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

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

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

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

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

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