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Unformatted text preview: AGRO 301 1/29/08 Chapter 2 Formation of Soils From Parent Materials I) Weathering of Rocks and Minerals A Weathering- the physical and chemical breakdown of particles B Characteristics of Rocks and minerals 1) Igneous rock is composed of primary minerals such as light-colored quartz, muscovite, and feldspars and dark-colored biotite, augite, and hornblende (a) Dark-colored igneous rocks are more easily broken down than are lighter- colored igneous rocks 2) Sedimentary rock consists of compacted or cemented weathering products form older preexisting rocks (a) Ex. Quartz sand weathered from a granite rock and washed into the ocean on the ocean floor and eventually become cemented into a solid mass called sandstone . Clays may be compacted into shale . 3) Metamorphic rocks are those that have formed by the metamorphism or change in form of other rocks (a) Ex. When the plates shift and a mountain range appears the igneous and sedimentary rock is under tremendous heat and pressure which melts them and they become new metamorphic rocks. C Weathering: A General Case 1) Weathering is a biochemical process that involves both destruction and synthesis. 2) Physical Disintegration- breaks down rock into smaller rocks and eventually into sand and silt particles 3) Chemical Decomposition- decomposes minerals into new minerals. The particle size is also decreased. D Physical weathering (Disintegration) 1) Temperature (a) Rocks heat up during the day and cool down at night, which expands and contracts the minerals that will eventually crack and break down the rock. (b) Because the outside is often a different temperature than the inside some rocks weather by Exfoliation- the peeling away of outer layers. 2) Abrasion by water, ice and wind (a) Wind causes abrasion and in a river the rocks are rounded 3) Plants and animals (a) Plant roots sometimes enter cracks in rocks and pry them apart (b) Burrowing animals may also help disintegrate rocks (c) Plants and animals have little impact compared to the rest E Biogeochemical Weathering 1) Biogeochemical weathering is the chemical breakdown of rocks 2) The 6 types of chemical weathering reactions (a) Hydration- adding water to the crystal/mineral itself (b) Hydrolysis- water is split into a Hydrogen and an OH (c) Dissolution(solution)- water hydrates ions until they become dissociated...
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This note was uploaded on 03/26/2008 for the course AGRO 301 taught by Professor Dr.tomhallmark during the Spring '08 term at Texas A&M.
- Spring '08