3842:278 Geomorphology

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38/42278Geomorphology

• Topic 2 Basic Concepts
• Chapter 1 Ritter et al.
• Chapter 2 Easterbrook
• Chapter 1 2 Trenhaile

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1) Equilibrium

• Balance or equilibrium exists between landform
  and process

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1) Equilibrium

• Gilbert (1877) - landforms reflect balance b/w
  climate and geology
• No change in landform if no change in process

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1) Equilibrium

• Davis (1909) cycle of erosion
• Landscapes evolve through stages
• Youth
• Maturity
• Old Age
• Replaces Gilberts theory

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Youth

• Medium relief
• Many small tributaries
• Turbulent flow
• Original surface preserved
• Deep narrow valleys
• Downwasting

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Maturity

• Maximum relief
• More tributaries
• Only crests represent original surface
• Wider valleys
• Meandering main-stem
• Deposition begins
• Rounded interfluves

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Old Age

• Low relief
• Gently inclined
• Meandering rivers
• More deposition
• Less erosion
• Wide flat valleys
• Approaching peneplain

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1) Equilibrium

• Equilibrium reintroduced by Hack
• Called dynamic equilibrium
• Landforms rapidly adjust to change
• Try to re-establish equilibrium
• Cause and effect relationship
• Systems approach

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1) Equilibrium

• However
• Systems often in state of disequilibrium
• Landforms represent relic conditions
• Processes responsible no longer exist
• Perception of equilibrium dependent on time

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1) Equilibrium

• Static equilibrium
• Occurs over steady time intervals (days, months)
• No perceivable change in process or landform

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Static Equilibrium
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1) Equilibrium

• Steady-state equilibrium
• Occurs over graded time (ka)
• Changes occur, but long-term average conditions
  are maintained via negative feedback

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Steady-state Equilibrium
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1) Equilibrium

• Dynamic equilibrium
• Occurs over cyclic time (Ma)
• Average conditions progressively change

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1) Equilibrium

• Dynamic metastable equilibrium
• Occurs over cyclic time (Ma)
• Sudden changes result of thresholds
• Irreversible

Metastable
Threshold
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2) Opposing Forces

• Balance b/w process and form is result of
  interaction of opposing forces
• Driving Forces vs. Resisting Forces

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Driving i) Climate

• Solar radiation drives exogenic processes
• Climate determines
• Temperature
• Precipitation
• Atmospheric Circulation
• Ocean Currents

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Driving ii) Gravity

• Impacts all exogenic and endogenic processes
• Imparts potential energy
• Determines/regulates
• Fluvial Processes
• Mass Wasting
• Glacial Movement
• Tidal Motions
• Even distribution of rocks

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Driving iii) Internal Heat

• Result of
• radioactive decay
• residual heat energy
• friction
• Fuels endogenic processes
• Responsible for
• Movement of lithospheric plates
• Earthquakes
• Volcanism

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Resisting i) Lithology

• Internal strength of rock
• Susceptibility to weathering
• Varies according to
• genetic origin
• mineralogy
• Considerable geographic variation

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Resisting ii) Structure

• Includes
• Bedding
• Jointing
• Faulting
• Determines
• Resistance of rock to failure
• Susceptibility to weathering processes

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Resisting iii) Friction Cohesion

• Significant in terms of slope processes
• Resist downslope movement
• Note gravity is a component of friction, so is a
  driving and resisting force

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3) Thresholds Complex Responses

• Threshold defined as limit of equilibrium
• Change in driving forces
• OR
• Change in resisting forces
• Response is often complex
• involves a series of reactions

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Extrinsic Thresholds

• Change in driving forces
• External limiting condition
• Climate change
• Human impacts

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Intrinsic Thresholds

• Change in resisting forces
• External variables remain constant
• Result of gradual deterioration of system
• Weathering of rock
• Friction/cohesion

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4) Linkages

• Complex responses involve chain reactions called
  process linkages
• Changes in one part of the system produce changes
  in another
• Domino effect
• Often unanticipated results

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5) Time

• Important consideration in the study of
  geomorphic systems
• Equilibrium state varies over intervals of time
• Steady time no change
• study process
• Graded time continual readjustment
• process response
• Cyclic time progressive, sometimes sudden
  change
• origin of landforms/landscapes

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Frequency and Magnitude of Events

• Frequency of high magnitude events is low, but
  impacts are significant

Frequency
Magnitude