubes of 1x1 m 24 sq. m to 48 sq. m MECHANICAL WEATHERING A. unloading joints Bethel Granite B. Frost wedging Water expands 9% upon freezing Longs Peak, CO Frost wedging has produced large blocks (you can still see planar nature of joint sets partially preserved) C. Macro- and micro-organism, including plant, BIOTURBATION physically “churn” and break apart sediment and rock • Burrowers • Borers • Root penetration Root Wedging http://www.mccullagh.org/image/10d-17/anthill.html CHEMICAL WEATHERING = chemical reactions (water-based) break down existing minerals, creating new minerals (CLAYS and oxides) and ions in solution (water) 4 examples of chemical weathering: A. Solution B. Oxidation C. Ionic Exchange D. Hydrolysis A. Solution • Ionic-bonding is only type affected • Presence of water is critical • Water contains CO2 CaCO3 Here, calcite-rich rock (limestone) is “dissolving” due to chemical solution weathering 1908 1969 Compare to the carbonate rock weathering of last slide B. Oxidation • O2 (in water or atmosphere) • strips an e- from Fe++ (ferrous iron) • results in disruption of crystal structure Oxidation of Febearing minerals along joint and fracture surfaces in a granite C. Ionic Exchange Ions removed from original mineral, replaced with different ions, of slightly different size often. This disrupts the crystal structure, weakening it. Most common in the sheet silicates • Mica + H2O HAlSiO*OH [clay mineral] • Clays are very common (soils, shales) D. Hydrolysis Hydrolysis = H+ is very aggressive and moves into crystal structure, disrupting and breaking down the mineral • a MAJOR hydrolysis reaction is: Feldspar + water KAlSiO + H2O Clay mineral + silica + free cation HAlSiO*OH + SiO2 + K Here, the effect of hydrolysis on feldspars and micas of a granite … has turned solid rock into loose grains that can be easily eroded (Quartz remains, its stable!) Granite, before hydrolysis Conway Granite boulder in the East Branch of the Pemigawasett River east of Lincoln, NH Granite, after hydrolysis … production of grus Weathered boulder East Branch of the Pemigiwasett River east of Lincoln, NH Differential weathering of pegmatite dike in diorite, Ontario, Canada Diorite - Pegmatite Origin of GRAINS of all sizes sandstones, siltstones, claystones (shales) !!! Weathered boulder East Branch of the Pemigiwasett River east of Lincoln, NH
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