agChapter 7

agChapter 7 - Chapter #7: A Surface VeneerSediments, Soils,...

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Chapter #7: A Surface Veneer—Sediments, Soils, and Sedimentary Rocks I.) Introduction *Sediment : consists of loose fragments of rocks or mineral broken off bedrock, mineral crystals that precipitate directly out of water, and shells (formed when organisms extract ions from water) *Sedimentary Rock : rock that form at or near the surface of the Earth by precipitation of minerals from water solutions, by the growth of skeletal material in organisms, or by the cementing together of shell fragments or of loose grains derived from preexisting rock *Where do they exists? The upper part of the crust form a surface veneer or “cover” *What is the “basement” or what does the veneer cover? Igneous rocks and metamorphic rocks *Sediments and sedimentary rocks cover more than 80% of Earth’s surface *Constitute less than 1% of Earth’s mass *Why important? They represent a uniquely important type, for they contain the bulk of our energy resources ( Coal, Petroleum, and natural gas ) and our groundwater II.) How Does Weathering Lead to Sediment Formation? **Rocks at or near the surface of the Earth undergo weathering. During chemical weathering, minerals dissolve and/or transform into new minerals (such as clay and iron oxide). During physical weathering, rock breaks down into smaller pieces** A.) The Mountains Crumble *If you ever have the chance to hike or drive through granitic mountains, like the Sierra Nevadas of California or the Coast Mountains of Canada, you may notice that in (1) some outcrops the granite surfaces look hard and smooth and contains shining crystals of feldspar, biotite and quartz while (2) in other outcrops the granite surface looks grainy and rock may peel apart like an onion… Why are these two types of outcrop different? (1) The first type exposes “fresh” or unweathered rock whose mineral grains have kept their original composition and shape (2) The second type exposes weathered rock that has reacted w/ air and/or water at or near the Earth’s surface and has thus been weakened *Weathering : the processes that break up and corrode solid rock, eventually transforming it into sediment *All mountains and other features on the Earth’s surface sooner or later crumble away b/c of weathering B.) Physical Weathering : breaks intact rock into unconnected grains or chunks, collectively called debris or detritus; physical process that do not change its chemical composition ( like a plumber’s snake, physical force ) 1.) Natural Zones of Weakness ( joints, fractures…etc ) *Jointing Joints : natural cracks that form in rocks due to removal of overburden or due to cooling *So how exactly do joints form? Rocks buried deep in the Earth’s crust endure: (1) a lot of pressure b/c of the overlying rocks, and (2) warm temperatures b/c of Earth’s geothermal gradient *Over long periods of time, moving water, air, and ice at the Earth’s surface grind away and remove overburden ( overlying rock ) so rock formerly at depth rises
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agChapter 7 - Chapter #7: A Surface VeneerSediments, Soils,...

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