Earth’s External Processes
- Weathering - the disintegration and decomposition of material at or near the surface
- Mass wasting - the transfer of rock material downslope under the influence of gravity
- Erosion - the incorporation and transportation of material by a mobile agent, usually water, wind, or ice
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Two Kinds of Weathering
- Mechanical Weathering - breaking of rocks into smaller pieces
- Chemical Weathering - chemical transformation of rock into one or more new compounds
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Four Important Processes of Mechanical Weathering
Chemical Weathering
Rates of Weathering
- Advanced mechanical weathering aids chemical weathering by increasing the surface area
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Important Factors on Rates of Weathering
- Rock Characteristics
- Mineral composition and solubility
- Physical features such as joints
- Climate
- Temperature and moisture are the most crucial factors
- Chemical weathering is most effective in areas of warm temperatures and abundant moisture
- Differential weathering
- Caused by variations in composition
- Creates unusual and spectacular rock formations and landforms
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Soil
An interface in the Earth system
Soil is a combination of mineral matter, water, and air
- Regolith
- Rock and mineral fragments
- Supports the growth of plants
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Soil Components
- Soil texture and structure
- Texture refers to the proportions of different particle sizes
- Sand (large size)
- Silt
- Clay (small size)
- Loam (a mixture of all three sizes) is best suited for plant life
- Structure
- Soil particles clump together to give a soil its structure
- Four basic soil structures
- Platy
- Prismatic
- Blocky
- Spheroidal
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Controls of Soil Formation
Parent material
- Residual soil – parent material is the bedrock
- Transported soil – parent material has been carried from elsewhere and deposited
Time
- Important in all geologic processes
- Amount of time to evolve varies for different soils
Climate
Plants and animals
- Organisms influence the soil’s physical and chemical properties
- Furnish organic matter to soil
Slope
- Angle
- Steep slopes often have poorly developed soils
- Optimum is a flat-to-undulating upland surface
- Orientation (direction the slope is facing) influences
- Soil temperature
- Moisture
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Soil Profile
- Soil forming processes operate from the surface downward
- Horizons – zones or layers of soil
- Horizons in temperate regions
- O – organic matter
- A – organic and mineral matter
- E – little organic matter
- Eluviation
- Leaching
- B – zone of accumulation
- C – partially altered parent material
- Topsoil
- Solum
- “True soil”
- O, A, E, B together
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Soil
Soil Erosion
- Recycling of Earth materials
- Natural rates of erosion depend on
- Soil characteristics
- Climate
- Slope
- Type of vegetation
- Processes that erode soil
- Water erosion
- Raindrops dislodge
- soil particles
- Flowing water carries soil away
- 2/3 of all U.S. soil erosion
- Wind Erosion
- Occurs with dry soil conditions
- Soil erosion and sedimentation can cause
- Reservoirs to fill with sediment
- Contamination by pesticides and fertilizers
- Controlling soil erosion
- Leave steep slopes undisturbed
- Terrace crop planting
- Grassed waterways
- Tree windbreak barriers
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Mass Wasting
- The downslope movement of rock, regolith, and soil under the direct influence of gravity
- Gravity is the controlling force
- Saturation of the material with water
- Destroys particle cohesion
- Water adds weight
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Mass Wasting Triggers
- Oversteepening of slopes
- Unconsolidated granular particles assume a stable slope called the angle of repose
- Stable slope angle is different for various materials
- Removal of anchoring vegetation
- Ground vibrations from earthquakes
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Classifying Mass Wasting
Each type is defined by
- The material involved – debris, mud, earth, or rock
- The movement of the material
- Fall (free-fall of pieces)
- Slide (material moves along a well-defined surface)
- Flow (material moves as a viscous fluid)
- Each type is defined by
- The rate of the movement
Slump
- Rapid movement along a curved surface
- Occur along oversteepened slopes
Rockslide
- Rapid
- Blocks of bedrock move down a slope
Debris flow (mudflow)
- Rapid flow of debris with water
- Often confined to channels
- Debris flows composed mostly of volcanic materials are called lahars
Earthflow
- Rapid
- Typically occur on hillsides in humid regions
- Water saturates the soil
- Liquefaction – a special type of earthflow sometimes associated with earthquakes
Creep
- Slow movement of soil and regolith downhill
- Causes fences and utility poles to tilt
Solifluction
- Slow movement in areas underlain by permafrost
- Upper (active) soil layer becomes saturated and slowly flows over a frozen surface below
