lect 8 1113 Weather Soils

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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: 

rock cycle weathering: slow and steady erosion: may be more dramatic

weathering ->: 

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


why can we find diamonds in Murfreesboro? diamonds are hardest substance we know and resistant to erosion everything else erodes first -- diamonds are left and concentrated


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


soils sedimentary rocks


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


soil - a layer of weathered, unconsolidated material on top of bedrock contains: • clay minerals • quartz • water • organic material


idealized soil profile not all horizons will be present everywhere downward motion of water “leached” from above


O: (organic horizon) uppermost portion of soil plant matter; 2 trillion bacteria, 400 million fungi, 50 million algae, thousands of insects in kilogram A: inorganic mixed with organic derived from O horizon) thickness depends on amount of decomposed vegetation …in tropical regions may be thick… E: light-colored with little or no organic material color from dissolution and removal of Fe and Al B: variety of types: depends on organic and oxide content where dissolved materials from upper levels collect


example of a soil profile


influences on soil formation • parent material • topography • climate • vegetation • time


differences in soils among Indonesian islands reflect parent materials parent material: bedrock from which soil develops minerals in parent material determine: • nutrient richness of soil • amount of soil produced Java: parent materials are volcanic ashes --nutrient rich, thick soils …population density is 460/square km Borneo: parent materials are granites, gabbros, and andesites --soils depleted of nutrients (lack of fresh ash) …population density is 2/square km


climate: controls precipitation/chemical weathering moderately wet climates: • more chemical weathering and thicker soils significant clay-rich layers -- may be solid enough to form hardpan arid climates: • less chemical weathering and thinner soils subsurface evaporation leads to build-up of salts calcite-rich accumulation zones may form -- solid enough for hardpan extremely wet climates (tropical rain forests): • highly leached and unproductive soils -- laterites most nutrients come from thick O/A horizons


hardpan (hard surface) forms at depth where annual rainfall penetrates arid climate: calcite horizon forms “caliche” moderately wet climate: clay that hardens “soil” is now “rock” hard


spectacular example of caliche formation in Kansas hardpan

laterite: unproductive, tropical soils: 

laterite: unproductive, tropical soils contain lots of Fe2O3 and Al2O3 -- what remains after rest is leached away very red; very oxidized (rusted) paradoxical? wet, yet not productive? homogenized


water flows quickly down steep slopes -- little soil formation topography: differences in elevation • relief (elevation change--valley bottoms to hill tops) • steepness of slopes think about what water does… water accumulates in low-lying areas -- high soil formation


vegetation: source of organic matter in soil • produces oxygen and carbon dioxide -- chemical weathering • yields H+ for ion exchange in feldspars/clays that gives plants Ca, Na, K time: dependent on other factors per se • warm, moist climate -- soils develop quickly • arid, dry climate -- soils form very slowly thicker soil is not necessarily older


soil classification residual soil - “what is left” -- weathering of bedrock transported soil - soil from “elsewhere” flood plain deposits (soils) from rivers. wind-transported deposits (soils) are called loess soil composition parent rock is deciding factor chemical weathering through time determines composition soil thickness time increases soil thickness wet climate increases soil thickness low slopes also increase thickness

US Soil map: 

US Soil map soft & organic rich strongly weathered & clay rich gray-brown & moist

paleosols: “paleo” -- old; “sols” -- soils: 

paleosols: “paleo” -- old; “sols” -- soils formed in the past and preserved in rock record --implies that rocks formed on surface in the past-- tells us when paleosol was exposed at Earth’surface

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