5Introduction: Earth’s Varied Landscapes Uplift:Processes that raise topographyByproduct of plate tectonics and thermal convection
6Introduction: Earth’s Varied landscapes Exhumation and denudation are closely related terms that imply different perspectives on erosional processesExhumationGradual exposure of subsurface rocks by stripping off (eroding) the surface layersDenudationThe transport of eroded material to another locationGeomorphology:The study of Earth’s landforms.
7Factors changing the landscape Interplay between the forces that raise the lithosphere (tectonic and isostatic uplift, volcanism) and the forces that level it (gravity, physical and chemical weathering processes)Is steady state reached?What is the topographic consequence when uplift dominates over denudation or vise versa?
8Factors Controlling Uplift Uplift occurs as a byproduct of mantle convection and plate tectonicsWe can identify several types of uplift:Collisional upliftIsostatic upliftExtensional uplift
9Collisional UpliftThe most natural uplifted landscape would be a long, narrow orogen that forms along a convergent plate boundaryResults from compression of relatively light crustal rock atop a continental plateWhen an oceanic plate subducts beneath a continental plate, an orogen can form from a rising wedge of marine sediments scraped from the downgoing plateThe island of Taiwan and the Olympic mountains of Washington State are examples of such orogens
10A: Collisional uplift in subduction zone; B: Isostatic uplift following the loss of a cool dense lithospheric “root” of a collisional mountain range; C: Isostatic uplift over a rising mantle plume; D: Extentional uplift
15Isostatic UpliftWhen continents collide, their plate boundaries tend to crumple somewhat, so the lithosphere locally thickensThe dense mantle lithosphere of a thickened plate boundary can peel off and be replaced by hot buoyant asthenosphereIsostatic uplift occurs locally wherever a hot, buoyant mantle plume rises to the lithosphere, as beneath the Hawaiian Island or Yellowstone National ParkUplift will occur to compensate for eroded materials
16A: Collisional uplift in subduction zone; B: Isostatic uplift following the loss of a cool dense lithospheric “root” of a collisional mountain range; C: Isostatic uplift over a rising mantle plume; D: Extentional uplift
18Isostatic UpliftMany geologists argue that the dense mantle lithosphere of a thickened plate boundary can peel off and be replaced by hot buoyant asthenosphere, followed with isostatic upliftIsostatic uplift occurs wherever a hot, buoyant mantle plume rises to the base of the lithosphere, as beneath the Hawaiian Island or Yellowstone National Park.
19Evolution of the Hawaiian Chain Midocean volcanic island rises up as lava extrudes from the seafloor at a hot spot.Underlying ocean crust subsides isostatically, limiting the maximum altitude of volcanos.Pacific Plate carries the volcanic island northwestward beyond the magma source.
20Extensional UpliftExtensional uplift occurs in regions where the ascent of hot buoyant mantle at the base of continental lithosphere induces broad uplift and stretching of crustIsostatic uplift can induce short-scale topographic relief by extension and normal faultingBathymetric scarps parallel the midocean ridgesThe Grand Teton Range of northwest WyomingThe Basin and Range Province (Nevada, Utah, Arizona)Horst-and-graben structures
21A: Collisional uplift in subduction zone; B: Isostatic uplift following the loss of a cool dense lithospheric “root” of a collisional mountain range; C: Isostatic uplift over a rising mantle plume; D: Extentional uplift
22Rapid uplifting mountain range in Wyoming, caused by a regional crustal extensional that surrounds the Yellowstone hot spot
28Factors Controlling Denudation: Climate Streams may be the primary agent that moves and deposits sediment in humid climatesWind assumes the dominant role in arid regionsGlaciers are dominant in high latitudes and high altitudes regionsClimate controls vegetation coverModern landscapes may reflect former conditions, such as glaciation, rather than current processes
29Factors Controlling Denudation: Lithology Some rock types are less erodible, and thus will produce greater relief and steeper landformGranite or hard sandstone can maintain steep slopes for a long timeSoft shale easily erodes into low-relief debrisHowever, a specific rock type may behave differently under different climatic conditionsLimestone in a moist climate may dissolve and erode into a terrain of sinkholesIn a desert, the same limestone may form bold cliff
30Differential Erosion: ridges are sandstones and conglomerates and valleys are shales
31Factors Controlling Denudation: Relief Tectonically active regions with high rates of uplift generate rapid erosionRelief is low and erosion rates are slower in tectonically inactive areas, but a rapid change of sea level or small regional isostatic movements may significantly change the denudation rateErosion in high-relief terrain. A stream transports coarse-gravel contributed largely by mass-wasting.(Hindu-Kush of northern Pakistan)
32Tectonic and Climatic Control of Continental Divides The line separating any two major drainage basins is a continental divideIn North America, continental divides lies at the head of major streams that drain into the Pacific, Atlantic, and Arctic oceansBecause continental divides often coincide with the crests of mountain ranges, and because mountain ranges are the result of uplift associated with the interaction of tectonic plates, a close relationship exists between plate tectonics and the location of primary stream divides and drainage basins
34Tectonic and Climatic Control of Continental Divides Climate can also influence the location of the divide.Prevailing winds may cause one side of a divide to be wetter than the other, and therefore more subject to denudation
38The Geographic Cycle of W. M The Geographic Cycle of W.M. Davis (Hypothetical Models For Landscape Evolution)Youthful landscape: streams downcut vigorously into the uplifted land surface to produce sharp, V-shape valleys, increasing the local reliefMature landscape: meander in their gentler valleys, and valley slopes are gradually worn down by mass wasting and erosionOld-age landscape: broad valleys containing wide floodplains, stream divides are low and rounded, and the landscape is slowly worn down closer to sea level
39Potential factors conflicting with Davis’s hypothetical cycle: Land equilibriumRelative contribution of uplift and erosion through timeComplex feedback among the atmosphere, hydrosphere, biosphere and lithosphere
40Steady-State Landscapes A landscape that maintains a constant altitude and topographic relief while still undergoing uplift, exhumation and denudationIt is not common for a landscape to achieve a steady state because of:A tectonic event that lifts up a landmassA substantial sea level dropA climate shiftA stream that erodes downward through weak rock and encounters massive, hard rock beneath
41Steady-State Landscapes If rates of denudation and uplift are stable for long periods, the landscape can adjust itself to a steady stateA steady state landscape maintains a constant altitude and topographic relief while still undergoing changeIf uplift ceases altogether, the average rate of change gradually decreases as the land surface is progressively eroded toward sea level
42Steady state equilibrium Dynamic equilibriumDynamic equilibrium with sudden changes whenever a threshold is reached; e.g. slope failure
43Rapid landscape Changes: Threshold Effects Sudden changes can occur once a critical threshold condition is reachedA threshold effect implies that landscape development, rather than being progressive and steady, can be punctuated by occasional abrupt changesA landscape in near-equilibrium may undergo a sudden change if a process operating on it reaches a threshold level
44How Can We Calculate Uplift Rates? Three methods are used to calculate uplift rates:Extrapolation from earthquake displacementsMeasurement of warped strataMeasurement of river terraces
45Extrapolation from Earthquake Displacements Measure how much local uplift occurred during historical large earthquakesExtrapolate the recent rates of uplift back in timeThis is based on the risky assumption that the brief historical record is representative of longer intervals of geologic timeOrganic processes can supplement historical record.In New Zealand, for instance, lichens grow on newly exposed rock surfaces at a predictable rate for several centuries, providing information about the age of the surface
46Calculation from Warped Strata This technique measures the warping (vertical dislocation) of originally horizontal strata of known ageFor this method, we need to know:The altitude difference between where the stratum is today and where it was at its formationA radiometric age for the stratum
47Calculation from River Terraces Rivers tend to carry material eroded from steep slopes and deposit it along flatter portions of river valleysIn an uplift region, a river may form a sequence of abandoned depositional surfaces called river terracesAs uplift raises a river valley, the river erodes into its former depositional surface and its terraces are abandoned
48Calculation from River Terraces If we can determine the age of the last sediment deposited, abandonment times and uplift rates can be calculatedHowever, interpretation requires some caution:Climate changes can alter streambed erosion rates and cause river terraces to form as wellMeasured uplift rates in tectonic belts are quite variable through time
52Measurement of exhumation rate: Radioactive - Decay Techniques Determination of the time when the rocks were located at a specific depthSome radioactive products, such as fission tracks and radiogenic helium, will not accumulate if the rock is too hotThey are retained only if the rock is cooler than a certain critical temperature, called the closure temperatureRadioactive decay products use thermal energy to escape their host rock
53Closure temperature and diffusion A spontaneous net atom-by-atom migration along a concentration gradient diffusion process
54Radioactive - Decay Techniques Each decay product has an independent closure temperatureEscape is easier if the decay product is a noble gas, like helium (He) or argon (Ar)Fission tracks damage the crystalline structure of minerals, but this damage will repair itself by annealing if the temperature is sufficiently high
55Zircon (Tc = 240oC) and apatite (Tc = 110oC) accumulate fission tracks The number of tracks increases with time, so track density can be used to date the timeThe length distribution reflects the cooling rate
56Cosmogenic NuclidesCosmogenic nuclides are radioactive isotopes that are not present in a rock at its formation, but rather are generated by high-energy subatomic particles emitted by the sunCosmic ray convert some atoms to rare isotopes, such as 36Cl, 26Al, 10Be, but the accumulation of cosmogenic nuclides is limited by the depth of burial.Their abundance in a surface rock can tell us how long a rock has been exposed to such radiation
58Denudation RatesTo calculate long-term denudation rates, we must know how much rock debris has been removed from an area during a specified length of timeFor areas drained by major streams, the volume of sediment reaching the ocean each year is a measure of the modern erosion rateWe can estimate the volume of sediment deposited during a specific time interval by using drill-core and seismic records of the seafloor
61World Sediment Yields The highest measured sediment yields are from: The humid region of southeastern Asia and OceaniaBasins that drain steep, high-relief mountains of young orogenic belts, such as the Himalayas, the Andes, and the AlpsLow sediment yields are from:DesertsThe polar and subpolar sectors of the northern continents
62World Sediment Yields Structural factors also play a role Rocks that are more highly jointed or fractured are more susceptible to erosion than massive rock formationDenudation is surprisingly high in some dry-climate regions, often because the surface lacks a protective cover of vegetationIn south-coastal Alaska, as in the most extensively glaciated temperate mountain regions in the world, denudation rates exceed those of comparable-sized nonglaciated basins in other region by factor of 10 or more
67Human Impact on Denudation Humans increase the rate of denudation by:Clearing forestsDeveloping cultivated landDamming streamsBuilding cities
68Human Impact on Denudation Areas cleared for construction produce 10 to 100 times more sediment than comparable rural areas or natural areas that are vegetated.The high Aswan Dam on the Nile River intercepts most of the sediment that the Nile previously carried to the Mediterranean sea
69Intensively farmed since the dawn of human history, topsoil is scarce now
70Ancient landscapes can be found in shield areas in Australia where ancient mountain roots have been exposed. Changes in plate motion lead to cessation of uplift, then long-term erosion gradually lowered land surface to low altitude.
71Extensive ancient landscape with low relief is preserved as widespread unconformity
72The Carbon CycleThe carbon cycle on Earth is driven by plate tectonic motionsUplift and exhumation are among the most important factors in determining the level of CO2 in the atmosphere
73Natural sources of atmospheric CO2: Volcanic outgassing and metamorphism of carbon-rich ocean sediments.Removal of CO2 from atmosphere: Weathering of silicate rocks and burial of organic carbon
74The Carbon Cycle: Sources of CO2 Global volcanism adds CO2 to the atmosphere from Earth’s interior.CO2 is released from metamorphism of carbon-rich marine sediments carried downward in subduction zones.Utilization of oil gas and coal.
75The Carbon CycleCO2 is removed from the atmosphere by weathering of surface silicate rocksRainwater combines with CO2 to form carbonic acid (H2CO3), and this acid weathers silicate rocks:CaSiO3 + H2CO3 CaCO3 + SiO2 + H2O
76The Carbon CycleLand plants synthesize other carbon-based acids in their roots to leach chemical nutrients from rocksStreams transport weathering products to the oceanThere the carbonate and silica are used by marine organisms and their remains accumulate on the seafloor, stored as sedimentHigh spreading rate -> high CO2 emission -> warm up the Earth’s surface -> Accelerate the weathering -> Increase CO2 removal
77The Carbon Cycle: An Alternate View An alternative hypothesis: uplift could increase the removal rate for atmospheric CO2, because it generates faulting, which exposes fresh, fractured rock to chemical weatheringA trend over the past 100 million years toward cooler global temperatures and reduced atmospheric CO2 has occurred at the same time as the uplift and erosion of several major continental plateaus, including Tibet in central Asia, the Altiplano in South America, and the Laramide uplift (Rocky Mountains) in North America