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F13 quiz 1 condensed notes

Terrigenous clastics are classified according to

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Unformatted text preview: ake caldera in Oregon. LECTURE 5: WEATHERING, SEDIMENTARY ROCKS Sedimentary rocks are recycled rocks that form by the processes of weathering, erosion, transport, deposition and lithification, or diagenesis, of sediments. They are typically stratified, or layered, and are broadly categorized into two groups—clastic and chemical sedimentary rocks. Minerals that crystallized under high ­T, high ­P conditions deep in the earth are unstable in the low ­T, low ­P, water ­rich environment at Earth’s surface. They respond to the new (surface) environment by experiencing mechanical or chemical weathering. Mechanical weathering is simply a reduction of size—larger particles becoming smaller particles. Chemical weathering is accomplished as water penetrates the rock, attacking mineral structures. The complex framework structure of feldspar is totally reconstituted into clay minerals (sheet silicates) as dissolved K, Na, and Ca ions are carried away in groundwater. Ferromagnesian minerals transform into clay, are partly dissolved, and are partly transformed into insoluble iron oxide minerals such as hematite (Fe2O3). Acidic rainwater dissolves the calcite (CaCO3) in limestone, creating caves, honeycomb rock, and other karst features common in areas with limestone bedrock like we have in central Texas. Inert, durable quartz is not chemically attacked except under prolonged humid tropical conditions. Weathering produces spectacular effects on granite, a rock that had originated far beneath Earth’s surface. Expansion associated with release of pressure, as overlying rock is removed by erosion, causes curved sheets to pop off of massive granite slabs in the process of exfoliation. These giant blocks of granite are further mechanically weathered when tree roots pry open fractures, and water that has seeped in freezes and wedges the rock apart. Weathering penetrates around all the mineral grains, freeing them into a mass of loose fitting crystals, called granite grus. Continued chemical weathering creates clay minerals that occupy a larger volume than the original feldspar. A mechanical effect (expansion) may have a chemical cause (weathering of feldspar into clay minerals). Soils form at the interface between weathered mineral material that has not been transported, and the biological communities that live on the land surface. Soils have vertical profiles with internal horizons that are composed of organic material at the surface, weathered rock material at the base, and a zone of chemical and physical interaction in between. Rock parent material, weathering time and climate (long ­term moisture and tempe...
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