13 What is Geomorphology? The study of LANDFORMS 1) Form 2) Process TheoreticalExperimentalDescriptive3) Geology and Climate (Physical)4) Geography (Spatial)5) History (Temporal)
14 Basic Principles of Process Geomorphology An equilibrium (“delicate balance”) exists between landforms and processesBalance between form and process created by interaction of force and resistanceDriving Forces and Resisting ForcesA coastal bluff in California is stable only if the driving forces (gravity, weight of sediments) is less than the resisting force (friction between particles and cohesion)Heavy rainfall on coastal bluff increases water table (adding weight), and decreases the cohesion of the sediment, and becomes a landslide (temporary disequilibrium)The new slope has adjusted to be closer to equilibrium
17 Endogenic and Exogenic processes Endogenic processes – energy source is internal, i.e. within the EarthVolcanismOrogeny and TectonicsExogenic processes – energy source is external to the systemGravitySunSolar constant of 2.0 cal/cm2/minClimateThe average of weather over 30+ yrBoth may combine to produce landforms
19 Volcan Iztaccíhuatl, Mexico Volcanism – composite volcanoTemperaturePrecipitationGlaciationGravityBasic form related to endogenic energy – volcanic activity and eruption of Lavas. Scalloping and slopes on the volcano are a result of atmospheric processes such as snowfall, low temperatures, and the formation of glaciers which are driven by gravity.
20 Thresholds and Equilibrium Thresholds represent the limits of equilibriumCan be measuredE.g. increasing slope angle, water content, and landslidingAn extrinsic threshold is where the threshold is crossed by exogenic processesincreased rainfall on a slope already near failureAn intrinsic threshold is crossed when endogenic processes result in a decrease in resisting forcesweathering of cements that bind grains together
21 Gullying in the Scottish highlands; Lachniet, 2001 Example of threshold responseGullying in the Scottish highlands; Lachniet, 2001
22 Driving Forces Solar Radiation Insolation – amount of energy absorbed by the systemThe sun drives the climate systemMain energy source for exogenic processes
25 Driving Forces Gravity Equal over surface of earth Controls processes such as fluvial action, glacial flow, tides
26 Driving Forces Internal Heat Radioactive decay Less than solar radiation heatCauses uplift and deformation
27 Resisting Framework Lithology and Structure Silicic and mafic Igneous, metamorphic, sedimentary (consolidated and unconsolidated)Variations in weathering due to climate and mineralogyFaults, folds, mountains, basinsStratigraphic variations in rock density
28 Complex Response and Process Linkage Threshold-crossing events initiate sequence of changesProcess Linkage“Cascading” effects in geomorphic systemHydraulic mining in Sierra NevadaIncreased sediment loadsBed aggradationSlope steepeningIncreased floodingGradual relaxation to original state (unfinished)“Recovery time”: time to revert to original state
29 Feedbacks Positive Negative Reinforcing “Snowball” effect Not judgmental!Uplift erosion overburden removal upliftNegativeDampeningThermostatFlood scouring of stream channel wider x-sectional area slower stream velocity more sediment aggradation smaller x-sectional area
30 Next Topic: Internal Forces and Climate Read Chapter 2.