Presentation on theme: "Structural Control of Landforms"— Presentation transcript:
1Structural Control of Landforms ESCI 307, Fall 2003, Lecture 3Structural Control of LandformsMostly Chapter 12Plus a review of folds and faultsPhoto from Drury: Two distinct units. One dendritic drainage pattern is sparsely vegetated. Parallel contours suggest it is horizontal. Other formation banded, with straight wooded ridges, controlled by steep dips. The boundary truncates the ridges. Horizontal unit lies unconformably on the steeply dipping strata (angular unconformity). The wide spacing of drainage in the younger unit suggests that it is a massive, coarse clastic rock. The older unit comprises shales and limestones. From Steve Drury, Image Interpretation in Geology, adopted for this courseSome photos in this PowerPoint made available online, courtesy of Steve Dutch, click hereSAND, HOSES, Slickensided Rock, Pencil, Rubber bandGum, Foam sediments, Cardboard fault models, 2 Plastic boxes, Food Coloring ,Paper, wood, IceFrom our lab workbook Image Interpretation in Geology by Steve Drury
2Erodability Relative Erodability Layered rocks = wide range ESCI 307, Fall 2003, Lecture 3ErodabilityRelative ErodabilityLayered rocks = wide rangeSedimentaryVolcanicMassive rocks = narrow rangeMetamorphicIntrusive igneousErodability is not absolute buttypically shale > limestone > sandstone ~ gneissCanadian Shield. Pale granite and darker metavolcanic rocks, the granite having resisted glaciation best. Drury IIG
3ESCI 307, Fall 2003, Lecture 3Erodability"… shale, limestone, marble and some types of [mica] schist are less resistant "valley-makers" in humid climates" …"whereas [quartz] sandstone, quartzite, [quartz] conglomerate and various igneous rocks [ granite has ~20% quartz H= 7] are resistant "ridge-makers" ….Easterbrook (1969) Principles of Geomorphology[words in brackets added]
4ESCI 307, Fall 2003, Lecture 3Lithology/ClimateErodability: shale > limestone > sandstone ~ gneissIn humid areas, weathering and erosion are faster, slopes are more eroded,gentler after the same duration of exposure to weathering
5Horizontally layered rocks – outcrops parallel topographic contours. ESCI 307, Fall 2003, Lecture 3Horizontally layered rocks – outcrops parallel topographic contours. In arid terrains (a) the intermittent violent erosion develops steep-sided gullies and valleys. Note differential erosionIn humid climate the topography is more muted.Monadnocks resistant rock ridges Colorado
7Vees are pointing in direction of dip ESCI 307, Fall 2003, Lecture 3Review: Stream VeesVees are pointing in direction of dip
8In horizontal beds, rock outcrops ESCI 307, Fall 2003, Lecture 3TablelandsIn horizontal beds, rock outcropswould follow contours
9Tablelands: note horizontal layers, differential erosion ESCI 307, Fall 2003, Lecture 3In horizontal beds, rock outcropswould follow contoursButte chimneyTablelands: note horizontal layers, differential erosionmesaInselbergPediment (gentle slope < 5%,erosional concave up surfacew thin veneer of gravel etc.)Dry Climate, intermittent strong stormsPlateau>mesa>butte>chimneyRatio surface area of top to height
10Desert Landforms near Mountains ESCI 307, Fall 2003, Lecture 3Alluvial Fan(often exposed bare rock with gravel veneer)Rain-shadow desert in the lee of mountainsMountains eventually erode away to Inselbergs
11Compression, Tension, and Shearing Stress ESCI 307, Fall 2003, Lecture 3Compression, Tension, and Shearing StressConvergent Divergent Transform
12Convergent Plate Boundaries and Folding ESCI 307, Fall 2003, Lecture 3Convergent Plate Boundaries and FoldingSubduction causes Arc: Under Ocean Lithosphere Japan,Aleutians, Cent. Am.; under continent Andes, CascadesContinent-Continentcollision formsFold and Thrust Mountains: Alps, Himalayans, Appalachians
13Strike and DipESCI 307, Fall 2003, Lecture 3Map Symbols: Strike shown as long line, dip as short line. Note the angle of dip shown: 45oStrike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface
14Tilted Strata Monoclinal folds, or one side (limb) of a fold ESCI 307, Fall 2003, Lecture 3Monoclinal folds, or one side (limb) of a foldName = f(dip angle)Cuesta (moderate dip)Hogback (steep dip)Flatiron remnant of dissected Hogback w triangular face
15Dip Slope vs. Scarp slope ESCI 307, Fall 2003, Lecture 3Dip Slope vs. Scarp slopeCuestaHogbackHogback dip slope greater 30° - 40° with near symmetric slope on each face
23ESCI 307, Fall 2003, Lecture 3Various Folds (cont'd)
24ESCI 307, Fall 2003, Lecture 3Various Folds (cont'd)
25Various Folds (cont'd) Axis ESCI 307, Fall 2003, Lecture 3Various Folds (cont'd)AxisAxial plane near axis should be close to horizontal
26Plunging Folds and Nose Rules ESCI 307, Fall 2003, Lecture 3Plunging Folds and Nose RulesDemo: Plastic box, water, paper foldsUpEndDownEndNose of anticline points direction of plunge, syncline nose in opposite direction
27ESCI 307, Fall 2003, Lecture 3Plunging FoldsNoseNoseNose
28Joints: Fractures – with no movement ESCI 307, Fall 2003, Lecture 3Joints: Fractures – with no movementvs. Faults with relative movementSandstone, note no streams here, too many cracksSource: Martin G. Miller/Visuals Unlimited
29ESCI 307, Fall 2003, Lecture 3Demo: Cardboard ModelsDip-Slip Faults
30Continental Rift into Ocean Basin - Tension => Divergence ESCI 307, Fall 2003, Lecture 3Continental Rift into Ocean Basin - Tension => DivergenceRift Valleys and Oceans are the same thingNormal Faults
31Normal Faults at Divergent Margins - Iceland ESCI 307, Fall 2003, Lecture 3Normal Faults at Divergent Margins - IcelandA new graben, down dropped hanging wall block - Normal Fault – divergent zone MOROverhangingBlockFootwall
32Fault Line scarp (High-angle Normal Fault) ESCI 307, Fall 2003, Lecture 3Fault Line scarp(High-angleNormal Fault)
35Lewis Thrust Fault (cont'd) ESCI 307, Fall 2003, Lecture 3Lewis Thrust Fault (cont'd)Source: Breck P. KentPreCambrian Limestone overCretaceous Shales
36Geologists are frequently called upon to find the ore body ESCI 307, Fall 2003, Lecture 3Geologists are frequently called upon to find the ore bodyYoungerThis guy is richWhat phase of magma fractionation would result in the placement of this ore body?Which formed first, the ore body or the fault?What common mineral is mostly likely in the ore body?ReverseMiners pay geologists to find their lost orebodyOne friend earned enough to buy a houseNormalThis poor guy is out of luck
37Horizontal Movement Along Strike-Slip Fault ESCI 307, Fall 2003, Lecture 3Horizontal Movement Along Strike-Slip Fault
38Landscape Shifting, Wallace Creek ESCI 307, Fall 2003, Lecture 3Landscape Shifting, Wallace CreekSan Andreas Fault
40Fracture Zones and Slickensides ESCI 307, Fall 2003, Lecture 3Fracture Zones and Slickensides
41Part 2 Structural Control of Streams mostly Ch. 12 ESCI 307, Fall 2003, Lecture 3Part 2 Structural Control of Streams mostly Ch. 12Consequent streams follow slope of the land over which they originally formed.Subsequent streams are streams whose course has been determined by erosion along weak strata.Resequent streams are streams whose course follows the original relief, but at a lower level than the original slopeObsequent streams are streams flowing in the opposite direction of the consequent drainage.
42resequent streams (original slope but lower level) subsequent (s along weak)ESCI 307, Fall 2003, Lecture 3consequent (c follow slope)Insequent (random dendritic)obsequent (o opposite main slope)resequent streams (original slope but lower level)
43Insequent Streams= Initial Consequent ESCI 307, Fall 2003, Lecture 3Insequent Streams= Initial ConsequentAlmost random drainage often forming dendritic patterns.Typically tributaries - developed by headward erosion on a horizontally stratified rocks, or a substrate with ~ constant composition.NOT controlled by the original slope of the surface, its structure or the type of rock.Headward Erosion
45Drainage Patterns with and without structural control ESCI 307, Fall 2003, Lecture 3Drainage Patterns with and without structural controlNone Joints fold limbsVolcano, exposed pluton, diapir
46Dendritic PatternsESCI 307, Fall 2003, Lecture 3Underlying bedrock has no structural control over where the water flows.Characteristic acute anglesNo repeating pattern.
47ESCI 307, Fall 2003, Lecture 3Trellis PatternsForm where underlying bedrock has repeating weaker and stronger types of rock.Streams cut down deeper into the weaker bedrockNearly parallel streamsBranch at higher angles.
48Rectangular patterns Branching of tributaries at nearly right angles ESCI 307, Fall 2003, Lecture 3Rectangular patternsBranching of tributaries at nearly right anglesForm in jointed igneous rocks or horizontal sedimentary beds with well-developed jointing or intersecting faults.
49ESCI 307, Fall 2003, Lecture 3Parallel ErosionForm on unidirectional regional slope or parallel landform features. Small areas.
50Radial Erosion Flow of water outward from a high point ESCI 307, Fall 2003, Lecture 3Flow of water outward from a high pointDown a volcano coneor an intrusive dome, ordown an alluvial fan.
51Annular patterns form on domes of alternating weak and hard bedrocks. ESCI 307, Fall 2003, Lecture 3Annular patternsform on domes of alternating weak and hard bedrocks.The pattern formed is similar to that of a bull's-eye when viewed from aboveweaker bedrocks are eroded and the harder are left in place.
52Centripetal patternsESCI 307, Fall 2003, Lecture 3Form where water flows into a central location, such as a round bowl-shaped watershed, or a karst limestone terrain where disappearing streams flow down into a sinkhole and then underground.
53Structural Control of Drainage ESCI 307, Fall 2003, Lecture 3Structural Control of DrainageContortedFolded Rocks
54ESCI 307, Fall 2003, Lecture 3Stream CaptureHeadward Erosion
55Stream Capture vs. Structural Control ESCI 307, Fall 2003, Lecture 3Stream Capture vs. Structural ControlSusquehanna captures headwaters of Beaverdam Creek, diverting upper Beaverdam trunk to Susquehanna channel.Subsequent Susquehanna does not reach Beaverdam Creek flowing through water gap
56Stream Capture Dry Valley Elbow of Capture Brodhead Creek ESCI 307, Fall 2003, Lecture 3Dry ValleyElbow of CaptureBrodhead CreekGodfrey RidgeHeadward erosion from Water Gap area cut through Godfrey Ridge and captured Brodhead Creek which was flowing east behind Godfrey RidgeStream Capture
571. Old river meanders across floodplain ESCI 307, Fall 2003, Lecture 3Terraces 11. Old river meanders across floodplain2. Base level drops (how?), or region uplifts. Area now much higher above sea level than before.Potential energy increases, water flows faster, better erosion, stream straightens and cut down to base level, less floodplain width and cut lower.3.Terrace forms from previous floodplain. Further incision cuts another terraceNext time Terraces 2 and 3: Isostatic Rebound and high water shorelines as glaciers meltPotential rgh to Kinetic Energy 1/2mV2
58A flight of river terraces ESCI 307, Fall 2003, Lecture 3A flight of river terraces
59River is older than uplift ESCI 307, Fall 2003, Lecture 3Antecedent Streams and Superimposed StreamsMeanders in steep, narrow valleysCaused by a drop in base level or uplift of regionDelaware Water GapIncised (entrenched) meandersRiver is older than uplift
60ESCI 307, Fall 2003, Lecture 3"In this panorama in southwestern Colorado, a stream flows from the right across an uplift (anticline) in the rocks. As soon as the stream enters the uplift, its canyon becomes deep. Note the entrenched [incised] meanders, a couple of which were cut through and abandoned when the canyon was about half its present depth. As soon as the river exits the uplift, the canyon once again becomes shallow. Clearly, the river was there first and the rocks arched upward across its course." Steve DutchSome photos in this PowerPoint made available online, courtesy of Steve Dutch, click here
61Pediments and Alluvial Fans Alluvial fans typically develop at the exits of intermittent streams draining arid mountainous regions.
63An example of a v-shaped stream, with fairly constant slope and cross section
64Conservation of Energy with frictional losses An example for the homework calc.A stream channel has been uplifted to 300 meters above base level. It’s cross sectional area, slope, and water depth is close to constant. The stream is full of large boulders. At 300 meters it flows out of an alpine lake, where it has an average velocity of meters/sec, that is, it has mostly potential energy. At base level it has a velocity of 15 meters per second (so all kinetic energy, plus frictional losses on the way down. Estimate the percent energy lost to friction.