Transcript Ch 14-Weathering and Erosion

Ch 14-Weathering and Erosion
• Objectives
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Identify agents of mechanical weathering
Compare mechanical and weathering processes
Describe chemical reactions that decompose rock
Explain how rock composition affects the rate of weathering
Discuss how surface area affects the rate at which rock weathers
Describe the effects of climate and topography on the rate of weathering
Summarize how soil forms
Explain how the composition of parent rock affects soil composition
Describe the characteristics layers of mature residual soils
Predict type of soil that will form in artic and tropical climates
Define erosion, list agents of erosion
Identify farming methods that conserve soil
Discuss 2 ways gravity contributes to erosion
Describe the major land forms shaped by weathering and erosion
Weathering Processes
• What is weathering?
– The natural process in which wind, rain, and
temperature changes, disintegrates and decomposes
rock
– 2 types of weathering-mechanical and chemical
• Mechanical weathering-rocks break down into
smaller pieces by physical means
– Do not cause change to composition to rock
– Ice, plants, animals, gravity, running water, wind
– Physical changes may affect mech weathering
• Joints-long, curved cracks that develop in rock
• Exfoliation-joints are parallel to surface of rock, breaks into
curved sheets
Mechanical Weathering
• Ice Wedging
– Occurs in cold climate, high elevations
– Water seeps into cracks
• Abrasion
– Collision of rocks
– Gravity, running water, wind
• Organic Activity
– Weathering caused by animal and plants
– Can weather rocks dramatically over a long
period of time
Chemical Weathering
• What is chemical weathering?
– Process by which rocks break down as a result of
chemical reactions
– Takes place when chemical reactions act on
minerals in rock
– Occur between rock, water, CO₂, oxygen, acids
– May lead to formation of new minerals
• Changes both chemical composition and
physical appearance of rock
• Oxidation, hydrolysis, carbonation, organic
acids, acid precipitation
Types of Chemical Reactions
• Oxidation
– Reaction that removes 1 or more electrons from substance.
– Metallic element combines w/ oxygen
– Occurs in rock w/ iron bearing minerals
• Hydrolysis
– Reaction between water and another substance to form 2 or more new
substances
• Carbonation
– Conversion of a compound into a carbonate
• Organic Acids
– Produced naturally by certain living organisms
– Lichens and mosses produce weak acids
• Acid Precipitation
– Precipitation that contains high concentration of acids
– Rain, sleet, snow
– Weathers rock faster than ordinary
Sec 2-Rates of Weathering
• The rate at which rock weathers-rock
composition, climate, topography
• Differential Weathering-Softer, less weather
resistant rocks wear away at faster rate than
harder, more weather resistant rocks do
• Rock Composition
– Weathering of rocks depends on the material that
holds the rocks together
– Limestone and sedimentary rocks
– Shales and sandstones
Surface Area
• Important factors in determining rate of
weathering-rock’s surface area, amount of
time rock is exposed
• Surface Area
– Part of rock that is exposed to air, water and
other agents of erosion
– Factures and joints increase surface area
Effects of Climate
• Alternating periods of hot and cold
weather allow fastest rates of weathering
• Freezing and thawing=mechanical
breakdown
• Rising temps=chemical weathering
• Humid climates=fairly rapid chemical
• Hot, dry climates=slow weathering
Topography and Elevation
• What is topography?
– Elevation and slope of land surface
• Ice wedging more common at high
elevation
• Gravity pulls rock fragments downhill
– Washed out by heavy rains
• Human Activities
– Mining and construction expose rock surfaces
to agents of weathering
Sec 4-Erosion
• Define erosion
– Materials of Earth’s surface are loosened, dissolved
or worn away and transported from one place to
another by natural agent
• Soil erosion
– Greatest environmental problem
– Farming and ranch practices increase erosion
• Gullying and sheet erosion
– Farming technique increases erosion
– Strips away layers of topsoil, exposes sub soil
• What is the result of erosion?
– Reduces fertility of soil, removes A horizon-contains
fertile humus
Farming Methods
• Contour plowing
– Soil is plowed in curved bands that follow contour, or shape of
land
– Prevents water from flowing directly down slopes
• Strip cropping
– Crops planted in alternating bands
– Corn and alfalfa planted next to each other
– Combined w/ contour plowing, reduces erosion by 75%
• Terracing
– Construction of step like ridges, follow contours of sloped field
– Prevent down slope of movement of water
• Crop rotation
– Plant 1 crop one yr and different one next yr
– How does this stop erosion?
Gravity and Erosion
• Mass movement-movement of large mass of sediment or
section of land down a slope
• Rockfalls and Landslides
– Rocks fall from steep cliff
– Masses of loose rock combined w/ soil suddenly fall down a
slope
– Triggered by heavy rain, spring thaws, volcanic eruptions,
earthquakes
• Mudflows and Slumps
– Rapid movement of large amount of mud
• Dry, mountainous regions during sudden, heavy rainfall
– Large block of soil and rock becomes unstable and moves
downhill in one piece
• Soilfluction
– Slow, downslope of soil saturated w/ water in
areas surrounding glaciers at high elevations
• Creep
– Slow downhill movements of weathered rock
material
– Moves most soil of all mass movements
Erosion and Landforms
• What is a landform?
– Physical feature of Earth’s surface
• Major landforms shaped by erosion
– Mts, plains, plateaus
– Hills, valleys, dunes
• Processes landforms go through
– Bends, breaks, lifts Earth’s crust, creates
elevated or uplifted landforms
– Weathering and erosion, wears down land
• Erosion of Mts.
– Tectonic forces uplift the mountain, faster than
erosion
– Tend to be rugged, sharp peaks, deep narrow valleys
– Forces stop=erosion and weathering
• Erosion of Plains and Plateaus
– Plain-relatively flat landform near sea level
– Plateau-broad, flat landform w/ high elevation
– Erosion depends on climate and composition and
structure of rock
Glacial Erosion
• What kind of landforms do glaciers create?
– Large lakes, solitary boulders on flat plains, jagged
ridges
• Landforms created by glaciers
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Mt ranges
Cirque-bowl shaped depression
Arete-sharp jagged edge that forms between cirques
Horn-sharp, pyramid peak, forms when several aretes
join
– U-shaped valleys
Effects of Wind Erosion
• Deserts and coastlines-most affected by wind
– Few plants, low moisture. How would this affect erosion?
• Desert pavement
– Deflation-wind removes top layer of fine, very dry soil or rock
particles and leaves behind large rock particles
• Deflation Hollows
– Wind strips off topsoil, shallow depression forms
– Expand width of several km and to depth of 5-20 m
Wave Erosion
• Sea Cliffs
– Waves slowly erode base of rock
– Waves cut under underlying rock, rock collapses to form seep
sea cliff
– Headlands-high and steep formation of rock that extends out
from shore into water
– Areas w/ less resistant rock form bays
• Sea Caves, Arches, Stacks
– Waves cut deep fractured and weak rock along base of cliff to
form large hole
– Sea arch forms when waves cut completely through headland
– Sea stacks are offshore columns of rock that once were
connected to sea cliff or headland
• How are beaches formed?
– Created by water eroding the land and depositing
sediment