Transcript Lecture 10
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 Sediment: weathered material derived from pre-existing rocks Sedimentary rock: consolidated sediment (compacted, cemented) plus fossils Clastic sediments seen during fieldtrip Clastic sedimentary rocks seen during fieldtrip 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) in quartz sand in clay in rust River sediments are consistent with the composition of the continental crust Effect of surface area on weathering Climate and Weathering Hot and wet favors chemical weathering Cold and snowy favors mechanial weathering Why erosion is important • • • • Life in the sea depends on it to supply critical nutrients. It is responsible for the salt content of the sea. It is the source of the basic materials to form sedimentary rocks. It continually reduces and shapes the surface of the land. Sea water Where’s the Cl come from? Why no silica? Mechanisms of Erosion Mass Wasting The often catastrophic (geologically rapid) movement of material on the Earth’s surface is referred to as mass wasting. Such features constitute the most widespread of the natural geological hazards. the Slumgullion earthflow/landslide, Hinsdale County, CO Nevado Huascaran, Peru, 1970 It began high in the mountains as an earthquake-induced avalanche of snow and ice, but picked up glacial sediment on its way. It hit the towns of Yungay and Ranrahirca, 18 km away, at around 150 km/hr. The former town was completely buried. An astounding 66,000 people were estimated dead as a result of this massive debris avalanche. Mudslides: Costa Rica, June, 2000 The June 27 slide killed 10. Six were killed in the same place in 1993 mudslides. landslides This is a typical landslide. Note that materials hold together in more-or-less singular blocks. As is common, when the slide blocks reach a lower slope, they break up (becoming a debris flow in this case). The La Conchita landslide, near Santa Barbara, CA, Spring, 1995. Glaciers (mechanical) weathering Freezing action Freezing action Frost wedging Joint-controlled weathering Roots (mechanical) weathering Root Wedging Chemical Weathering Chemical 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) Differential Weathering Differential Weathering Differential Weathering Differential Weathering Differential Weathering Resistant cap rock Clastic Sediments from “clast” … little piece Can be associated with rivers, glaciers, wind Clastic Sediments and Clastic Sedimentary Rocks A. Sediments B. Sedimentary Rocks Energy and Depositional Environment Coarse-Grained Sediments Breccias Conglomerates Tillite Brian J. Skinner Worldwide sediment yield of major drainage basins Where erosion occurs … elevation a factor … higher more erosion Meaning of rate 100 tons per sq km per year 1000 meters 100 tons Rock density about 2.5 tons per cubic meter so 100 tons is about 40 cubic meters (a cube 3.4 m on edge) 1 sq km is 1,000,000 sq meters Spread 40 cu meters over 1,000,000 sq meters and get a layer 40/1,000,000 = 0.00004 meters = 0.04 millimeters thick So in 100 years, wear away 4 mm Rivers and Sediments crossbed from fieldtrip Migration of meanders leads to cross-bedding Deltas 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 Human Influence • Human beings move more sediment through mining and building than rivers do. • Agricultural practices also increase erosion in rivers. Rates thousands of years ago were less than today. • Empounded water from lakes behind dams also prevent sediment from reaching the sea. Existence of Lakes • Largely due to glaciation disrupting drainage networks. • North American lakes mostly at latitudes greater than 45°, where glaciers were during Pleistocene. • Will slowly fill up with sediment and disappear Removal from sea water • Particles settle and are deposited as sediment. – Sedimentary rocks consisting of fine-grained particles (< .06 mm) are called shale. – Sedimentary rocks made of medium-grained particles (.06-2 mm) are called sandstone. – Sedimentary rocks consisting of coarse-grained particles (> 2 mm) are called. conglomerates. • Calcium and bicarbonate are removed from seawater by organisms to form shells made of CaCO3 (limestone). • SiO4 is removed from solution by organisms to form silica “ooze” which lithifies to chert (arrowheads). • Mg++ and SO4-- are removed from ocean water as that water is heated and pumped through mid-oceanic ridges. • Na+, K+, and Cl- are removed from seawater sporadically in evaporite deposits. Formation of sedimentary rocks Cementation of grains by CaCO3 or SiO2. • Names of clastic rocks depend on size of grains (big to small). – Conglomerates - tend to be poorly sorted. – Sandstones - often well-sorted. – Shales - made of clay minerals. • Non-clastic rocks: from dissolved load – Limestones - CaCO3, usually removed from water by a biological process (e.g. corals and sea-shells). – Evaporites - NaCl and CaSO4 from evaporation of seawater in enclosed basins (e.g. Utah's Great Salt Lake and the Mid-East's Dead Sea). Shale Formation Bioclastic Limestone Fine-Grained Limestone Deep sea limestones will not have visible fossils, because they are made up of the shells of little dead bugs like these: foraminifera. This specimen (viewed by scanning electron microscope) is about 100 microns long (0.1 millimeter). Coccoliths – another type of organisms with a CaCO3 test Chert Some microorganisms secrete silica shells. When these pile up on the deep ocean floor, they lithify to become a micro-crystalline quartz rock called chert (the same stuff as the substance flint). diatoms and radiolaria Diatoms – organisms with silica tests Radiolaria – another silica test Chert arrowhead Chert beds, now deformed Near Golden Gate Bridge Evaporites Calcite, halite and gypsum are common precipitates from dried lakes in arid environments. the white is halite, the red are bacteria that love salt Owens Valley, CA Evaporites in Death Valley Martin G. Miller