Weathering, Erosion, & Deposition part 1
Part I. WeatheringA. Weathering is the physical or chemical break-
down of rocks or minerals at or near the earth’s surface - Breaking rock into pieces
B. ONLY rocks at the surface can weather
C. TypesA. Physical – breaking rocks w/o changing
composition
B. Chemical – breaking rocks by chemically changing the minerals
Chemical Weathering Agents
A. Water – dissolves certain rocks (salt)
B. Aira) Oxidation (rusting) – O2 combines w/
water and reacts with minerals containing iron
b) CO2 – combines w/ water to make carbonic acid, dissolves limestone
c) Plants – lichens make acid that dissolves rock
Acid rain at work!
Review of chemical weathering• Oxidation (rust)• Acid rain• Carbonic acid
Water + Carbon dioxide Carbonic acid• Water dissolving rocks with acids mixed in it• Hydrolysis-water reacting with rock causing chemical
change
feldspar + water = clay
Physical WeatheringA. Frost action
1. Water enters cracks in rocks, as it freezes it expands breaking the rock
Physical WeatheringB. Abrasion
1. Grinding, rolling & scraping of rocks together. Breaks off sharp edges, rounding the rock
2. Caused by running water, waves, wind and glaciers
Physical WeatheringC. Plant action
1. Plant roots work into rocks breaking the rock apart as it grows
Physical WeatheringD. Exfoliation
1. Peeling off the outer layers of rock.
2. Caused by repeated heating or cooling
Review of physical weathering
• Frost action-alternating temperatures above and below freezing causes rock to crumble
• Abrasion-the physical grinding of rocks along a surface
• Root action -roots break a part land as they grow into the ground
• Burrowing animals• Exfoliation-peeling of rock layers due to crustal
unloading
Frost action at work!
Factors Affecting WeatheringA. Climate
1. Hot, dry climate – very little weathering
2. Warm, moist climate – rapid chemical
3. Cold, moist climate – strong frost action
Factors Affecting Weathering
B. Type of Rock1. Igneous and Metamorphic – react
SLOWLY, more dense
2. Sedimentary – weather rapidly
Factors Affecting WeatheringC. Particle Size
1. As rocks break into smaller pieces, it weathers faster (more surface area exposed)
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Particle Size
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Review Factors affecting rate of weathering
• Exposure to the elementsThe closer a rock is to the earth’s surface, the more weathering will occur
• Particle sizeWhen the rock particles are smaller, the rate of weathering increases
• Mineral compositionSome minerals are more resistant to weathering than others
• ClimateWarm, moist climates chemicalCold, dry climates physical
Soil
A. Final product of weathering
B. Soil composition1. Inorganic material – pieces of weathered
rock
2. Humus – organic material from decayed plants and animals; provides nutrients for growth
Classes of Soil
A. Soil Types1. Residual Soil
a. Soils that stay where they are formed
b. Soil composition matches the rocks underneath, granite rock = granite in soil
2. Transported Soila. Soils that were carried to other locations
b. Soil does NOT match, granite rock = NO granite in soil
Part 2 - Erosion
A. Carries away pieces from weathering
B. Most soils are different from bedrock under them (transported soils)
C. Gravity is MAIN force behind erosion
What does erosion mean?
• Transport of weathered material.
• Agents of erosion are: wind, water, ice (glaciers) and gravity.
• Water is the most common erosional agent
What is mass movement?
• Mass movement: When gravity pulls geologic materials down slope
• Driving force gravity
• Resisting force friction
Conclusion
• Mass movement is the downward movement of rock, snow, soil and ice as a result of gravity.
• When the driving force>resisting force slope failure occurs
• Order of increasing speed:
soil creepdebris flowmud flowrock fall
Agents of Erosion
A. Gravity (Review)1. Landslides – rapid movement of rock
material down hill
2. Slumping – small landslides
3. Creep – VERY slow mvmt of material
4. Talus – piles of rock found at base of very steep slopes
Agents of ErosionB. Wind
1. Carries fine grained sediments about 1 meter above ground
2. Grains are lifted and bounced along
3. Wears away rocks at the base
4. Dunes – piles of wind blown sand
Agents of ErosionC. Glaciers
1. Masses of frozen water on ground
2. Valley (alpine) glaciersa. “rivers of ice”
b. Found at high altitudes in mountains
Agents of Erosion
3. Continental glaciersa. Large sheets of ice covering large areas of
surface (Greenland, Antarctica)
Agents of Erosion
4. Formed in areas where snow does not melt, keeps building up and is compacted into ice
5. As Ice builds up, pressure increases at bottom, causing ice to flow (cm to m per day)
Agents of Erosion
6. As glaciers move, they carry materials of ALL sizes (sand to boulders)
7. Materials carried cause bedrock under glacier to be gouged and scraped
Agents of Erosion
8. Glacier Landscapesa. U-shaped valleys
b. UNSORTED rock material when glacier melts (till)
c. Drumlins – groups of long oval hills
d. Eskers – winding ridges formed in tunnels under ice
e. Kettles – large piece of ice breaks off glacier making a hole, ice melts leaving the hole. Kettle lake – hole is filled with water (Beaver Lake)
Agents of Erosion
D. Running Water1. MOST important (moves most material)
2. Ways of carrying sedimentsa. Solution – dissolved materials (salt)
b. Suspension – very fine particles (silt) suspended in water, looks muddy
c. Bouncing – pebbles bounced along stream
d. Rolling and sliding – largest particles moved without being lifted
Particle Transport
Agents of Erosion
3. Velocitya. Speed of the water flow
b. Depends on slope and volume (stream discharge)
c. Greater slope = greater velocity
d. Greater volume = greater velocity
Agents of Erosion
4. Sediments Carrieda. Size
1) FAST streams can carry LARGER sediments
b. Quantity (amount)1) MORE volume carries MORE sediment
2) Even though a fast moving stream can carry larger rocks, a slow moving stream with greater volume can carry more sediment (Mississippi River)
Agents of Erosion
5. Effects of moving streamsa. Carry sediments
b. Deepen and widen stream bed as it drags and carries its sediments
c. Abrasion happens (rocks becoming rounded)
Agents of Erosion6. Stages of Stream Development
a. Youth1) Steep gradients
2) Rapid downcutting (carries rocks and pebbles)
3) V shaped valley
4) Has waterfalls or rapids
5) Straight path
Agents of Erosionb. Mature
1) Less steep, less velocity
2) Wind around obstructions forming loops
3) Carries silt and clay (no rocks)
4) Larger volume = more sediment than youth
Agents of Erosionc. Old
1) Very small gradient
2) Only carries finest sediments
3) Can flood
4) Makes Oxbow Lakes
Agents of Erosiond. Old streams can
become young again if the area is uplifted and gradient becomes steeper
e. Most streams have youth at source, mature in middle and old age at their mouth (where river dumps into a lake/ocean)
Factors Affecting Deposition
A. Size1. Large particles settle the fastest
Settling Time
Particle Size
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Settling Rate
Factors Affecting Deposition
B. Shape1. Round particles settle faster than flat
particles
C. Density1. More dense particles settle faster
Sorting of Sediments
B. Horizontal Sorting1. Large particles settle first as a stream
enters a body of water
Glacial Deposits
A. Till1. Dropped from glacier.
2. UNSORTED!!!!! (all sizes mixed)
B. Outwash material1. Deposited from meltwater
2. SORTED
Energy Changes
A. Kinetic Energy – energy of movement
B. Potential Energy – stored energy
C. High velocity = High KE = erosion
D. Low velocity = Low KE = deposition1. Streams slow when:
a. Slope or volume decreases
b. Moves into a body of water
c. On the inside of a curve
Energy Changes
E. Streams have GREATEST PE at their source (highest point)
1. Loses PE as it turns to KE on the way down
Greatest PE, Less KE
Greater KE, Less PE
KE
PE
Erosion – Deposition System
A. Side View (profile)
Erosion Dominates – water moving fast
Delta can form
Source
Mouth
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B
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Ocean
Deposition – river slows down
Erosion – Deposition System
B. Top ViewInside of curve, water moves SLOWER, deposition, shallow
Outside of curve, water moves FASTER, erosion, deeper water
OCEAN
A B C
Delta
Velocity within a Stream
A. Water moves fastest in the middle towards the top of the stream (less friction)
Stream Cross Section
A
B
CD
D – Water moves fastest, smallest friction
Velocity within a Stream
B. Cross Section of a Stream Corner
Outside = Faster = More Erosion = Deep
Inside = Slower = More Deposition = Shallow
Effects of Climate on Landscape
A. Humid (wet)1. Worn down and rounded (more
weathering)
B. Arid (dry)1. More angled, sharper edges