90Tectonic and Structural GeomorphologyI. Introduction To Plate TectonicsA. "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 lithosphericplates, physical interaction between the plates, and the resultant deformation that isincurred by the crustal rocks during this process.B. Modern Plate Tectonic Theory1. Plate Mobility - it is recognized that each moves as a distinct rigid unit in relation toother plates. These plates move on top of a semi-plastic aesthenosphere, andinteract with one another along their boundaries. We find most of tectonicdeformation of plates along their boundaries where plates crush and grind past oneanother leading to such occurrences as earthquakes, volcanism, and mountainbuilding.2. Plate Boundaries and Nature of Interaction between Plates3 types of plate boundary interaction: Divergent, Convergent, or Transform fault boundariesa. Divergent Boundaries - boundary condition in which tectonic plates moveapart, resulting in upwelling of magma and volcanic material to create newseafloor: i.e. creation of new crust.(a) Seafloor spreading- process of plate divergence andinjection of magma. b. Convergent Boundaries- plate boundaries in which two plates movetoward one another or collide.(1) subduction zone- a zone of plate convergence in which where anoceanic plate descends into the upper mantle beneath the overridingplate.(a) Oceanic-Continental Plate Convergence-(b) subduction-related volcanism and volcanic arcs(c) volcanic arc - an arcuate chain of volcanoes on continentalcrust that result from subduction of oceanic crust beneathcontinental crust.(2) Oceanic-Oceanic Convergence- (3) Continental-Continental Convergence-c. Transform Fault Boundaries: condition where plates slide horizontallypast 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).91II. Mountain Building A. Mountains: topographically elevated portions of the earth that rise thousands to 10's ofthousands of feet above surrounding low lands.1. Mountains may be single isolated masses (e.g. Kilimanjaro a volcanic peak in EastAfrica, or Mt. Rainier) or2. Mountains may form continuous chains extending for thousands of miles (e.g.western Cordillera region of SAM and NAM, near continuous mountain chainrunning 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 landareas to form mountains.B. Crustal Uplift: Elevating the Crust Through Tectonic Process1. Evidence that continental crust of earth has been uplifted in mountainous areas:a. presence of fossilized shells of sea critters found on top of mountainsb. wave-cut terraces - Ancient benches that were cut by wave erosion are nowelevated hundreds of meter above sea level.C. Isostasy- the concept of floating lithosphere in gravitational balance. 1. density contrasts between less dense crust/lithosphere and more denseaesthenosphere results in the lithosphere "floating" on top of the aesthenosphere.2. mountainous areas are thickened portions of continental crust that extend deep intothe aesthenosphere (roots of mountains).a. Granitic Crust: average density = 2.6 gm/cu. cm3. oceanic crust on the other hand is denser than continental crust and is thinner, thusowing to its relatively low isostatic position relative to the aesthenosphere.a. Basaltic Crusts: average density = 3.0 gm/cu. cm4. 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 ofthe crust will result in uplift or bobbing up of crust (glacial melt, erosional denudationof landscape).a. As Mountain Ranges undergo denudation, erosional unloading results inisostatic adjustment and continued uplift of the mountains, isostaticadjustments, and further erosion as mountainous crust is thinned anddeepest portions of mountains are exposed through time.92III. Rock Deformation and Structural Geology- process of rocks becoming physically deformedas they are subjected to tectonic/crustal stressA. Plastic vs. elastic vs. brittle deformation of rocks: rocks may respond to stress in theform 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 shapeafter stress is removed)1. brittle deformation of rocks is rather easy to recognize, analogous to hittingconcrete with sledge hammer. Conditions of stress result in fracturing or rupturingof rocks.2. elastic: stress is applied slowly under constant pressure, rocks return to originalsize and shape after stress is removed.3. plastic deformation: a set of conditions must be met before rocks will deformplasticallya. relative heat, constant pressure, and time4. Generally: as stress is applied to rocks at low temp, and low press, rocks will firstdeform elastically (with ability to return to original size and shape once stress isremoved), 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 theneither deform plastically or brittally.B. Folding of Rock Strata1. Under components of horizontal stress: flat-lying layers of sedimentary/volcanicrocks may become bent into a series of folds (analogous to pushing and foldingsheet of paper).a. folding process results in shortening and thickening the crust2. Fold Typesa. Anticlines-upfolded forms, results in older rocks becoming enclosed withinyounger stratab. synclines-downfolded forms, results in younger rocks becoming enclosedwithin older strata.c. symmetrical folds - both limbs of the fold dipping at same angle away fromfold axisd. asymmetrical folds - both limbs of the fold not dipping at same angle awayfrom fold axise. overturned folds - one limb of fold has been tilted beyond vertical93f. plunging folds- axis of fold is tiltedg. Domes- more or less circular equivalent of anticline, oldest rocks exposedin center of domeh. Structural Basin- more or less circular equivalent of syncline, youngestrocks exposed in center of dome (not to be confused with depositionalbasin)3. Outcrops Patterns