Weathering, Soil, and Mass Wasting

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

\ Four Important Processes of Mechanical Weathering

• Frost wedging - expansion of freezing water
• Salt Crystal Growth
• Sheeting - generates onion-like layers and exfoliation domes
• Biological activity   * Plants can break rocks

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Chemical Weathering

• Alters the internal structures of minerals by removing or adding elements
• Most important agent is water   * Oxygen dissolved in water oxidizes materials   * Carbon dioxide (CO2) dissolved in water forms carbonic acid and alters the material
• Weathering of granite   * Weathering of potassium feldspar produces clay minerals, soluble salt (potassium bicarbonate), and silica in solution   * Quartz remains substantially unaltered   * Weathering of silicate minerals produces insoluble iron oxides and clay minerals
• Spheroidal Weathering   * Generates rounded rocks   * Weathering works inward from exposed surfaces

Rates of Weathering

• Advanced mechanical weathering aids chemical weathering by increasing the surface area

\ 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

\ 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

\ 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   * Zones O and A together
• Solum   * “True soil”   * O, A, E, B together

\ \ Soil

• Classifying soils - Soil Taxonomy   * Emphasizes physical and chemical properties of the soil profile   * Names of the soil units are combinations of syllables of Latin and Greek origin

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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

\ 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

\ 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

\ 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       * Fast       * Slow
• 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