F13 quiz 1 condensed notes

Metamorphism generally occurs in a closed chemical

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Unformatted text preview: rature conditions) control the formation of different soil types. Products of weathering are eroded, usually by gravity or flowing water, wind or ice, transported, and then deposited as sediment. Sedimentary rocks are classified as clastic or as chemical. Terrigenous (“derived from the land”) clastic (“broken”) sediment consists of particles derived by weathering and erosion of a source rock, later transported and deposited. Terrigenous clastics are classified according to particle size irrespective of the composition of the particles. Chunks of the source rock may be pebble ­ sized or larger, up to cobbles, up to boulders, and rarely pieces even kilometers on a side, that are deposited together with grains of sand, silt, and clay to form conglomerate. Sand ­sized grains (2 ­0.62 mm) in sandstone have a familiar gritty feel. Because of their resistance to weathering, quartz grains survive abundantly as sand particles, but sandstone may also contain sand ­sized particles of feldspar or rock fragments. With continued transportation, for example with distance down a stream system, the clastic particles become better rounded (i.e., less angular), and better sorted (particles more nearly of a uniform size). Abrasion during transportation generally does not reduce material to smaller than silt ­sized particles (siltstone). Consequently, the microscopic particles of clay minerals in shale and mudstone, the most abundant terrigenous clastic sedimentary rocks, must have originated by chemical weathering of feldspar, not mechanically by knocking against one another. Clastic sediments are laid down in layers, or strata, in many different types of depositional environments (ex. beach, alluvial fan, river delta, desert sand dune field). The processes by which loose clastic grains are compacted and cemented together to become sedimentary rock are together called diagenesis. Chemical sediments originate by direct precipitation of dissolved ions. Evaporite chemical sediments precipitate in reverse order of their solubility (calcite first, then gypsum, then halite) as the concentration of dissolved ions in seawater becomes highly concentrated through intense evaporation and loss of water vapor. Biochemical sediments are composed of the undissolved remains of organisms and minerals that were precipitated by biological processes. Calcite and quartz are the most common minerals in biochemical sedimentary rocks. Carbonate sediments contain the CO32 ­ (carbonate) ion, combined with Ca2+ to make the chemical substance calcium carbonate, which is also the mineral calcite. A sedimentary rock comp...
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This document was uploaded on 03/03/2014 for the course GEO 303K at University of Texas.

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