Weathering and erosion Chapter 12 Weathering and erosion

Weathering Process Weathering changes the physical form of chemical composition of exposed rock. Mechanical weathering physically breaks down rock into smaller pieces agents include ice, plants & animals, gravity, running water, wind Physical changes in the rock can increase weathering exfoliation

Mechanical weathering Ice wedging occurs when water gets into cracks and then freezes, causing cracks to widen Thawing allows more water in Rocks eventually split apart Organic activity of plants and animals: Plant roots get into cracks and widen as they grow Animals dig – expose rock to weathering Abrasion occurs when rocks collide with each other Gravity, water, wind

Chemical Weathering (Decomposition) Occurs when chemical reactions take place between rock minerals and water, CO2, O, and acids Change the chemical composition and physical appearance Hydrolysis – change in mineral composition due to chemical reaction with water Minerals are dissolved in water and carried to lower rock layers = leaching -> mineral ore deposits

Chemical Weathering Carbon dioxide dissolves in water to form carbonic acid H2O + CO2 -> H2CO3 Some minerals combine with H2CO3 to form new substance = carbonation Calcite reacts with H2CO3 to form calcium bicarbonate which dissolves easily in water -> underground caverns in limestone form this way

Chemical Weathering Oxidation occurs when metallic elements combine with oxygen Iron combines with oxygen to form rust 4Fe + 3O2 -> 2Fe2O3 Red soil in SE US Rain water is slightly acidic naturally but is a damaging weathering agent in industrial areas Some plants such as lichens and mosses produce acids that can dissolve the surface of rock and seep into cracks to break rock.

Rates of Weathering Major factors: Composition Exposure Climate Topography

Composition Sedimentary weathers fastest of 3 types Limestone & others with calcite fastest carbonation Rate of mechanical weathering depends on what is holding sediments together Igneous and metamorphic Quartz is affected least (one of hardest minerals)

Exposure More exposure = faster weathering Most rock at surface are broken by fractures an joints -> split into smaller pieces Fractures & joints increase surface area to allow more rapid weathering

Climate Rainfall and freezing/thawing have biggest effect Variable weather causes ice wedging fractures Exposes new rock surface Chemical weathering attacks fractured rock more quickly Temperatures changing daily or seasonally speed up chemical reactions Hot, dry climates slow weathering because of lack of water for reactions Weathering is very slow in cold climates – cold slows rxns Warm, humid climates are most damaging Faster chemical rxn rate, moisture is damaging

Cleopatra’s Needle Cleopatra's Needle, an obelisk with inscribed Egyptian hieroglyphs, remained virtually unweathered for more than 3,000 years in Egypt (A), but then began to weather rapidly when brought to New York City's Central Park in the late 1800s (B).  The difference in climate (arid versus humid) accounts for the change in weathering rates. 

Topography & Elevation Ice wedging increases with elevation Rock fragments on steep slopes are pulled down by gravity and washed along by heavy rain.

Weathering and Soil Weathering breaks and alters all exposed rock Forms regolith (rock fragments) over solid bedrock Upper regions of regolith weather faster and form a layer of fine particles = basic components of soil Soil = mixture of minerals (from rock), water, gasses, and humus (organic matter from decaying organisms)

Soil Composition Rock material in soil consists of sand, silt, & clay (in that order from largest to smallest) Proportions of these particles depends on the “parent rock” = the rock it was weathered from Rich in clay = rich in feldspar Sandy soils = rich in quartz Silt – gives soil a gritty feel. Often carried by river currents and deposited on soils near river banks Minerals may be carried by wind or water away from the parent rock = transported soil may have a different composition than underlying bedrock.

Soil Profile Shows the layers of soil and the underlying bedrock 3 main layers = horizons A horizon = topsoil – contains humus, other organic matter, and living organisms B horizon = subsoil – minerals leached from topsoil, clay, little humus C horizon = bottom layer – partially weathered bedrock (will eventually weather into B then A horizon) Transported soils do not have horizons

Soil Profile A horizon B horizon C horizon

Climate Influences Soil Formation Humid tropical climate – chemical weathering causes thick laterites to develop Contain iron and aluminum – do not dissolve easily Rain leaches mineral form A & B horizons and washes away topsoil (\ A is very thin) Dense vegetation continuously adds organic material to topsoil to support plants Desert climate – chemical weathering is slow/ mechanical weathering forms a thin soil mostly regolith Arctic – chemical weathering is slow/ mechanical weathering forms a thin soil mostly regolith

Climate Influences Soil Formation Temperate climate – 2 main soil types: Pedalfer Forms with more than 65 cm of rain annually Contains clay, quartz & iron compounds Gulf Coast States & States East of Mississippi Pedocal Forms with less than 65 cm of rain annually Contains much calcium carbonate – combines with soil H to make soil less acidic and very fertile SW States and States West of Mississippi

Topography Affects Soil Formation Rainwater washes soil from steep slopes causing topsoil to be thicker at the bottom than on the slope Topsoil left on slopes is often too thin and dry to support plant growth \ does not accumulate organic matter = poor for cultivation Lowlands that hold water have thick, wet soils with much organic matter to form humus = good for cultivation

Erosion Soil erosion occurs about as slowly as residual soil forms Unwise use of land and unusual climate conditions accelerate erosion Unwise farming and ranching Clearing forests & overgrazing Expose soils for increased erosion Gullying = water runs through furrows in plowed land washing away soil until furrows become deep gullies

Gullying

Erosion Sheet erosion strips away layers of soil to expose subsoil Continuous rainfall evenly washes away topsoil which may end up clogging streams or changing their course In dry periods wind carries away loose soil as clouds of dust and drifting sand

Erosion Constant erosion reduces fertility by removing the A horizon and exposing the B horizon B horizon resists plant growth \ has no protection All soil layers could be removed within a few years

Rapid Erosion Can Be Prevented by Soil Conservation Cover crops can protect soil Some crop planting methods can reduce erosion Contour plowing Strip-cropping Terracing Crop rotation

Contour Plowing Plowing follows the shape of the land and prevents water from running directly down slope

Strip-Cropping Crops are planted in alternating bands to protect the soil by absorbing and holding water. Ex. Corn bands alternating with alfalfa (cover crop) When combined with contour plowing soil erosion can be reduced 75%

Terracing Cutting step-like ridges that follow the contour of the land Slows the downslope movement of water

Crop Rotation Farmers alternate crops every year to stop erosion early and repair the damage already caused by erosion

Urban Conservation In urban areas clearing vegetation and removing soil to build houses, roads, etc contributes to erosion thereby making conservation measures necessary.

Mass Movement Gravity causes rock fragments to move downhill = mass movement Rocks break into fragments which accumulate at the base of slopes in piles called talus

Rapid Mass Movements Rockfall – the fall of rock from a steep cliff is the most rapid mass movement Landslide – masses of loose rock and soil move downslope May be triggered by heavy rains, spring thaw, volcanic eruptions and earthquakes

Rapid Mass Movements Mudflows - masses of mud flow downslope and spread out in a fan shape at the bottom Occur in dry mountainous regions during heavy rains Slump – large block of soil and rock becomes unstable and moves downhill in one piece

Slow Mass Movements Solifluction – in regions where soil is permanently frozen, thawing of the topsoil makes it muddy and it slowly flows downslope. Creep – extremely slow downhill movement of weathered material Most effective mass movement Usually undetected

Landforms Shaped by weathering and erosion Result from two opposing forces which bend, break and lift the crust then wear it down Mountains - steep landforms of high elevation uplifted by tectonic forces Plains – flat landforms usually near sea level Plateaus – broad, flat landforms at high elevations

Mountains Youthful – still being uplifted Rugged shape, sharp peaks & deep, narrow valleys Mature mountains – no longer rising Rounded peaks and gentle slopes Peneplain – old stage, almost featureless Usually low rolling hills

Peneplain Knobs of hard rock that resist weathering and protrude above peneplain = monadnocks May be mistaken for depositional plain but underlying rock is folded and tilted

Plateaus Young plateaus have deep stream valleys separating broad, flat regions Mature plateaus are eroded into rugged hills & valleys As plateaus age they may form smaller, table-like areas = mesas Mesas weather into small, narrow topped formations - buttes

Landforms In dry regions landforms have steep walls and flat tops. In humid climates they are more rounded due to increased weathering Minor landforms = hills, valleys and dunes