Chapter 5

Breaks rocks down, but doesn’t change mineral composition Increases surface area of rocks Caused by 3 processes: Frost wedging Unloading Biological Activity Transforms rock into 1 or more new compounds

Frost wedging – mostly in the mountains Water seeps into rock cracks, freezes, splits rocks Piles of broken rocks are called talus Unloading – uplift and erosion expose buried mass of igneous rock Often produces large dome shaped formations Biological Activity – plants or roots grow into rock cracks, which causes rocks to crack apart Animals can also cause via burrowing or humans

Water is the most important agent of chemical weathering Water causes: Rust Carbonic acid Clay is product of most chemical weathering Spheroidal weathering is when rock changes into a more round shape b/c of chemical weathering

Rock characteristics Cracks influence ability of water to get into rock Mineral composition – marble faster than granite Climate Temperature & Moisture Controls freeze-thaw cycles (frost wedging) Vegetation present – more = more weathering High temps & high moisture most favorable for chemical weathering

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

Weathering produces layer of rock and mineral fragments called regolith Soil is the part of regolith that supports growth of plants

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***

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

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

***Depends on climate*** Pedalfer – forms in temperate Eastern US (forests) Pedocal – found in dry Western US Laterite - forms in hot, wet tropical areas Becomes very hard when dried Used to build structures w/ dried bricks Useless for crop growing

Sheet erosion, rills, trenches (gullies) Rate of soil erosion depends on soil characteristics, climate, slope, and type of vegetation Accelerated b/c of human activities

What happens if soil erodes faster than it is formed? Lower productivity Poor crop quality Decreased food supply Preserving the soil is the key to feeding the rapidly growing world

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

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

Over steepened slopes – if steepness exceed stable angle (25-40), mass movements are likely Creek can undercut a bank or humans excavate during construction of roads or building

Removal of vegetation – roots bind soil and regolith together This makes more stable slopes When removed, mass movements increase

Earthquakes - most dramatic Can dislodge rocks and unconsolidated material

Classified by material moved, how it moves, & speed of movement Rockfalls – rocks or pieces of rock fall through the air Common on over steepened slopes Result of mechanical weathering: freeze- thaw or root-pry Can trigger other mass movements

Slides – block of material moves suddenly along a flat, inclined surface Rockslide – bedrock included (fastest mass movement) Slumps – downward movement of block material along a curved surface Not fast or far Leaves a crescent shaped cliff Common on over steepened slopes

Flows – material containing lots of water, thick fluid Mudflows – quick moving Arid regions Takes boulders and trees with it Earth flows – slower moving Wet regions Continuous over years

Creep – slowest mass movement Contributing factor is freeze-thaw Freeze expands soil particles, lifts Thaw causes contraction, lower level Each cycle moves particles slowly downhill Cause of tilt of gravestones