weathering - Weathering and Soils Page 1 of 7 EENS 1110...

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This page last updated on 30-Jan-2012 EENS 1110 Physical Geology Tulane University Prof. Stephen A. Nelson Weathering and Soils Earth is covered by a thin ±veneer² of sediment. The veneer caps igneous and metamorphic ±basement.² This sediment cover varies in thickness from 0 to 20 km. It is thinner (or missing) where igneous and metamorphic rocks outcrop, and is thicker in sedimentary basins. In order to make this sediment and sedimentary rock, several steps are required: z Weathering ³ Breaks pre-existing rock into small fragments or new minerals z Transportation of the sediments to a sedimentary basin. z Deposition of the sediment z Burial and Lithification to make sedimentary rock. Each Step in the process of forming sediment and sedimentary rocks leaves clues in the sediment. These clues can be interpreted to determine the history of the sediment and thus the history of the Earth. Weathering Geologists recognize two categories of weathering processes 1. Physical Weathering - disintegration of rocks and minerals by a physical or mechanical process. 2. Chemical Weathering - chemical alteration or decomposition of rocks and minerals. Although we separate these processes, as we will see, both work together to break down rocks and minerals to smaller fragments or to minerals more stable near the Earth's surface.Both types are a response to the low pressure, low temperature, and water and oxygen rich nature of the earth´s surface. Physical Weathering The mechanical breakup or disintegration of rock doesn't change mineral makeup. It creates broken fragments or ±detritus.² which are classified by size: z Coarse-grained ³ Boulders, Cobbles, and Pebbles. z Medium-grained ³ Sand z Fine-grained ³ Silt and clay (mud). Physical weathering takes place by a variety of processes. Among them are: z Development of Joints - Joints are regularly spaced fractures or cracks in rocks that show no offset across the fracture (fractures that show an offset are called faults). { Joints form as a result of expansion due to cooling or relief of pressure as overlying rocks are removed by erosion. Page 1 of 7 Weathering and Soils 1/30/2012
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{ Igneous plutons crack in onion like ±exfoliation² layers. These layers break off as sheets that slide off of a pluton. Over time, this process creates domed remnants. (See figure B.4 in your text) Examples: Half-Dome (CA.) (see figure 22.12a in your text) and Stone Mountain (GA.). { Joints form free space in rock by which other agents of chemical or physical weathering can enter. z Crystal Growth - As water percolates through fractures and pore spaces it may contain ions that precipitate to form crystals. As these crystals grow they may exert an outward force that can expand or weaken rocks. z
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This note was uploaded on 02/01/2012 for the course EENS 1110 taught by Professor Staff during the Fall '10 term at Tulane.

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weathering - Weathering and Soils Page 1 of 7 EENS 1110...

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