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Published byCarson Lamkins Modified over 9 years ago
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Weathering
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Sedimentary Cover + Sedimentary rocks form a veneer over much older igneous and metamorphic “basement” rocks. + This veneer varies in thickness from 0 to 20 km
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Types of Weathering Weathering + Physical – Mechanical breakage, disintegration. + Chemical *- Decomposition by reaction with water. Produces dissolved matter and an insoluble residue. + Biological – Action of organisms often combines physical and chemical weathering processes. + Coffee metaphor for weathering: ! Physical – Grinding coffee beans. ! Chemical – Leaching coffee with hot water 8 Generates dissolved material (coffee), and 8 Depleted residue (coffee grounds).
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Rock Disintegration + Weathering – Processes that break-up solid rock. ! Weathered Granite ! Fresh Granite
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Rock disintegrates to form sediments which can form sedimentary rocks.
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Physical Weathering aka mechanical weathering -Mechanical breakup with no change in mineral composition. -Often occurs with chemical weathering, except at climate extremes. -Physical weathering dominates in polar and desert regions. -Everywhere else, chemical weathering dominates. + Types of Physical Weathering -Jointing (release of overburden pressure) -Frost Wedging -Root Wedging -Thermal Weathering -Alternate wet / dry
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Jointing Rocks expand when rock erodes off of the top of a formation from reduction in pressure. + Rocks develop cracks called joints. + Igneous rocks crack in parallel layers,
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Frost Wedging + Water enters a rock fracture, freezes in fractures. + Ice expansion wedges the fracture open. + During thaw, water infiltrates deeper into crack. + Repetition results in rock disintegration. + Dominant process in mountainous regions.
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Frost wedging -Rocks wedged loose by freeze-thaw accumulate as talus at the base of steep slopes. - Talus (broken rock) accumulate at the bottom of the slope
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Chemical weathering -Breaks minerals into chemical components. -Requires water “the universal solvent”. -Significant in humid (water-rich) landscapes. -Chemical weathering virtually absent in deserts. - 3 major processes 1. Dissolution 2. Oxidation 3. Hydrolysis
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Types of Chemical Weathering Dissolution- -Some minerals (halite, gypsum, calcite) dissolve. -Acidity (i.e. acid rain) enhances this effect. Oxidation- - A reaction whereby a metal loses electrons. -Important in decomposing minerals. -Rusting is a familiar example of oxidation Hydrolysis- -Breaking (lysis) by water (hydro). -Water breaks apart elements that hold silicates together. Alteration minerals (residues) such as -Clay minerals -Iron oxides (rust)
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Biological weathering Mosses and plants release acids during decomposition that cause rock to deteriorate
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Effects of weathering Unstable minerals decompose. Rock volume is reduced. Corners and edges attacked rapidly = spheroidal weathering.
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Chemical Weathering in Granite + Biotite and feldspar weather to clay minerals. + These minerals are removed by running water. + Mineral removal causes the rock to crumble. + Quartz, resistant to weathering, is left over.
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Weathering Rates -Temperature and wetness extremes Hot and Wet – Chemical weathering at a maximum Cold and Wet – Physical weathering at a maximum Hot and Dry – Physical weathering dominates Cold and Dry – All weathering at a minimum -Biological effects – Active biota accelerates weathering. -Geologic past – Rates of weathering have varied with atmospheric and biotic changes.
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Differential weathering - Rock masses do not weather uniformly. -This results in many unusual and spectacular rock formations, landforms and landscapes.
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