structural control of landforms sand, hoses, slickensided rock, pencil, rubber bandgum, foam...
TRANSCRIPT
Structural Control Structural Control of Landformsof 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
ErodabilityErodability
• 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
ErodabilityErodability
• "… 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]
Lithology/ClimateLithology/ClimateErodability: 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
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
Undisturbed Sediments Undisturbed Sediments showing differential erodabilityshowing differential erodability
Dry Climate, intermittent strong storms
Tablelands: note horizontal Tablelands: note horizontal layers, differential erosionlayers, differential erosion
• Plateau>mesa>butte>chimney
• Ratio surface area of top to height
Dry Climate, intermittent strong storms
In horizontal beds, rock outcropswould follow contours
Pediment (gentle slope < 5%,erosional concave up surface w thin veneer of gravel etc.)
Inselbergmesa
Butte chimney
Desert Landforms Desert Landforms near Mountainsnear Mountains
Alluvial Fan
(often exposed bare rock with gravel veneer)
Mountains eventually erode away to Inselbergs
Rain-shadow desert in the lee of mountains
Compression, Tension, Compression, Tension, and Shearing Stressand Shearing Stress
Convergent Divergent Transform
Convergent Plate Boundaries Convergent Plate Boundaries and Foldingand Folding
Subduction causes Arc: Under Ocean Lithosphere Japan,Aleutians, Cent. Am.; under continent Andes, Cascades
Continent-Continentcollision formsFold and Thrust Mountains: Alps, Himalayans, Appalachians
Strike and DipStrike 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: 45o
Tilted StrataTilted 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
Dip Slope vs. Scarp slope
Hogback
Cuesta
Hogback dip slope greater 30° - 40° with near symmetric slope on each face
RidgesRidges• Dip of Cuesta < Hogback
Copyright © J. Michael Daniels 2002http://www.alperry.com/coal/grand_hogback.html http://www.aureo.org/conference/boulderconference.html
Folds are typical of convergenceFolds are typical of convergenceFolded Rock Before ErosionFolded Rock Before Erosion
Folded Rocks, Hwy 23 Folded Rocks, Hwy 23 Newfoundland, New JerseyNewfoundland, New Jersey
Source: Breck P. Kent
Adjacent Anticline and Syncline
Note highest point
Folded Rock After ErosionFolded Rock After Erosion
Eroded Anticline, older rocks in center. Syncline is opposite.
Topography may be opposite of Structure Topography may be opposite of Structure
AnticlineAnticline Before/After Erosion Before/After Erosion
Notice center rock oldest
Topography may be opposite of Structure Topography may be opposite of Structure
Syncline Before/After ErosionSyncline Before/After Erosion
Notice center rock youngest
Various Folds (cont'd)Various Folds (cont'd)
Axial plane near axis should be close to horizontal
Axis
Plunging Folds and Nose RulesPlunging Folds and Nose Rules
Nose of anticline points direction of plunge, syncline nose in opposite direction
UpEnd Down
End
Demo: Plastic box, water, paper folds
Joints: Fractures – with no movementJoints: Fractures – with no movement
Source: Martin G. Miller/Visuals Unlimited
vs. Faults with relative movement
Sandstone, note no streams here, too many cracks
Continental Rift into Ocean Basin - Tension => Divergence
Rift Valleys and Oceans are the same thing
Normal Faults
Normal Faults at Divergent Normal Faults at Divergent Margins - IcelandMargins - Iceland
A new graben, down dropped hanging wall block - Normal Fault – divergent zone MOR
Overhanging Block
Footw
all
Convergent MarginsConvergent MarginsShallow Reverse Fault = Thrust FaultShallow Reverse Fault = Thrust Fault
Lewis Thrust Fault (cont'd)Lewis Thrust Fault (cont'd)
Source: Breck P. Kent
PreCambrian Limestone over Cretaceous Shales
Geologists are frequently called upon to find the ore bodyGeologists are frequently called upon to find the ore body
Younger
Miners pay geologists to find their lost orebodyOne 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
Normal Fault Quake - NevadaReverse Fault Quake - Japan
Strike Slip Fault Quake - California
HW Down
HW UpConvergent
Divergent
Transform
Fracture Zones and SlickensidesFracture Zones and Slickensideshttp://pangea.stanford.edu/~laurent/english/research/Slickensides.gif
Part 2 Structural Control Part 2 Structural Control of Streams mostly Ch. 12of 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.
consequent (c follow slope) subsequent (s along weak)
obsequent (o opposite main slope) resequent streams (original slope but lower level)
Insequent (random dendritic)
Insequent Streams= Initial ConsequentInsequent 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
Drainage Patterns with and without structural control
None Joints fold limbs
Volcano, exposed pluton, diapir
Dendritic PatternsDendritic Patterns• Underlying bedrock has no structural control
over where the water flows. • Characteristic acute angles
• No repeating pattern.
Trellis PatternsTrellis 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.
Rectangular patternsRectangular patterns
• Branching of tributaries at nearly right angles
• Form in jointed igneous rocks or horizontal sedimentary beds with well-developed jointing or intersecting faults.
Parallel ErosionParallel Erosion
• Form on unidirectional regional slope or parallel landform features. Small areas.
Radial ErosionRadial Erosion• Flow of water outward from a high point
• Down a volcano cone
• or an intrusive dome, or
• down an alluvial fan.
Annular patternsAnnular 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.
Centripetal patternsCentripetal 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.
Stream Capture vs. Structural ControlStream 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.
Dry Valley
Godfrey RidgeBro
dhead Cre
ek
Elbow of Capture
Stream Capture
Headward erosion from Water Headward erosion from Water Gap area cut through Godfrey Gap area cut through Godfrey Ridge and captured Brodhead Ridge and captured Brodhead Creek which was flowing east Creek which was flowing east behind Godfrey Ridgebehind Godfrey Ridge
1. 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 terrace
Terraces 1
Next time Terraces 2 and 3: Isostatic Rebound and high water shorelines as glaciers melt Potential gh to Kinetic Energy 1/2mV2
• 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
"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
PedimentsPediments and Alluvial Fans and Alluvial Fans
Alluvial fans typically develop at the exits of intermittent streams draining arid mountainous regions.
Conservation of Energy with frictional lossesConservation 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.