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Monument Valley, Utah !Chapter 5: Weathering and soils !erosion is physical collection of rock particles by water, ice, or wind!weathering, erosion, and transportation!weathering is the group of destructive processes that change the! physical and chemical character of rocks at or near Earth’s surface!rocks on Earth’s surface are constantly changed by "water, air, temperature changes and other factors!transportation is the movement of eroded particles by water, ice, or wind!rock cycle!weathering: slow and steady! erosion: may be more dramatic!weathering ->!results in both positive and negative effects!positive: generates soils!negative: causes deterioration of buildings!marble!slate!tombstones from 1870’s!if you want to be remembered, what will you choose?!weathering is divided into two classes:!both go on continuously and usually together!• mechanical weathering!-- breaks rock into smaller pieces!-- does not change chemical makeup!-- causes physical disintegration only!• chemical weathering!-- changes chemical composition of minerals/rocks!(exposure to atmospheric gases)!-- alters rocks that are unstable at Earth’s surface!to more stable substances!(new chemical compounds -- minerals -- form)!more resistant sandstone cap rock!less resistant shale!mechanical weathering!Grand!Canyon!chemical weathering!marble statue!loss of detail!weathering and Earth’s systems!hydrosphere!atmosphere!oxygen and carbon dioxide critical to chemical weathering!water cycled through atmosphere essential to mechanical weathering!chemical weathering!oxygen dissolved in water oxidizes iron in rocks!carbon dioxide dissolved in water creates carbonic acid!mechanical weathering!running water loosens and abrades particles!weathering and Earth’s systems!biosphere!plant root growth widens cracks!animal movement and human activity break rocks!decaying organic matter in soils yields acids!cryosphere!mechanical weathering!glacial ice removes and abrades particles!freeze/thaw cycling breaks rocks apart!mechanical weathering!chemical weathering!frost action: mechanic effect of freezing (and expanding) !" " "water on rocks!mechanical weathering: processes!water expands about 9% when it freezes!• upper surface freezes first!"(contact with atmosphere)!• water below freezes later and cannot!"expand upward!• ice expands and fractures rock!where?!pressure release: removal of mass of overlying rocks, allows!" " for expansion of buried rock and fracturing!• mass presses down on buried rock!• removal releases pressure!• rock expands!classic example: Half-Dome in Yosemite National Park!plant growth: growing roots !" "widen fractures!burrowing animals: activity breaks down rock!thermal cycling: large temperature variations fracture rocks!" " "from expansion and contraction!• different minerals will expand different amounts as T increases!"(e.g. quartz expands much more than feldspar)!• important where days are hot and nights are cool!• water likely is necessary!similar to frost action, but freezing is not required!surface area to volume ratio increases!(volume remains constant at 1 m3 )!what happens during mechanical weathering?!rock breaks down into smaller pieces...!for a cube that is 1 m on each side…mechanical weathering!breaks it down into smaller pieces, exposing more surfaces!over time, can make rectangular pieces “spheroidal”!spheroidal weathering!granite that has undergone spheroidal weathering!oxidation: chemically active oxygen from atmosphere!" "reacts with Fe and oxidizes (“rusts”) it!chemical weathering: processes! 4 Fe + 3 O2 = 2 Fe2O3! Iron Oxygen Hematite !rust is very stable at the Earth’s surface; !remember “Rust Never Sleeps…”!acid dissolution: atmospheric gas dissolved in water yields acid!• atmospheric carbon dioxide forms carbonic acid!• sulfur and fluorine from volcanic eruptions form!"sulfuric and hydrofluoric acid!• some minerals, e.g. calcite, will completely dissolve!• human activity from burning fossil fuels, mining, etc. can also!"produce acids in atmosphere -- “acid rain”!caves in limestone !in Saudi Arabia!etched !by carbonic acid!acid leaching from mining!industrial pollution -- generating acid rain!what is acid rain?!pH -- concentraion of H+ ions!larger number is greater concentration - acid!pH = - log [H+]alteration of feldspars: ! feldspars easily broken !"down by ! acidic rain water!• alteration of feldspars forms !"clay minerals!• feldspars are most common! minerals in crust!• K, Na, Ca ions released into!"water!• SiO2 also released into water!"and carried away!pathway on right is for calcite!-- calcite dissolves completely --!what happens to feldspars as they alter?!water is added to crystal structure of feldspar to yield clay!carbonic acid essential for weathering (carbon cycle)!carbon (inorganic) cycle!• carbon dioxide in atmosphere combines with water to make carbonic acid!• carbonic acid weathers rocks!• limestone (calcium carbonate) forms in bodies of water !• plate tectonics returns limestone to deeper in Earth !• volcanic eruptions send carbon dioxide back to atmosphere !crystals growing in cracks put pressure on walls!example is Cleopatra’s Needle! …survived Egypt for > 3,000 years! …removed for transport to New York City! …stored at site where salty groundwater penetrated column!an example of both mechanical and chemical weathering!another famous example:!Mesa Verde, Colorado!process builds ledges!that!cliff dwellers preferred!for habitation!weathering does not occur at same rate everywhere!factors:!climate: heat, humidity increase chemical weathering!…warmer water, increased plant growth…!living organisms: surface exposure increases from breakdown!…average earthworm colony brings 7-18 tons!of soil per acre to surface each year!time: rock must be exposed; if not, more time required!mineral composition: stability of minerals at Earth’s surface!…minerals formed at high temperatures/pressures!are not stable at Earth’s surface!e.g. olivine, pyroxene!33!soils! sedimentary rocks!34!what happens to rock after it weathers?!chemical and mechanical weathering of! sediment and bedrock (pre-existing hard, rock)!"breaks rock into regolith (fragmented rock)!upper few meters!of regolith!is!soil!35!soil - a layer of weathered, unconsolidated material !" "on top of bedrock!contains: !• clay minerals!• quartz!• water!• organic


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