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Chapter 14: Weathering and Erosion
Transcript of Chapter 14: Weathering and Erosion
What are some natural agents that may be able to break rock apart into smaller pieces?
14-1: Weathering Processes
Weathering: Physical breakdown or chemical decomposition of rock materials exposed at earth's surface
Process by which rock is broken down into smaller pieces by physical means
Types of Mechanical Weathering
Exfoliation: Rock Breaks into curved sheets that peel away when pressure of overlying rock is removed.
Ice Wedging: Water seeps into cracks and freezes
Water expands by 10% when frozen
Abrasion: The collision of rocks that results in breaking and wearing away.
The process by which rock is broken down because of chemical interactions with environment.
Types of Chemical Weathering
Oxidation: Elements combine with oxygen
Ex: Iron in rock will rust
Hydrolysis: Minerals react with water
Can dissolve and be carried to lower layers of rock (Leaching)
Carbonation: minerals react with carbonic acid (from CO2 + H2O in atmosphere)
From Egypt to Paris, NY, London
Carved in 1450 BC
moved in the late 1800's
Extreme chemical weathering
Agents: ice, plants, animals, gravity, running water, wind
Caused by gravity, running water, and wind
Organic activity: Plant roots grow and expand in cracks
Animals dig and burrow, scrape rocks
Changes both chemical composition and physical appearance of rock.
Organic Acids: produced by living organisms (lichens, mosses)
Natural rainwater is slightly acidic (pH 5.6), but mixes with nitrogen oxides and sulfur dioxides from burning fossil fuels and becomes more acidic (pH 5.0-4.5)
14.2: Rates of Weathering
Generally mech. & chem. weathering are very slow
ex: Carbonation of Limestone = 1/20 cm per 100 years
Rate depends on rock composition, climate, topography
Softer rock wears away and leaves harder, more resistant rock behind.
Ayer's Rock (Uluru) Australia: resistant core of ancient mountain.
Factors that affect Weathering
1. Rock Composition
Quartz most resistant
Limestone / Calcite weathered most rapidly
In general, sedimentary rock weathers faster than igneous or metamorphic
2. Amount of Exposure
Amount of time, amount of rock surface exposed determine rate of weathering.
Surface area increases as rock breaks into smaller pieces
Fractures / joints increase surface area
open new areas to flow of water (faster weathering)
Climates with alternating periods of hot and cold have fastest overall weathering rates
Chemical reactions occur faster at higher temps
Chemical weathering fastest in humid climates
Hot / Dry or cold / Dry climates have slowest weathering
4. Topography and Elevation
Topography: Elevation and slope of the land surface
Ice wedging more common at height elevations (colder)
Steep slopes make weathered fragments move faster and abrade more
5. Human Activities
Mining: Exposes new rock, acids and chemicals used
Construction: Removes Soil
6. Plants and Animals
Along with plant roots and digging animals, biological wastes can accelerate weathering
Ex: Bat caves... guano speeds up chemical weathering.
What factors affect the rate at which weathering occurs?
In what ways might the climate affect the soil?
Regolith: Layer of weathered rock fragments that cover most of the surface
Bedrock: Solid, unweathered rock beneath regolith
Soil: small rock particles mixed with minerals, water, gases, and organic remains
Characteristics of Soil
Depends on parent rock
If soil forms and stays directly over parent rock: Residual Soil
If soil forms in one place by then moved and deposited someplace else: Transported soil
Rich in feldspar / aluminum: Clay Soil
Rich in quartz: Sandy Soil
Rich organics: Black Color
Rich in iron: Red Color
3 main horizons (layers of soil):
A (Topsoil): organics and rock particle
B (subsoil): minerals leached from topsoil
C : partially weathered bedrock
Humus: Dark, organic material from decaying organisms
Soil and Climate
Climate most important factor in determining soil formation
Lots of leaching
Thin layer of humus
Poor for farming
Both mechanical and chemical weathering
All 3 horizons thick
Pedalfer soil: Forms with >65cm rain / year
less fertile, but okay; more acidic
Pedocal soil: Forms with <65cm rain / year
Very fertile, less acidic
Desert / Arctic Climate:
Thin soil, mostly regolith
Soil & Topography
Soil thicker at top & bottom of slope
Why do the Rocky Mountains have jagged, rugged peaks, while the Appalachian Mountains have more rounded, gentle slopes?
Process by which the products weathering are transported
Agents: Gravity, wind, glaciers, water
Erosion of fertile topsoil one of the world's greatest environmental concerns
Usually due to farming & ranching techniques
Clearing native plants
Furrowing (plowing long, narrow rows)
Sheet erosion: Process by which water flows over a layer of soil and removes topsoil
Can be caused by wind too (dust storms)
Proper planning by land developers and farmers
Contour plowing: Plant in curves to follow land
Strip-Cropping: crops planted in alternating bands
Terracing: Step-like ridges on slopes
Crop rotation: Plant different crops each year to give soil time to recover
Gravity + Erosion
Mass Movement: Movement of large mass of sediment or a section of land down a slope
Some fast, some slow
Talus: Material collected at the base of a slope
Landslides: Rock + Soil
Slump: Large block of Soil moves as unit
Creep: Extremely slow downhill movement of soil
Solifluction: slow, downhill flow of soil saturated with water (arctic areas)
Homework on Pg. 364: 1-8, 10, 11