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:
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
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
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]
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
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
Review: Stream Vees Vees are pointing in direction of dip
Tablelands In horizontal beds, rock outcrops would follow contours
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
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
Compression, Tension, and Shearing Stress Convergent Divergent Transform
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
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
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
Dip Slope vs. Scarp slope Hogback Cuesta Hogback dip slope greater 30° - 40° with near symmetric slope on each face
Lewis Thrust Fault (cont'd) Source: Breck P. Kent PreCambrian Limestone over Cretaceous Shales
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
Landscape Shifting, Wallace Creek San Andreas Fault
Normal Fault Quake - Nevada Reverse Fault Quake - Japan Strike Slip Fault Quake - California HW Down HW Up Convergent Divergent Transform
Fracture Zones and Slickensides http://pangea.stanford.edu/~laurent/english/research/Slickensides.gif
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.
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 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 Patterns Underlying bedrock has no structural control over where the water flows. Characteristic acute angles No repeating pattern.
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.
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.
Parallel Erosion Form on unidirectional regional slope or parallel landform features. Small areas.
Radial Erosion Flow of water outward from a high point Down a volcano cone or an intrusive dome, or down an alluvial fan.
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.
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.
Structural Control of Drainage Contorted Folded Rocks
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.
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
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
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
Pediments and Alluvial Fans Alluvial fans typically develop at the exits of intermittent streams draining arid mountainous regions.
An example of a v-shaped stream, with fairly constant slope and cross section
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.