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CHAPTER 5: The Formation of Sediment and Sedimentary Rocks 5.1 How and Why Do Rocks Disintegrate to Form Sediment?Sediment results from the disintegration of rocks exposed to weather and is then redistributed on Earth’s surface by wind, water, and glaciers.It gets buried and transformed to hard rock.Weathering is the process that break down preexisting rocks at Earth’s surface, reducing them to loose particles, dissolving some minerals and forming new ones.Erosion refers to the process that pickup sediment particles.Weathering occurs when the geosphere interacts with the atmosphere, hydrosphere, and biosphere.o Physical: Break large rocks into smaller fragmentso Chemical: Reactions among minerals and water and gases that dissolve some minerals and produce new onesPhysical WeatheringWater freezing into rock cracks causes the rock to break moreSalt during the winter causes concrete to crackExpansion and contraction of minerals in rocks causes rock disintegrationChemical WeatheringThe key to understanding chemical weathering lies in figuring out how minerals react with oxygen and waterThe reactions that take place in oxygen and water are important also in the shallow subsurface where ground water interacts with mineralsDissolution: reactions break apart mineral molecules and these molecules then disperse in waterHydrolysis: reactions that consume both mineral and some of the water molecules and form new solid minerals and dissolved ionsOxidation: the process in which substances react with oxygen to form new substances by exchanging electronsPlutonic and metamorphic rocks form below Earth’s surface in the absence of oxygen gas, so they are especially prone to oxidation reactions on the surfaceWhat Minerals Survive Weathering Minerals dominated by ionic bonds most readily dissolve in water, especially if the water is acidicSilicate minerals are generally more resistant to chemical weatheringo Those with more siliconoxygen bonds weather more slowly5.2 What Is the Link Between Weathering and Sediment?
Recall:Weathering is the process that forms sedimentTwo things are produced – solid minerals and dissolved ionsMaking Clastic SedimentThey are of solid mineral particles that are left behind as rocks weatherThey form by chemical weatheringQuartz is the most resistant to weathering because of its covalent bonds and the lack of ironMaking Chemical SedimentChemical sediment: dissolved ions precipitate as solid ionic compoundsMost precipitation of chemical sediment takes place in lakes and oceansIn some case, biochemical processes also cause mineral precipitationo Ex: formation of shells and bones5.3 How Does Loose Sediment Become Sedimentary Rock?The first step in forming sedimentary rock: CompactionThe first step of lithification of compaction: sediment particles are weighed down with previous sediment and pressure to break bondso Decrease the volume of pore space and packs grainsStickiness of compacted sediment is explained by the observation that natural broken surfaces on minerals expose atoms with unbalances charges that tent to attract oppositely charged atoms in adjacent grainsThe second step: CementationCementation: Filling pore spaces with precipitated minerals to make rock hardMinerals precipitate from water in the pore spaces between sediment particles and cement them together into rockCement minerals grow to fill pore spaces and surround the sediment particles Most common cementing agents are calcite, quartz, clay minerals, and hematitePrecipitation of mineral cements is caused partly by and increase in ion concentration in the water, but affected by temperatureCalcite precipitates and forms cement readily at depth because it relies on high heat to precipitate5.4 How Are Sedimentary Rocks Classified?1. Clastic Sedimentary Rocks are composed primarily of mineral grains remaining from or produced by weathering of preexisting rocks and cemented by minerals that precipitated from pore water.2. Chemical and biogenic sedimentary rocks composed of minerals that were precipitated from water or that are remains of organisms.
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School: Western Michigan University
Course: Physical Geology
Term: Winter 2016
Tags: Geology and Chemistry
Name: GEOS 1300 Week 5 Notes
Description: These notes cover the formation of different sedimentary rocks. Beginning with how the components of sedimentary rocks are broken down by different weathering. They go over how sedimentary rocks are classified, how fossil fuels are found in sedimentary rocks, and what sedimentary rocks reveal about ancient environments.
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