Chapter 5

Weathering, Soil, & Mass Movements

Weathering

Mechanical vs. ChemicalBreaks rocks down,

but doesn’t change mineral composition

Increases surface area of rocks

Caused by 3 processes:Frost wedgingUnloadingBiological Activity

Transforms rock into 1 or more new compounds

Frost wedging – mostly in the mountainsWater seeps into rock cracks,

freezes, splits rocksPiles of broken rocks are called

talus Unloading – uplift and erosion

expose buried mass of igneous rockOften produces large dome

shaped formationsBiological Activity – plants or

roots grow into rock cracks, which causes rocks to crack apartAnimals can also cause via

burrowing or humans

Mechanical

Water is the most important agent of chemical weatheringWater causes:

RustCarbonic acid

Clay is product of most chemical weatheringSpheroidal weathering is when rock changes

into a more round shape b/c of chemical weathering

Chemical

Rock characteristicsCracks influence

ability of water to get into rock

Mineral composition – marble faster than granite

ClimateTemperature &

MoistureControls freeze-thaw

cycles (frost wedging)

Vegetation present – more = more weathering

High temps & high moisture most favorable for chemical weathering

What Affects the Rate of Weathering

Differential Weathering – different parts of a rock weathers at different rates

Bryce Canyon

Soil

Weathering produces layer of rock and mineral fragments called regolith

Soil is the part of regolith that supports growth of plants

Soil

1. Composition (4 components, vary for different soils)

1. Mineral matter – broken down rocks

2. Organic matter – humus (decayed remains of organisms)

3. Water 4. Air***good quality soil is

made up ½ of mineral matter and organic matter and ½ of air and water to circulate***

Characteristics of Soil

2. Texture – proportions of different particle sizes

USDA categorizes soil based on percent clay, silt, or sand in the soil

Influences the ability to support plant life

3. Soil structure – determines how easily soil can be cultivated & how susceptible to erosion

1. Affects how easily water penetrates the soil

**5 factors interact during formation**1. Parent Material – source of mineral matter

in soil Affect weathering and rate of soil formation

2. Time – longer a soil forms, the thicker it gets

Soil Formation

3. Climate – variations in temp. & precipitation influences rate, type, and depth of weathering

Big effect on type of organisms living in soil

4. Organisms Plants are main source of

organic matter in soil Organic matter releases

nutrients as it decays Microorganisms (fungi,

bacteria, protozoans) are decomposers and convert nitrogen gas into compounds

Burrowing animals mix minerals and organic matter and helps water and air to penetrate soil

5. Slope Steep slopes –

erosion is accelerated thus soils are thin or nonexistent

Flat areas have little erosion

Dark color indicates lots of organic matter

Soils on south facing slopes are warmer and drier and influences the type of plants

**varies by depth – divided into soil horizons

1. A Horizon – “topsoil,” mostly composed of leaves and plant structures (organic material) also has microorganisms and insects

Lower part is mixture of minerals and organic matter

2. B Horizon – “subsoil,” fine clay particles washed trough A horizon

Can form hardpan (compact, impermeable clay layer)

Lower limit of plant roots and animals

3. C Horizon – partly weathered parent material

b/w B horizon and unweathered parent material

Soil Profiles

***Depends on climate***Pedalfer – forms in temperate Eastern US

(forests)Pedocal – found in dry Western USLaterite - forms in hot, wet tropical areas

Becomes very hard when driedUsed to build structures w/ dried bricksUseless for crop growing

Soil Types

Sheet erosion, rills, trenches (gullies)Rate of soil erosion depends on soil

characteristics, climate, slope, and type of vegetationAccelerated b/c of human activities

Soil Erosion

What happens if soil erodes faster than it is formed?Lower productivityPoor crop qualityDecreased food supply

Preserving the soil is the key to feeding the rapidly growing world

Mass Movements

Mass Movements – rock and soil move down slope b/c of gravity, landforms are product

Water – heavy rains and rapid snow melt saturates surface materialsClay and sand become very slick when pores are full of

water

4 Factors that Trigger Mass Movements

Over steepened slopes – if steepness exceed stable angle (25-40), mass movements are likelyCreek can undercut a bank or humans excavate

during construction of roads or building

Removal of vegetation – roots bind soil and regolith togetherThis makes more stable slopesWhen removed, mass movements increase

Earthquakes - most dramaticCan dislodge rocks and unconsolidated material

Classified by material moved, how it moves, & speed of movement

Rockfalls – rocks or pieces of rock fall through the airCommon on over

steepened slopesResult of

mechanical weathering: freeze-thaw or root-pry

Can trigger other mass movements

Types of Mass Movements

Slides – block of material moves suddenly along a flat, inclined surfaceRockslide – bedrock included (fastest mass

movement)Slumps – downward movement of block

material along a curved surfaceNot fast or farLeaves a crescent shaped cliffCommon on over steepened slopes

Flows – material containing lots of water, thick fluidMudflows – quick moving

Arid regionsTakes boulders and trees with it

Earth flows – slower movingWet regionsContinuous over years

Creep – slowest mass movementContributing factor is freeze-thaw

Freeze expands soil particles, liftsThaw causes contraction, lower levelEach cycle moves particles slowly downhill

Cause of tilt of gravestones