Concepts in Geomorphology

Geomorphology Study of landforms

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

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

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

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

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

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

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

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 geometries within the landsurface Climate: Determines what geological ‘agents’ are available to interact with the exposed rocks/sediment and influences the rate of physical and chemical reactions Geomorphic Agents and Processes: Affect how rocks and seds are eroded Also includes biologic factors

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

Themes in Geomorphology 4. Thresholds – limits to equilibrium conditions 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 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)

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

Concept of a Geomorphic Threshold - Schumm

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

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

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

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

WM Davis (1850-1934) - Harvard “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

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”

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