Transcript Lecture 20
Weathering and Erosion Formation of Sedimentary Rocks • Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) of rock at or near Earth’s surface • Erosion – the physical removal of material by agents such as water, wind, ice, or gravity insoluable basalt (Mg,Fe)2SiO4 (Mg,Fe)SiO3 pyroxine H4SiO4 in solution Mg2+ in solution Fe (III) hydroxide (insoluble, rust) CaAl2Si2O8 Ca-feldspar and NaAlSi3O8 Na-Feldspar Ca+2 in solution Na+1 in solution Al2Si2O5(OH)4 (insoluble, “clay”) granite SiO2 quartz SiO2 (insoluble, “sand”) CaAl2Si2O8 Ca-feldspar; NaAlSi3O8 Na-Feldspar KAlSi3O8 K-Feldspar Ca+2 , Na+1, K+1 in solution Al2Si2O5(OH)4 (insoluble, “clay”) (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH)2 amphibole (and also mica) Mg+2, Ca+2 , Na+1 in solution Al2Si2O5(OH)4 (insoluble, “clay”) Fe (III) hydroxide (insoluble, rust) Climate and Weathering Hot and wet favors chemical weathering Cold and snowy favors mechanial weathering Differential Weathering and Erosion creates topography Slowly weathered and eroded - high (Morningside Heights, Palisades, Ramapo Mountains) Quickly weathered and eroded - low (sediments beneath Hudson River and west of Palisades) uplift erosion Hill formed by differential erosion Residual topography Clastic Sediments and Clastic Sedimentary Rocks A. Sediments B. Sedimentary Rocks Energy and Depositional Environment Worldwide sediment yield of major drainage basins crossbed from fieldtrip Migration of meanders leads to cross-bedding Cross-section of Delta note that delta grows (progrades) towards sea Hjulstrom Curve Hjulstrom Curve Pebbles and cobbles: hard to get moving, an hard to keep moving Pebbles and cobbles Hjulstrom Curve Sand Sand: easy to get moving, a fairly easy to keep moving Hjulstrom Curve Silt and Clay Silt and Clay: hard to get moving, but very easy to keep moving Ocean Sediments Part 1 Evapotite: common during with continental rifting Fossil Fuels Solid Earth System petroleum Organic-rich source rock, e.g. shale Maturation through burial at the right temperature Collection in a porous reservoir rock Concentration in trap through buoyancy Formation of Ores Some unusual process must: 1) remove specific elements, compounds or minerals from ordinary rock, 2) transport these elements, compounds, or minerals 3) concentrate the elements, compounds, or minerals preferentially at one spot or zone where the transport stops. the primary mechanisms for concentrating minerals into ores involves either: sorting by density sorting by solubility. Concentration through liquid immiscibility High T Low T Desirable element preferentially concentrated into low-volume melt Aqueous fluids in magma As magma cools, the volatiles (mostly water and carbon dioxide) that they contain can form super-critical fluids. supercritical fluids are on the verge of making the phase transition from liquid to gas. because of their extremely high temperature, many elements are soluble. These fluids can concentrate copper, molybdenum, gold, tin, tungsten and lead. The fluids from a large pluton can invade surrounding rocks, along cracks called hydrothermal veins). Aqueous fluids from granitic magma have invaded surrounding rock porphery copper ore Mechanisms that involve oxidation state of the water Ground water can carry dissolved materials. These can precipitate out of solution if the water becomes more or less oxidizing. Example: uranium ore soluable U6+ is produced during the weathering of igneous rocks. U6+ was transported by groundwater until it encounters reducing conditions. It is reduced to U4+ and precipitates as uranium oxide. Mineral Commodities Solid Earth System Geothermal Power 6.5 km – expensive but routine, areas of western US are hot Solution to low permeabiliy Artificially increase permeability by creating fractures “Hydrofracture” … pressurize well until you crack the surrounding rock, routinely used in oil extraction, at least for small volumes of rock Fresh Water Possibly the most Limiting Resource US Water Usage, billion gallons / day Public Supply Domestic Supply 27.3 0.6 Irrigation Livestock & Aquaculture Industrial Mining 80 3.4 14.9 1.2 Thermoelectric Power 135 Total 262 How much irrigation water does the world need? 2000 calories/day minimum At 3 cal/liter 670 liters/day 6 billion people 365 days/year = 1.46 1015 liters/year = 14700 cubic kilometers per year About 46,000 cu km available Global impoundments of water 8400 km3 Not much growth in last decade, except in AsiaAustralia Good luck with the final best wishes for 2009