Tectonic and Structural Geomorphology I Introduction To Plate Tectonics A Tectonics is a term that refers to the deformation of the earth s crust 1 Plate Tectonics refers to the formation and migration of these lithospheric plates physical interaction between the plates and the resultant deformation that is incurred by the crustal rocks during this process B Modern Plate Tectonic Theory 1 Plate Mobility it is recognized that each moves as a distinct rigid unit in relation to other plates These plates move on top of a semi plastic aesthenosphere and interact with one another along their boundaries We find most of tectonic deformation of plates along their boundaries where plates crush and grind past one another leading to such occurrences as earthquakes volcanism and mountain building 2 Plate Boundaries and Nature of Interaction between Plates 3 types of plate boundary interaction Divergent Convergent or Transform fault boundaries a Divergent Boundaries boundary condition in which tectonic plates move apart resulting in upwelling of magma and volcanic material to create new seafloor i e creation of new crust a Seafloor spreading process of plate divergence and injection of magma b Convergent Boundaries plate boundaries in which two plates move toward one another or collide 1 2 3 c subduction zone a zone of plate convergence in which where an oceanic plate descends into the upper mantle beneath the overriding plate a Oceanic Continental Plate Convergence b subduction related volcanism and volcanic arcs c volcanic arc an arcuate chain of volcanoes on continental crust that result from subduction of oceanic crust beneath continental crust Oceanic Oceanic ConvergenceContinental Continental Convergence Transform Fault Boundaries condition where plates slide horizontally past one another along a fault or fracture along which there is movement Crust is neither consumed nor destroyed e g San Andreas Fault in CA 90 II Mountain Building A Mountains topographically elevated portions of the earth that rise thousands to 10 s of thousands of feet above surrounding low lands B 1 Mountains may be single isolated masses e g Kilimanjaro a volcanic peak in East Africa or Mt Rainier or 2 Mountains may form continuous chains extending for thousands of miles e g western Cordillera region of SAM and NAM near continuous mountain chain running from Tierra Del Fuego to Alaska 3 Mountains may be youthful steep and of large relief e g Himalaya s or mature gently sloping and of relatively low relief e g Appalachian s 4 Orogenesis refers collectively to the processes that result in the elevation of land areas to form mountains Crustal Uplift Elevating the Crust Through Tectonic Process 1 Evidence that continental crust of earth has been uplifted in mountainous areas a presence of fossilized shells of sea critters found on top of mountains b C wave cut terraces Ancient benches that were cut by wave erosion are now elevated hundreds of meter above sea level Isostasy the concept of floating lithosphere in gravitational balance 1 density contrasts between less dense crust lithosphere and more dense aesthenosphere results in the lithosphere floating on top of the aesthenosphere 2 mountainous areas are thickened portions of continental crust that extend deep into the aesthenosphere roots of mountains a Granitic Crust average density 2 6 gm cu cm 3 oceanic crust on the other hand is denser than continental crust and is thinner thus owing to its relatively low isostatic position relative to the aesthenosphere a Basaltic Crusts average density 3 0 gm cu cm 4 thus according to idea of isostacy loading of crust will result in subsidence volcanic piling deposition of sediment in a basin thrust loading and unloading of the crust will result in uplift or bobbing up of crust glacial melt erosional denudation of landscape a As Mountain Ranges undergo denudation erosional unloading results in isostatic adjustment and continued uplift of the mountains isostatic adjustments and further erosion as mountainous crust is thinned and deepest portions of mountains are exposed through time 91 III Rock Deformation and Structural Geology process of rocks becoming physically deformed as they are subjected to tectonic crustal stress A B Plastic vs elastic vs brittle deformation of rocks rocks may respond to stress in the form of folding like paper plastic deformation or fracturing into blocks brittle deformation or may deform elastically i e given volume of rock will return to its original size and shape after stress is removed 1 brittle deformation of rocks is rather easy to recognize analogous to hitting concrete with sledge hammer Conditions of stress result in fracturing or rupturing of rocks 2 elastic stress is applied slowly under constant pressure rocks return to original size and shape after stress is removed 3 plastic deformation a set of conditions must be met before rocks will deform plastically a relative heat constant pressure and time 4 Generally as stress is applied to rocks at low temp and low press rocks will first deform elastically with ability to return to original size and shape once stress is removed once the level of stress exceeds the elastic limit of a given type of rock i e the point or strength of a rock with stress beyond which rock will fail it will then either deform plastically or brittally Folding of Rock Strata 1 Under components of horizontal stress flat lying layers of sedimentary volcanic rocks may become bent into a series of folds analogous to pushing and folding sheet of paper a 2 folding process results in shortening and thickening the crust Fold Types a Anticlines upfolded forms results in older rocks becoming enclosed within younger strata b synclines downfolded forms results in younger rocks becoming enclosed within older strata c symmetrical folds both limbs of the fold dipping at same angle away from fold axis d asymmetrical folds both limbs of the fold not dipping at same angle away from fold axis overturned folds one limb of fold has been tilted beyond vertical e 92 3 f plunging folds axis of fold is tilted g Domes more or less circular equivalent of anticline oldest rocks exposed in center of dome h Structural Basin more or less circular equivalent of syncline youngest rocks exposed in center of dome not to be confused with depositional basin Outcrops Patterns Associated with Folded Rocks a As rocks are folded and subsequently subjected to erosion regular patterns become evident in relation