GEOG 1113 Unit 3
GEOG 1113 Unit 3 GEOG 1113
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WEATHERING, EROSION, AND DENUDATION GEOG 10-13-15 Denudation = weathering + erosion There are two kinds of weathering 1) Mechanical or physical disintegration a. Breaking a rock with physical force b. The material is not changed chemically, just broken into smaller pieces 2) Chemical or decomposition a. Red clay was first a rock, but was decayed into the clay b. Substance is changed into a new chemical compound Dilatation: release of pressure that is built up in the rock Picture: tube from can to outside the box – remove air from can – can implodes because air pressure in container is higher than inside can remove air from container – can explodes because air pressure is higher in can than container Stone Mountain Stone Mt. has its own internal pressure with overlying material when first formed Overlying material was eroded over time Constant internal pressure is still pushing out outwards (like an expanding balloon) Top of mountain experiences no more pressure from lack of earth on top of it Stone Mountain is cracking due to expansive forces Cracks develop at right angles to the forces Cross Section of Stone Mountain dilatation fractures (sheeting) form of Stone Mountain is referred to as “whaleback” cracks produce layers of rock on top layer of Stone Mountain “Inselberg” – island mountains types: domes, whalebacks GEOG 10-22-15 Thermoclastis 1. Breakage due to heating and cooling 2. Causes rock spalling or flaking of slabs 1-3 cm thick Minerals have different: 1. Albedo 2. Specific heat 3. Rates of expansion and contraction 4. Rock temperatures in fires can reach several hundred degrees centigrade and can break rocks Rocks heat up in desert because the rock absorbs heat, but does not transfer the heat to something else Stresses of earth under surface layer because upper portion is trying to contract due to cold temp while lower portion is expanding due to heat (only a few cm) Albedo of a substance: extent of absorbing/reflecting heat from solar energy i.e. black absorbs a lot of heat while white reflects heat specific heat- amount of solar energy needed to raise the substance 1 degree centigrade a rock in the sun with mixed minerals will break up due to minerals differences in specific heat and albedo because some minerals are expanding more than others and produce micro fractures (Granular disintegration) when there is a fire, it is almost certain there will be breakage of rock Chemical Weathering Equations in book Carbonation: involves dissolved carbon from CO2 giving H2CO3 or carbonic acid rainfall When orthoclase feldspar is exposed to water, small clay particles (kaolinite) (a hydrated aluminosilicate) form and float in the mixture of potassium oxide and silica (last two are soluble in water) Kaolinite is Georgia red clay Orthoclase feldspar is Carbonic acid is rainfall If carbonic acid replaces water in hydrolysis (that’s what happens in the real world), then potassium carbonate is an output of the reaction instead of potassium oxide Solution – form of carbonation CaCO3 + H20 + CO2 Ca(HCO3)2 Limestone or calcite + H2CO3 (carbonic acid) Calcium Bicarbonate Other highly soluble rocks: Gypsum CaSO4 Halite NaCl Dolomite CaMg(CO3)2 If you evaporate water out of calcium bicarbonate, calcium carbonate would be precipitated GEOG 10-27-15 Inselbergs Island mountains first described by the German, Bergassor Bornhardt, working in East Africa Bornhardts (a whole class of inselbergs) (one whole rock, as opposed to boulder hills) 1) Domes 2) Whalebacks 3) Turtlebacks a. Domes with a lower inclination 4) Ruwares or rock pavements Boulder hills (looks like a hill of blocks, joints are visible) 1) Tors (very small hills made of boulders) 2) Castle kopjes / koppies (castle-like hill, can be quite large) Etchplain – hardly any weathered rock, almost completely exposed basal surface of weathering, creating a sea of boulders. GEOG 10-29-15 The Hydrological Cycle 1. Cycle by which water moves from the ocean to the atmosphere over the land and back to the ocean 2. Cycle operates at the global scale Runoff and groundwater ocean evaporated vaporized into atmosphere winds carry moisture condensation of vapor precipitation (either early on in stages near ocean, or occurs later on over land) runoff The water from precipitation can be snow or rain and adds back to rivers (leads to runoff), lakes (leads to evaporation), or sink into the ground (groundwater – which become springs under ocean floor) Plants suck up water through roots and then the water evaporates into atmosphere from leaves via transpiration 1. Each year, the amount evaporated from the oceans is abut 1 meter of water, lowering sea level 2. If not returned to the oceans, they would dry up in 4,000 years 3. A major change in the balance of the cycle occurred during glacial events when water was locked up as ice in continents. This led to a drop in the ocean of about 150-180 meters 4. Planets where evaporated water is lost to outer space become dry… e.g. Mars where only water is in frozen form below ground Groundwater Porosity – percent of void space in a rock Permeability – capacity of a rock to transmit fluids Primary: ability of a massive rock to transmit water through pore space Secondary: faults, joints, spaces produced by cracking of the rock, in which the water can enter very easily due to the large openings Water percolates into the ground until it is stopped Water will move as deep as possible due the effects of gravity Once the porosity becomes zero, water will stop moving down water table is the upper surface of the zone of saturation zone of saturation (phreatic zone) – area in rock in which water has filled every possible void zone of aeration (vadose zone) – above phreatic zone, contains air due to lack of saturation weathered rock is called regolith Aquifer Aquifer – a rock through which water can move readily Unconfined – connected to the surface Confined – enclosed by impermeable rock layers Rocks: limestone, sandstone, gravel, sand, basalt Aquiclude – impermeable rock layer preventing flow of water through it, hardly transmit water Rocks: shale, chert, mudstone Aquitard – retards or slows down the flow of water through the ground, relatively slowly compared to aquifer Rocks: shale, chert, mudstone Unconfined Aquifer 1. Vadose zone (zone of aeration) 2. Zone of Fluctuation 3. Zone of Saturation (Phreatic Zone) Zone of fluctuation – range of water table due to variability from wet and dry seasons (during wet season, water table rises into aeration zone; during dry seasons, water table drops into zone of saturation) GEOG 11-3-15 Unconfined aquifer is open to the atmosphere (air contact between air and water) Level of water in zone of saturation will keep rising as you add water Cannot rise beyond the lowest point on the surface When it reaches a topographic low (such as a dip between two hils) the water flows out onto the surface, forming a lake or river, etc. Water table tends to mirror topography (where there is a hill, the water table is heightened) After rain, water will percolate down through vadose zone via gravity, until it hits the water table The water table acts as the border between vadose zone and zone of fluctuation If permeability of rock and soil is high, water table will drop quickly during dry season Wells should be dug deep in the phreatic zone during the dry season to ensure access to water during all parts of the year so as to not run out of water in the fluctuation zone in the dry season (this would happen due to lowered water table) When water enters back into the ground: recharged 1) Hydrolic head: difference in elevation between parts of the water table Determines velocity and direction of ground water flow Top of hill to bottom of hill where water runs off into a body of water Water flows from point of high pressure to point of low pressure The bigger the head and the higher the permeability, the faster the water flow 2) Typical velocities: 1m/day fast velocity: 1m/hour 3) In very permeable materials like sand and gravel, may be 250m/day 4) In limestones velocities are much higher because water flows in caves as underground rivers Effluent seepage: e.g. flow of ground water into rivers and lakes maybe as springs or seeps (outward flow) Influent seepage: flow from rivers etc. into the ground (common in deserts) (inward flow) Base flow of a stream is the flow that is provided by ground water If a drought occurs, rivers do not dry up because they are supplied by ground water when there is a lack of water from precipitation (effluent seepage) In many dry area, as it rains, the water is contained in the river channel and begins to enter back into the ground towards the water table (influent seepage) Most rivers in the desert don’t reach the sea because they lose all their water when it seeps into the ground Dry river channels are called Wadi Artesian Water and Confined Aquifers Water is confined in an aquifer between impermeable beds Water table (unconfined aquifer) = Piezometric surface Potentiometric surface (artesian aquifer) = level to which water would rise if free to do so Water in a well will rise as high as the hydrolic head (recharge area) is (if the well is drilled down hill of the hydrolic head, the pressure downhill from the head will push water up through the well until it reaches the same planar level as the head) An aquifer is sandwiched between two aquicludes, so the only way for water to enter the aquifer is via recharge at a point that is unconfined because it is exposed to air (the only way the water enters here is because of the fact that it is an unconfined section of the same aquifer, which is classified as confined as soon as it starts to be sandwiched between the the aquicludes) This point is the considered the hydrolic head, where it is highest in water pressure When ground water is over used and pumped out, the hydrolic head will drop and eventually the artesian aquifer will no loner be above ground (which causes the water to no longer freely flow upward above ground through wells to the potentiometric surface – as explained by hydrolic head pressure and the artesian pressure surface) The Potentiometric Surface With no flow resistance the water level will be the same in all of the tubes If we add flow resistance and tilt the tube system, the water will rise to a lower and higher height Water can rise above the ground surface in places Floridian Aquifer (“Southeastern aquifer”) Extends across much of the southeast In Georgia, upper aquifer is in the Ocala limestone Aquifers of Egyptian Western Desert 1. Nubian aquifer 2. Oases of: a. Baharia b. Farafra c. Dakhla d. Kharga e. Siwa The Atlantic and Gulf Coast states are characterized by Tertiary and Cretaceous rocks dipping uniformly toward the ocean. Water enters permeable beds where they are exposed and becomes confined down the dip to form a large artesian system Nubian aquifer lies under the desert. Due to faults in the surface, the confined aquifer becomes connected to the air (thus becoming unconfined at that point) and forms oases in the desert because the water rises to the surface Recharge area is in the south of the desert Fluvial Systems The slope Catena with laminar and turbulent flow Sheetflow becomes rillflow and then as rills combine gullies are formed and ultimately streams and rivers Within a drainage basin: Watershed / interfluve – high point of water flow, like a roof on a house (water flows in both directions from the peak) Laminar flow – layered flow (like when a sheet of water flows down the windshield when its raining and you come to a stop) Upper layers sheer across the lower layers Linear flow Occurs with very thin layers As you move down the slope, amount of water is increasing Each section of a hill receives its own direct rain fall and also the overland flow of water from the prior section Hilltop is the driest part and the base of the hill is the wettest After laminar flow, the flow becomes turbulent Causes more erosion Gullies can be 1-2m deep Often flow into bigger streams and then stream flow into rivers Catena – entire system of gradation Hydrolic - drainage basin Vegetation – grass shrubs Increase in flow further down the hill is reflected in the vegetation, more dense vegetation further down the hill due to more water (grass near the hilltop, shrubs at the bottom) The drainage Basin – Strahler Ordering System This is a dendritic or tree-like drainage pattern Areas between stream valleys are interfluves Head waters are first order stream When two first order streams combine, it becomes a second order stream A stream only becomes larger in order if two of the same order stream are combined (1+1=2, 1+2=2, 2+2=3) First order: right at head of stream, smallest of streams Dynamics of Streamflow 1. Discharge – cubic meter /sec (base flow is flow supplied by groundwater) 2. Stream gradient – slope of the stream a. headwaters 50m/km b. Mississippi gradient is 1-2cm/km (very small) c. Steep gradients in headwaters will cause more erosion in the beginning of the stream, thus the remaining sections of the stream will remain fairly 3. Velocity – few cm/sec t 10 m/sec (35 km/hr) GEOG 11-5-15 Sediment Transport 1. Dissolved load = solution load (5-50%; Mississippi=30%) 2. Suspended load = 10-60% (10% is more typical of mountainous areas where weathering is not very advanced) 3. Bedload: a) Saltation load (7-10% up to 50%) b) Traction load (7-10% up to 50%) dissolved load is 60mg/L which is a very small amount, and therefore is not seen suspended load is made up of visible particles in the water saltation load is when sand particles are carried up due to turbulence, are in suspension for bit, and then when the turbulence slows, they fall once the particle falls, it may hit boulders on the floor of the stream and bounce back up into another current of turbulence within the traction bedload, the water’s force can push the material or roll it over due to water getting under the rock and uplifting it (heavy rocks only really move in an exceptional flood) bedload is typically high in the headwaters of streams higher order streams will tend to have lower bedloads because the boulders become finer over time as they move throughout a system (from lower order to higher order streams) because they bang into each other and are broken up into small pebbles 1. Abrasion of transported sediment leads to rounding and reduction in particle size downstream 2. Stream capacity = amount of sediment carried, where capacity (alpha) (velocity cubed). So, if velocity is doubled, then stream can move about 8 times as much sediment 3. Stream competence = size of largest particle a stream can carry. Also (alpha)(velocity cubed) Sediment transport is relational to the cubed velocity of stream 1. Threshold velocity is the minimum velocity at which a stream can pick up and move a particle of a given size 2. Easiest size to move is 0.6-0.7 mm or fine sand 3. Deposition of gravel occurs at about the same velocity as erosion 4. Deposition of silt ad clay only occurs at very low velocity – hence deposition only in laes and reservoirs or the sea Minimum velocity needed to erode particles is 10cm/sec o At this rate, the size of the particles is roughly 0.6-0.7 mm, which is fine sand o Easiest size to move because of small size As size of particle increases, a higher velocity is needed to erode them Very fine particles are more resistant to water erosion more so than coarser particles Clay is slippery due to charges Velocity for transportation and for deposition are so similar o If the velocity drops even a small bit, then the sand will fall to the floor of the stream Most reservoirs have a lifespan of about 100 years because as water enters and the velocity slows, sediment is deposited into the reservoir and eventually fills it up Hjulstrom curve shows relationship between velocity and particle size in regards to sediment transport (for both water and air) The Graded Stream Profile Adjustments of a stream to re-establish equilibrium Stream erodes faster on steeper slopes and deposits on shallow slopes Once a stream reaches the ocean, there is less velocity and deposition occurs heavily Graded profile – gradual decrease in energy If a fault occurs, water will run down the hill like normal until it reaches the fault scarp (i.e. waterfall or rapids) o The velocity increases much more and can carry much larger sediments and erode more o When the stream gets to the bottom of the scarp, the velocity decreases, and sediment is deposited (gravel will come out first) Solution and suspension load will probably not come out Deposition of coarse materials only Erosion occurs above the fault scarp, and deposition occurs downstream o Eventually balances out the gradient again o Eroded particles get dropped downstream and fill in the fault scarp area Levels out the gradient by moving particles from higher spots to lower spots In glaciated areas, the water contains a lot of rock flour (powder produced from rubbing rocks together) Glaciers carry rocks and force rocks to rub against one another, creating the rock flour in that area The Geographical Cycle: William Morris Davis 1930 Davis suggested a cycle of erosion of landscapes Starting with mountainous landscapes and end with flat surfaces Low energy areas will not experience any erosion Stages (1,2,3) show levels of erosion As stages increase, velocity decreases over time because slope of profile decreases due to erosion As you get closer to sea level, down cutting is not as common Stream is no longer straight, it starts to wonder across valley floor and widens the valley Interfluves are weathered at the same time Natural Levees Before flood: water is constricted to water channel During flood: water level in channel rises rapidly and carries large amounts of sediment over the edge of the channel (large sediment because high velocity) As water goes over edge of channel, velocity drops immediately because flat surrounding land, and drops sediment immediately on edge of channel Deposits form a bank on the water channel The channel alleviates water and eventually the water bursts through the levee and forms another path for a new river, in which will create more natural levees Human-constructed Levees Mississippi River floodplain GEOG 11-10-15 Flow of water around meander bend Water velocity is faster on the outside of the bend Water hits bank on outside curve and is redirected to follow the curve Water erodes on the outside of the bend due to water rushing into bank Creating deeper river beds on the outside curve of the bend As water flows through the channel, the water moves elliptically within the flow Water on the inside of the bend, water is much slower As water velocity decreases around the bend, large sediments are deposited on inside of bend The area inside the bend that is the point of deposition is the point bar Lines and ridges on the point bar is referred to as scroll bars Due to a compilation of floods over time ever-increasing on the point bar further inland each time Major Features of a Floodplain Floodplain- covered by water during floods Floodplains are clay-rich, and because clay does not let water permeate through it, the water remains in the floodplain for a while Higher parts of floodplain: levees and sand Lower parts: backswamps Meander – large bend in the river where stream may actually flow up-valley Point bar / scroll bar- deposits on the inside of the bend due to slower flow Oxbow lake – cutoff meander Meander loop is isolated and eventually plants grow and fill in the opening on one end Natural levee- embankment adjacent to the stream channel due t deposition of sand during flooding Backswamp- low area of floodplain at lower elevation than levees Yazoo streams- parallel to the main channel in backswamp area until they can join main stream The low areas of the floodplain (backswamp) prevent streams from reaching water channel because it is at a higher elevation than the floodplain Streams cannot flow across floodplain directly towards the main channel because it would have to flow uphill Braided stream- produced by streams with a great deal of sediment, particularly coarse sediment Stream piracy one stream can “capture” another stream one stream at a lower elevation on shale will work its way through the escarpment (which it currently is flowing parallel to) and eventually intersect another stream on the upper-level escarpment the upper-level stream will be diverted into the newly eroded path and the remaining part of the stream down flow of the diversion continues as beheaded stream Brevard Fault Zone, Georgia Intense zone of faulting Rivers that originally flowed across the fault zone from north to south in Georgia were captured by the Chattahoochee River Now, the rivers that originally flowed across, stop at the Chattahoochee and start again south of the fault zone Evolution of Stream Terraces A) Stream cuts a valley by normal downcutting and headward erosion processes B) Changes in climate base level or other factors that reduce flow energy cause the stream to partially fill its valley with sediments, forming a broad, flat floor C) An increase in flow energy causes the stream to erode through previously deposited alluvium. A pair of terraces is left as a remnant of the former floodplain D) The stream shifts laterally and forms lower terraces as subsequent changes cause it to erode through the older valley fill A terrace is an ancient floodplain that has been incised by the river, it is higher than the current river In a floodplain, a river erodes the ground, and works its way down Peneplanation and Pediplanation 1. Peneplanation (Peneplain) – William Morris Davis His model can be explained by looking at a cross-section of an interfluve (a watershed divide – the ridge that sends water in two directions) Occurs to humid areas due to downwasting On a hill, there are tress and other vegetation covering the soil atop the hill Moisture is retained evenly across the interfluve, causing even erosion of the interfluve Downwasting – erosion of interfluve further and further down 2. Pediplanation (Pediplain) – Lester King Occurs in arid/semiarid areas due to constant slope retreat Constant slope retreat- interfluve did not get low as fast as it gets smaller As interfluve is eroded, it mainly moves inward towards the center rather than downward There is no vegetation on hilltops, therefore water flows to the edges and is held there by vegetation that grows near the edges. Water does not erode downward because it is not held at the hilltop, but rather erodes inward from the sides because water is concentrated at the edges due to vegetation only growing there Erosion of Surfaces and Isostasy Erosions surfaces can be flat land that is actually over very folded and faulted land Isostasy- the rise and fall of the land Sedimentoisostasy- Glacioisostasy- When land is loaded with ice, the land is depressed (more so where the ice is thickest) Isostastic rebound- when land rises after ice has melted Rises due to less weight of ice Rebound occurs higher where ice is thicker (works as a gradient) As rebound increases across the land, it causes the basin to tilt Water level at one of the basin rises, while dropping at the other end Stream and Landscape Rejuvenation Kick point- the point at which the erosion stops due to a resistant rock i.e. Niagara Falls GEOG 11-12-15 Stream Drainage Patterns Dendritic Tree like Typically forms on homogenous rocks Rectangular More common Control of stream position by faults and joints (weak areas the stream can erode quickly – adjust underlying bedrock) Parallel One prominent drainage orientation Trellised Very obvious lineation of the stream Deranged River flows all over the place Not controlled by structure Rock Control of Drainage Patterns Consequent - Direction is determined by slope of surface Subsequent - Resequent – stream that flows in the direction of the consequent Obsequent – stream that flows opposite to consequent END OF NOTES ON STREAMS KARST LANDFORMS Disappearing streams, springs, and other quirks of nature Definition: landscapes formed mainly by the process of solution although other processes can be involved (e.g. mechanical weathering, fluvial, periglacial, glacial, arid) Characteristics: 1. Caves a. Mammoth-Flint Ridge Cave system, Kentucky (more than 250 miles long) b. Ellison’s Cave, Georgia (400ft vertical entrance) 2. Closed depressions colloquially called “sinkholes” 3. Sinking streams and springs. Underground water is common and the largest springs in the world are in karst areas Common Rock Types: 1. Limestone – CaCO3 2. Dolomite – CaMg (CO3)2 3. Gypsum – CaSO4 4. Halite – NaCl In caves, higher levels are the oldest, while lower levels are the youngest because water table works its way down More than 12% of the earth’s surface is covered by karst landforms Classic area for karst is in eastern Europe (Slovenia and surrounding areas) The Solution Process CaCO3 + H2O + CO2 Ca (HCO3)2 H2CO3 Carbonic Acid Pure water can dissolve 15 mg/liter of Calcium Carbonate (CaCO3) Water in contact with atmosphere (all rain water) (assume 0.03% CO2 by volume) will dissolve 65 mg/liter BUT, some spring waters have about 350 mg/liter CaCO3 AND some have 1,000 mg/liter of dissolved salts like CaSO4 and NaCL Soil CO2 Biogenic CO2 1. Microbial decay of organic material 2. Root respiration a. Elevates level of CO2 in soil because roots give off CO2 at certain times of the day 3. Atmospheric CO2 = 0.03% 4. Soil CO2 = 1% up to 20% At 20% CO2 concentration in soil, will kill plants Water in contact with atmosphere can dissolve limestone quicker Rain falls to ground, percolates through soil and picks up more CO2 and becomes much more acidic and then runs into limestone and chemical reaction is almost immediate Cave Deposits Flowstone: Travertine – Can be date up to 500,00 years Dripstone: Stalagmite – growing up from ground Stalactite – growing down from ceiling Column – stalagmite and stalactite that have joined together Shield – similar to a stalagmite but with a broad head After a solution cave develops, water level drops When the cave is air-filled, water can pass through the soil and migrate to the cave Formation of Speleothems CaCO3 + H20 + CO2 Ca (HCO3)2 Water droplet forms at a crack in the rock Water droplet is at 1.0% CO2 while surrounding atmosphere is at 0.03% Degassing of CO2 causes precipitation of calcium carbonate (degassing occurs to create equilibrium (of CO2) between atmosphere and water droplet) Carbonate will precipitate on nearest surface (where drop touches ceiling) Stalactites, Stalagmites, and Tufas Water droplet is beginning of straw stalactite Straw breaks and water runs down outside to form pointy stalactite Drips form point stalactite form broad stalagmite when water hits the cave floor and splashes Waterfall tufa: turbulence at waterfall speeds up CO2 degassing and precipitation of CaCO3 on rocks and vegetation As water moves down falls, it is aerated (water folds in oxygen and traps air bubbles in water) In spring tufas, degassing also occurs and forms tufa around the spring Waterfall tufas usually occur downstream of streams GEOG 11-17-15 Karst Hydrology 1. Single aquifer a. Water body is uniform within the ground b. One unit of water c. Everything would be polluted in a case of contamination 2. Multiple aquifer a. Exactly like the water pipe system of a house i. No mixing of water for different sources and destinations 3. Compromise aquifer a. In karst areas, many drilled wells are totally dry b. Some wells provide abundant water Scientists in southern England wanted to learn more about sinking streams The streams sink into a cave that cannot be accessed They wanted to follow the spring but could not They put dye in each sinking spring and observed the springs when they came out of the escarpment Dyes came out in an order which they had not expected Researchers found that springs under ground did not mix because each dye came out separate Each cave system was an individual aquifer and paths for water cross over each other but never mix conduit recharge – water sinks into an underground cave at specific points When the water comes back out, it is called resurgence Diffuse recharge – water sinks into soil everywhere When the water exits the springs, it is called exsurgence Compromise Model 1. Starts with single aquifer situation 2. Multiple aquifers, or caves, extend and become integrated producing a single aquifer although there is no true water table Multiple Integration Single This has happened in the Ocala Limestone aquifer of SW Georgia Fracture traces – Karst Water Table 1. This is a integrated aquifer where the level of water at one point influences the level at other points 2. Pollution at one point can also affect other parts of the aquifer Karst Landforms 1. Most landscapes in the world are dominated by fluvial valleys 2. In contrast, karst areas are dominated by closed depressions a. Dolines (colloquially called “sinkholes”) b. Uvalas – formed by the coalescence of two or more dolines c. Poljes – the largest of the karst depressions i. “field” ii. biggest pijoles is about 65 km Dolines 1. collapse} 2. solution} develop in bedrock 3. suffosion } 4. subsidence} developed in overburden 1. Cenotes are common in the Yucatan Peninsula of Mexico 2. Cenotes are collapsed dolines that contain water at the ground water table level A collapse will often seal the depression to where you cannot get further into the opening past the fallen rock The doline will get bigger over time as walls fall in GEOG 11-19—15 2. Solution Dolines developed by water sinking under ground, weathering limestone, creating depression occur where there is intense cracking and fractures in limestone reduced size of joint blocks depression sinks down when rocks decrease in size runoff into depression and water is focused on it creates a pond and water escapes underground and comes out as a spring soil is usually clay-rich all limestones have insoluble materials that produce clay when limestone is dissolved clay covers base of doline causes water to pond up above clay and create a pond doline different from cenotes in that solution dolines are created by water getting into cracks and moving down and weathering the opening, cenotes are produced by caves underground collapsing KARST LANDFORMS 2 3. Suffosion doline suffosion is the mechanical washing of sediment by water to create a cavity water can move a lot of sediment over time because of velocity movement of sediment can create holes or pipes = called “piping” underneath overburden (insoluble), there is limestone (soluble) in the limestone, caves are developed upper surface of limestone becomes depression in rock sediment is accumulated at bottom of cave over time depression drags down overburden and causes a funnel-shaped hole because it is now sloping in and being washed into the cave (sometimes visibly) 4. Subsidence Doline typically formed rapidly and catastrophic original water table was so high that all of the cave used to be water filled causing an upward force on ceiling of cave and allowing overburden to de damp and stick together new water table is low more water is being used than is being recharged (due to irrigation) lower part of cave is still filled with water but upper part is filled with air water table no longer supports ceiling water makes its way through overburden, and then reaches ceiling of caves and drips through into cave through air filled portion dripping causes erosion of ceiling weight of surface above collapses the overburden and everything falls into to the cave WINTER PARK, FLORIDA Subsidence dolines in the West Rand of South Africa Area famous for gold River flows down valley Broad, flat valley floor Ridges of higher rock to north and south of valley Underlying rock is dolomite (soluble) Dolomite is crossed by cyanite dikes As water sinks into overburden, It fills up until it spills over dike Miners have to pass through dolomite to get to the gold reef But if water fills up the dolomite, it has to be pumped out to open up the mine Major collapse of surface into underlying cave Overburden slumped into sinkhole West Driefontein Mine Disaster 1. To mine gold from the Olberholzer Compartment, they pumped water from the dolomites 2. On December 12, 1962, a three-story high crusher plant was absorbed by a sinkhole and 29 people died 3. In the mid-1960s they decided to extend tunnels through the Bank dike into the Bank Compartment to access the gold reef there 4. Between October 26 and November 18, 1968, 2,000 million gallons of water entered the West Driefontein Mine through the tunnels in the Bank Compartment 5. The film “Gold” starring Roger Moore was based on this tragedy Sinking stream and Blind Valleys Autogenic water = rainfall 1. Dry valley 2. Semi-blind valley – flow beyond the threshold only during floods 3. Blind Valley never flow beyond the valley 1. Disorganized stream system – produced by water being lost to underground drainage routes along solutionally-enlarged fractures in the limestone 2. Water is “pirated” underground Blind valley – all water goes underground Semi-blind valley – Characteristics of limestone is valleys with no streams in them Karst Uvalas and Poljes 1. The word “polje” is the Slavik word for field 2. Livino Polije in Slovenia is 65 km long 3. Grassy Cove in Tennessee is one of few depressions in the USA that might be polije Formation of Poljes from dolines: 1. Uvalas form by the gradual coalescence of dolines to form composite depressions called uvalas 2. At a later stage uvalas grow and coalesce to form polies 3. Poljes formed in this way are generally small, just several km 2 in area Poljes flood often, and therefore must be fairly close to the water table 1. The largest poljes form from tectonic or structural depressions 2. If the rocks around the depression are soluble they develop underground drainage routes via solution caves and thus preserve the initial form of the depression GEOG 12-1-15 Morphology and Hydrology of a Typical Polje Shale on top of limestone Shale does not dissolve or transmit water very readily After rain stream of water flow from shale onto limestone (allogeneic streams) If rain and allogeneic flow is high enough, caves will fill up As flooding occurs, sediment is evenly placed in a flat manner Leaves, twigs, even ice, block the drainage systems of ponors and lake develops (similar to a bathtub being blocked by hair) Estavelle – reversing sink spring When conditions are relatively dry, the water drains into ponor (sink) When conditions are extremely wet, water flows out of ponor due to saturated caves underground (stream) Tropical Karst Landforms Temperate Karst – Dolines 1. Dolines are circular or elliptical in shape 2. There is an upper surface and the dolines are formed in it 3. You can walk around the dolines on the plateau Tropical Karst – Dolines, Cockpits, Cones, Towers 1. Cockpits are star-shaped and surrounded by 3-4 conical hills 2. There is no upper surface just a series of hills 3. The cone-cockpit relief is generally more accentuated than with dolines 4. Peak clusters are called Fencong in China 5. Isolated peaks are called Fenglin in China Evolution of Tropical Karst Comical Karst: Shape of hills due to short vertical joints and well-developed bedding planes. Weathered residue accumulates in cockpits and erodes bases of hills Why does cockpit and tower karst form only n the himud tropical areas? 1. High rainfall means a lot of water for solution and dissolving limestone 2. High temperatures speed up chemical reactions 3. High rain and temperatures means a dense vegetation cover and a lot od CO2 4. There, denudation more rapid I the humid tropics Problem is: 1. Denudation is actually higher in temperate areas rather than tropical areas, so what now? 1. Rainfall intensity is probably important because this determines the distribution of solution across the landscape 2. In doline areas solution is fairly evenly distributed so only shallow depressions are produced 3. In the tropics, rainfalls are intense and focused on depression because of rapid runoff. Hence deeper depressions forming hills nearby END OF NOTES ON KARST LANDFORMS GLACIERS Types of Glaciers 1. Ice sheet – can be very thick a. Antarctic b. Greenland 2. Ice Cap – can be very extensive a. Mountain i. i.e. Columbia Ice Field, southern Canada b. Lowland i. i.e. Barnes Ice Cap, Canada 3. Valley Glacier a. Alpine i. Including cirque glaciers e.g. Teton Glacier, Wyoming b. Outlet i. i.e. Athabasca Glacier, southern Canada Types of ice: 1. Polar ice – frozen to its base, temperature well below the pressure melting point 2. Temperate ice – may have meltwater at it’s base, temperature close to the pressure melting point Types of Valley Glacier Cirque glaciers are preferentially oriented in the northeast (because it’s the coldest orientation) Extent of Pleistocene Glaciation in the Northern Hemisphere Ice sheets: Laurentide Ice sheet Scandanavian Ice Sheet Siberian Ice sheet The shoreline moved as much as 200 km seaward along the Atlantic coast of USA Sea ice covered most of the Arctic Ocean and extended into the Atlantic well to the south of Iceland. Ocean temperatures dropped by ca. 10 C. Glacial Modification of Drainage 1. Prior to the Pleistocene Ice Age drainage was mainly towards Hudson Bay 2. After glaciation much of this drainage was diverted to the Mississippi River area
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