chapter 5. breaks rocks down, but doesn’t change mineral composition increases surface area of...
TRANSCRIPT
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