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Structural Control of Landforms SAND, HOSES, Slickensided Rock, Pencil, Rubber bandGum, Foam sediments, Cardboard fault models, 2 Plastic boxes, Food Coloring,Paper,

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Presentation on theme: "Structural Control of Landforms SAND, HOSES, Slickensided Rock, Pencil, Rubber bandGum, Foam sediments, Cardboard fault models, 2 Plastic boxes, Food Coloring,Paper,"— Presentation transcript:

1 Structural Control of Landforms SAND, HOSES, Slickensided Rock, Pencil, Rubber bandGum, Foam sediments, Cardboard fault models, 2 Plastic boxes, Food Coloring,Paper, wood, Ice Photo 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 course Mostly Chapter 12 Plus a review of folds and faults Some photos in this PowerPoint made available online, courtesy of Steve Dutch, click here From our lab workbook Image Interpretation in Geology by Steve Drury

2 Erodability Relative Erodability –Layered rocks = wide range Sedimentary Volcanic –Massive rocks = narrow range Metamorphic Intrusive igneous –Erodability is not absolute but typically shale > limestone > sandstone ~ gneiss Canadian Shield. Pale granite and darker metavolcanic rocks, the granite having resisted glaciation best. Drury IIG

3 Erodability "… 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]

4 Lithology/Climate Erodability: shale > limestone > sandstone ~ gneiss In humid areas, weathering and erosion are faster, slopes are more eroded, gentler after the same duration of exposure to weathering

5 In arid terrains (a) the intermittent violent erosion develops steep-sided gullies and valleys. Note differential erosion Horizontally layered rocks – outcrops parallel topographic contours. In humid climate the topography is more muted. Monadnocks resistant rock ridges Colorado

6 Undisturbed Sediments showing differential erodability Dry Climate, intermittent strong storms

7 Review: Stream Vees Vees are pointing in direction of dip

8 Tablelands In horizontal beds, rock outcrops would follow contours

9 Tablelands: note horizontal layers, differential erosion Plateau>mesa>butte>chimney Ratio surface area of top to height Dry Climate, intermittent strong storms In horizontal beds, rock outcrops would follow contours Pediment (gentle slope < 5%, erosional concave up surface w thin veneer of gravel etc.) Inselberg mesa Butte chimney

10 Desert Landforms near Mountains Alluvial Fan (often exposed bare rock with gravel veneer) Mountains eventually erode away to Inselbergs Rain-shadow desert in the lee of mountains

11 Compression, Tension, and Shearing Stress Convergent Divergent Transform

12 Convergent Plate Boundaries and Folding Subduction causes Arc: Under Ocean Lithosphere Japan, Aleutians, Cent. Am.; under continent Andes, Cascades Continent-Continent collision forms Fold and Thrust Mountains: Alps, Himalayans, Appalachians

13 Strike and Dip Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface Map Symbols: Strike shown as long line, dip as short line. Note the angle of dip shown: 45 o

14 Tilted Strata Monoclinal folds, or one side (limb) of a fold Name = f(dip angle) –Cuesta (moderate dip) –Hogback (steep dip) –Flatiron remnant of dissected Hogback w triangular face

15 Dip Slope vs. Scarp slope Hogback Cuesta Hogback dip slope greater 30° - 40° with near symmetric slope on each face

16 Ridges Dip of Cuesta < Hogback Copyright © J. Michael Daniels 2002 http://www.alperry.com/coal/grand_hogback.htmlhttp://www.aureo.org/conference/boulderconference.html

17 Folds are typical of convergence Folded Rock Before Erosion

18 Folded Rocks, Hwy 23 Newfoundland, New Jersey Source: Breck P. Kent Adjacent Anticline and Syncline Note highest point

19 Folded Rock After Erosion Eroded Anticline, older rocks in center. Syncline is opposite.

20 Topography may be opposite of Structure Anticline Before/After Erosion Notice center rock oldest

21 Topography may be opposite of Structure Syncline Before/After Erosion Notice center rock youngest

22 Various Folds

23 Various Folds (cont'd)

24

25 Axial plane near axis should be close to horizontal Axis

26 Plunging Folds and Nose Rules Nose of anticline points direction of plunge, syncline nose in opposite direction Up End Down End Demo: Plastic box, water, paper folds

27 Plunging Folds Nose

28 Joints: Fractures – with no movement Source: Martin G. Miller/Visuals Unlimited vs. Faults with relative movement Sandstone, note no streams here, too many cracks

29 Dip-Slip Faults Demo: Cardboard Models

30 Continental Rift into Ocean Basin - Tension => Divergence Rift Valleys and Oceans are the same thing Normal Faults

31 Normal Faults at Divergent Margins - Iceland A new graben, down dropped hanging wall block - Normal Fault – divergent zone MOR Overhanging Block Footwall

32 Fault Line scarp (High-angle Normal Fault)

33 Convergent Margins Shallow Reverse Fault = Thrust Fault

34 Lewis Thrust Fault (cont'd) Same layer

35 Lewis Thrust Fault (cont'd) Source: Breck P. Kent PreCambrian Limestone over Cretaceous Shales

36 Geologists are frequently called upon to find the ore body Younger Miners pay geologists to find their lost orebody One friend earned enough to buy a house This poor guy is out of luck What 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? This guy is rich Normal Reverse

37 Horizontal Movement Along Strike-Slip Fault

38 Landscape Shifting, Wallace Creek San Andreas Fault

39 Normal Fault Quake - Nevada Reverse Fault Quake - Japan Strike Slip Fault Quake - California HW Down HW Up Convergent Divergent Transform

40 Fracture Zones and Slickensides http://pangea.stanford.edu/~laurent/english/research/Slickensides.gif

41 Part 2 Structural Control of Streams mostly Ch. 12 Consequent 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 slope Obsequent streams are streams flowing in the opposite direction of the consequent drainage.

42 consequent (c follow slope) subsequent (s along weak) obsequent (o opposite main slope)resequent streams (original slope but lower level) Insequent (random dendritic)

43 Insequent Streams= Initial Consequent Almost 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

44

45 Drainage Patterns with and without structural control None Joints fold limbs Volcano, exposed pluton, diapir

46 Dendritic Patterns Underlying bedrock has no structural control over where the water flows. Characteristic acute angles No repeating pattern.

47 Trellis Patterns Form where underlying bedrock has repeating weaker and stronger types of rock. Streams cut down deeper into the weaker bedrock Nearly parallel streams Branch at higher angles.

48 Rectangular patterns Branching of tributaries at nearly right angles Form in jointed igneous rocks or horizontal sedimentary beds with well-developed jointing or intersecting faults.

49 Parallel Erosion Form on unidirectional regional slope or parallel landform features. Small areas.

50 Radial Erosion Flow of water outward from a high point Down a volcano cone or an intrusive dome, or down an alluvial fan.

51 Annular patterns form on domes of alternating weak and hard bedrocks. The pattern formed is similar to that of a bull's-eye when viewed from above weaker bedrocks are eroded and the harder are left in place.

52 Centripetal patterns Form 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.

53 Structural Control of Drainage Contorted Folded Rocks

54 Stream Capture Headward Erosion

55 Stream Capture vs. Structural Control Subsequent Susquehanna does not reach Beaverdam Creek flowing through water gap Susquehanna captures headwaters of Beaverdam Creek, diverting upper Beaverdam trunk to Susquehanna channel.

56 Dry Valley Godfrey Ridge Brodhead Creek Elbow of Capture Stream Capture Headward erosion from Water Gap area cut through Godfrey Ridge and captured Brodhead Creek which was flowing east behind Godfrey Ridge

57 1. Old river meanders across floodplain 2. 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 terrace Terraces 1 Next time Terraces 2 and 3: Isostatic Rebound and high water shorelines as glaciers melt Potential  gh to Kinetic Energy 1/2 mV 2

58 A flight of river terraces

59 Antecedent Streams and Superimposed Streams Meanders in steep, narrow valleys –Caused by a drop in base level or uplift of region Delaware Water Gap River is older than upliftRiver is older than uplift Incised (entrenched) meanders

60 "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 Dutch Some photos in this PowerPoint made available online, courtesy of Steve Dutch, click here

61 Pediments and Alluvial Fans Alluvial fans typically develop at the exits of intermittent streams draining arid mountainous regions.

62 And on Mars … Link courtesy Melissa Hansen

63 An example of a v-shaped stream, with fairly constant slope and cross section

64 Conservation of Energy with frictional losses 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 0.01 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. An example for the homework calc.


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