2 Concepts, Thresholds, Equilibrium (1).pptx

Geomorphology

1. Concepts in Geomorphology

2. Geomorphology Study of landforms

3. Includes study of current processes at work to create landforms and the evolution of landscapes

4. Geomorphology Combo of: physical geography = landforms surficial geology = processes

5. Landscapes or Land forms ~ ~ (geology, surficial processes, time) Bedrock structure Material properties Tectonics Geologic history Climate Biology Weathering Transport Hydrology erosion Controls others

6. Themes in Geomorphology Landforms Processes Equilibrium between process and form, cause and effect Time dependent Study deposits to understand processes controlling landform Equilibrium between landforms and processes that create the landform as long as the processes and their climatic and tectonic controls do not change

7. Types of Equilibrium : Static Steady State Dynamic Time, major factor in equilibrium

8. Dynamic Equilibrium JT Hack – recognized there was considerable variability in driving forces over time, l eads to a dynamic equilibrium, whereby following a change in forcings (internal or external), the system tends to re-establish a new equilibrium

9. 2. Driving Forces Resisting Forces Themes in Geomorphology Climate, Gravity, internal heat Lithology, structure

10. Controlling Factors Tectonic Setting: Determines what materials are exposed at the Earth’s surface, the manner in which they are distributed, and rate of change in g eometries within the landsurface Climate: Determines what geological ‘agents’ are available to interact with the exposed rocks/sediment and influences the rate of p hysical and chemical reactions Geomorphic Agents and Processes: Affect how rocks and seds are eroded Also includes biologic factors

11. Themes in Geomorphology 3. Climate Tectonics Dominant driving forces Control processes and landforms Studies in “ “ Geomorphology

12. 4. Thresholds – limits to equilibrium conditions Themes in Geomorphology Extrinsic thresholds – caused by external factors (driving forces) Intrinsic thresholds – caused by instability in a system (due to deterioration of resisting forces rather than driving forces) Once crossed, system tends towards a new equilibrium condition (part of the concept of Dynamic Equilibrium) During period of adjustment process and form in disequilibrium

13. Patton and Schumm , 1975 Relationship between valley slope and drainage area, suggesting a threshold separating gullied and non-gullied catchments Schumm and Hadley 1957 threshold

14. Concept of a Geomorphic Threshold - Schumm

15. Response Time and shift to a new dynamic Equilibrium State (Bull)

16. Evolution of geomorphic thought (foundation) James Hutton (1726-1797) – concept of an ancient Earth sculpted by water John Playfair (1748-1819) – Law of accordant stream junctions Charles Lyell (1797-1875) – Law of Uniformitarianism Uniformity of Law, Process, Rate and State “Present is the Key to the Past” Louis Agassiz (1807-1873) – Glacial geology – erratic boulders from ice, not great flood

17. JW Powell (1834 – 1902) 1869 – Exploration of the Green and Colorado River – concept of base level Second Director of the USGS

18. GK Gilbert (1843 – 1918) - interaction of driving forces of erosion and resisting forces of rocks (Equilibrium) - relationship between present form and processes - 1890 USGS Monograph 1 – Lake Bonneville

19. “cycle of erosion” Davis 1889 The Rivers and Valleys of Pennsylvania The father of American geography ‘time’ dependant cycles of erosion – ‘Davisian’ concept of landscape evolution WM Davis (1850-1934) - Harvard

20. JT Hack (1913-1991) – USGS Proposed alternative model to Davis equilibrium as a fundamental, time-independent geomorphic model Processes of uplift and erosion eventually reach equilibrium “Dynamic Equilibrium”

21. Stanley A. Schumm (1927-2011) – Colorado State Univ - Concept of Thresholds in geomorphology - Quantitative approach