Weathering,Soil,

& Erosion

Chapter 12

300

Weathering sec. 1

• What is weathering?– the breakdown of rocks (into sediments)

due to exposure to processes that occur at Earth’s surface

• The 2 major types of weathering?–mechanical (physical)

• a.k.a. disintegration

• chemical

• a.k.a. decomposition

Mechanical (Physical) Weathering• What happens during mechanical

(physical) weathering?– rock is split, cracked or broken into

smaller pieces (disintegration)• mineral composition is NOT changed– just physical structure

Types of Mechanical (Phys.) Weathering• What is frost wedging (a.k.a. frost action)?– water gets into cracks in rock water freezes &

expands widens cracks ice melts more water gets in enlarged cracks water freezes & expands… & so on…. until rock breaks

• Where is frost wedging most common?– where temp. varies between below and above freezing

• What is abrasion?–when rocks rub or

bounce against each other as moved by wind, water, ice, or gravity

• & get broken into smaller pieces (sediments)

Types of Mechanical (Phys.) Weathering

–What do rocks abraded by wind (& the sediments they form) look like?• flat/angular surfaces, cuts, grooves

(known as ventifacts)

Types of Mechanical (Phys.) Weathering

–What do rocks abraded by water (& the sediments they form) look like?• smooth, rounded particles

Types of Mechanical (Phys.) Weathering

–What do rocks abraded by ice (glaciers) look like?• striations (grooves) showing direction

of ice flow

Types of Mechanical (Phys.) Weathering

Types of Mechanical (Phys.) Weathering• How do plants & animals cause weathering?– allow water and air to reach bedrock or may

bring rocks to surface by:• plant roots growing into cracks & expanding cracks• animals digging holes & burrows in the soil

Types of Mechanical (Phys.) Weathering

• What is exfoliation (jointing)?

– overlying layers of rock are removed pressure is released rocks below expand & break along curved joints (parallel to surface) rock peels off like layers of an onion

Chemical Weathering

• What happens during chemical weathering?–rock is broken down by chemical

reactions & its minerals are changed into new substances (decomposition)

Chemical Weathering

• Where is chemical weathering most common?–occurs almost everywhere because

of presence of water or water vapor• other agents include acids and oxygen

Types of Chemical Weathering• What is hydrolysis?–chemical weathering by reaction of water

with other substances• minerals dissolve into ions & react with water

to form clay minerals

• Water’s chemical effects are increased by presence of acids dissolved in water• CO2 dissolved in water produces carbonic acid– reacts with & dissolves some rocks & minerals

Types of Chemical Weathering

Types of Chemical Weathering–Which rocks

would be most susceptible to weathering by acidic water? Why?• limestone &

marble– b/c calcite-based

– Other rocks are very resistant to acid & do not dissolve.• quartzite

• How can air pollution increase chemical weathering?– gases from volcanoes, cars (nitrogen oxides), or

power plants (sulfur dioxide) can make rainwater even more acidic (ACID RAIN)

Types of Chemical Weathering

• How can plants & animals can contribute to chemical weathering?– Acids formed by the decay of dead organisms

can be incorporated into rainwater percolating into the ground.

– Lichens (fungi + algae) make acids which can dissolve rock.

Types of Chemical Weathering

• What is oxidation?–when oxygen reacts with minerals in rock• ex. iron + oxygen = iron oxide (rust)– red hematite– yellowish-brown limonite

Types of Chemical Weathering

Rates of Weathering• Normal conditions weathering is slow• What factors affect weathering rate?– surface area–mineral composition– climate

• How does surface area affect weathering rate?–smaller pieces have more surface area

(than a large piece with the same total volume) and causes faster weathering

Rates of Weathering

• How does the mineral composition of a rock affect the weathering rate?– quartz-based rocks weather slower• more resistant

– feldspar, hornblende, biotite mica, augite, calcite, gypsum-based rocks weather faster• less resistant

Rates of Weathering

• How does climate affect weathering rate?–How does an arid (dry) climate

affect weathering rate (& type)?• slow weathering

– mostly mechanical (by wind)

Rates of Weathering

–How does a cold & wet climate affect weathering rate (& type)?• faster weathering–mostly physical (frost action)

Rates of Weathering

–How does a warm & wet climate affect weathering rate (& type)?• faster weathering–mostly chemical (higher temperature

speeds up reactions)

Rates of Weathering

Soil sec. 2

• What is soil?–mixture of loose weathered rock &

organic material that covers bedrock & in which plants with roots can grow• typically 3 major layers (horizons)

• What is the material that soil forms from called?– parent material

• What is a residual soil?– soil whose parent material is the (local) bedrock

beneath the soil

• What is a transported soil?

– soil formed from parent material in different areas & brought into a new area by wind or water

• Most soils in CT are transported soils.

How Soil Forms

Stages of Soil Formation

• “Mature” soils:– have had a long time to form.– have 3 distinct zones.• can be seen in a soil profile

(cross section exposed by digging down to parent material)

Soil Profiles

• What is the O horizon (litter layer)?–partly decomposed

organic matter

• What is the A horizon (topsoil)?– rich in organic remains

(humus)

–gray-black color

Soil Profiles• What is the B horizon

(subsoil)?– rich in minerals (washed down

from A horizon)• iron, clay, calcium, magnesium

– red-brown color

• What is the C horizon?– slightly weathered parent

material– color depends on parent

material

Soil Composition• How is soil classified?–by composition (make-up)• strong link between

composition & climate

Mass Movements & Erosion Sec. 3

• What is erosion?– the removal & transport of materials by wind,

moving water, or ice• can only take place after weathering has occurred

Agents of Erosion• What are “agents” (causes) of erosion?– rivers, streams, glaciers, wind, and ocean

waves and currents

Erosion & Landforms• How does erosion help in shaping the landscape?– Topography (“shape of land”) depends on the “balance”

between uplifting forces & erosion.• Ex. Pressure builds mountains; erosion wears them down.

Mass Movements• What is a “mass

movement”?–downward

transportation of weathered material by gravity• What is talus?– pile of

weathered fragments at the bottom of a cliff

Types of Mass Movements• What are slow-

moving types of mass movements?– creep– slump

• What are fast-moving types of mass movements?– flows

• mudflows

• earthflows

– landslide/rock fall

• What is creep?–very low velocity (slow), gradual, steady

movement of surface soil• imperceptible, but effects can be seen– fence posts, poles, & trees to lean downhill

Types of Mass Movements

• What is slump?– blocks of land tilt & move

downhill along a surface that curves into slope• b/c bottom of the slope

has become too steep to support soil at top

Types of Mass Movements

• What is a mudflow?–rapid movement of

water containing large amounts of suspended clay & silt

–What is a lahar?•mudflow

accompanying volcanic eruption

Types of Mass Movements

• What is an earthflow?–soil saturated w/ water

flows downhill• slower & less fluid than

mudflow–affected by amount of

water present, composition of soil, & steepness of slope

Types of Mass Movements

• What is a landslide/rock fall?–general name for movement of a

mass of bedrock or loose soil & rock down slope of a hill, mountain, or cliff• high velocity event•material is generally loose & dry •most likely on steep slopes

Types of Mass Movements

Soil as a Resource sec. 4

• What is soil fertility?–ability of soil to grow plants

• What affects soil fertility?–proportion of minerals, water & organic

matter determines fertility

Threats to Soil Fertility• What is soil depletion/degradation?– when nutrients are lacking or taken out of soil to

point where it can no longer grow a usable crop

• How can depletion/degradation be reduced?– by allowing field to lay fallow or rotating crops• However, fertilizers are often added instead.

Threats to Soil Fertility• What is salinization?– water from irrigation leaves behind minerals as

it evaporates & soil cannot support crops

• Some ways to reduce/prevent salinization?– Use drip irrigation or soaker hoses (less water).– Avoid crops that require a lot of water.– Plant crops that remove salt from soil.– Improve drainage.

Erosion & Human Activity•How is erosion accelerated by human activity?–farming–construction–mining

Soil Conservation Methods

• What are windbreaks/ shelterbelts?– trees planted to reduce wind erosion• act as barrier to wind

• What is terracing?–“steps” cut into

land to slow water flow down slope

• What is contour farming?–plow parallel to

land shape, slows water flow

• What is strip cropping?–alternating rows of little ground

cover & complete coverage• planted perpendicular to dominant

wind direction

• What is no-till farming?–plowing, planting, fertilizing, &

weed control done at same time• land not disturbed again until harvest

Soil Conservation Methods