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Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) of rock at or near Earth’s surface Erosion – the physical.

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Presentation on theme: "Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) of rock at or near Earth’s surface Erosion – the physical."— Presentation transcript:

1 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 Weathering and Erosion Formation of Sedimentary Rocks

2 Sediment: weathered material derived from pre-existing rocks Sedimentary rock: consolidated sediment (compacted, cemented) plus fossils

3 Clastic sediments seen during fieldtrip

4 Clastic sedimentary rocks seen during fieldtrip

5 insoluable

6 basalt (Mg,Fe) 2 SiO 4 (Mg,Fe)SiO 3 pyroxine H 4 SiO 4 in solution Mg 2+ in solution Fe (III) hydroxide (insoluble, rust) CaAl 2 Si 2 O 8 Ca-feldspar and NaAlSi 3 O 8 Na-Feldspar Ca +2 in solution Na +1 in solution Al 2 Si 2 O 5 (OH) 4 (insoluble, “clay”)

7 granite SiO 2 quartz SiO 2 (insoluble, “sand”) CaAl 2 Si 2 O 8 Ca-feldspar; NaAlSi 3 O 8 Na-Feldspar KAlSi 3 O 8 K-Feldspar Ca +2, Na +1, K +1 in solution Al 2 Si 2 O 5 (OH) 4 (insoluble, “clay”) (Ca,Na) 2 (Mg,Fe,Al) 5 (Al,Si) 8 O 22 (OH) 2 amphibole (and also mica) Mg +2, Ca +2, Na +1 in solution Al 2 Si 2 O 5 (OH) 4 (insoluble, “clay”) Fe (III) hydroxide (insoluble, rust)

8 in quartz sand River sediments are consistent with the composition of the continental crust in clay in rust

9 Effect of surface area on weathering

10 Climate and Weathering

11 Hot and wet favors chemical weathering

12 Cold and snowy favors mechanial weathering

13 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.

14 Sea water Where’s the Cl come from? Why no silica?

15 Mechanisms of Erosion

16 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

17 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. Nevado Huascaran, Peru, 1970

18 Mudslides: Costa Rica, June, 2000 The June 27 slide killed 10. Six were killed in the same place in 1993 mudslides.

19 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.

20 (mechanical) weathering Glaciers

21 Freezing action


23 Frost wedging

24 Joint-controlled weathering

25 Roots (mechanical) weathering

26 Root Wedging

27 Chemical Weathering


29 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)

30 Differential Weathering





35 Resistant cap rock

36 Clastic Sediments from “clast” … little piece Can be associated with rivers, glaciers, wind


38 Clastic Sediments and Clastic Sedimentary Rocks A. Sediments B. Sedimentary Rocks

39 Energy and Depositional Environment

40 Coarse-Grained Sediments Breccias Conglomerates

41 Tillite Brian J. Skinner

42 Worldwide sediment yield of major drainage basins

43 Where erosion occurs … elevation a factor … higher more erosion

44 Meaning of rate 100 tons per sq km per year 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 1000 meters 1000 m

45 Rivers and Sediments

46 Migration of meanders leads to cross-bedding crossbed from fieldtrip

47 Deltas

48 Cross-section of Delta note that delta grows (progrades) towards sea

49 Hjulstrom Curve

50 Pebbles and cobbles Pebbles and cobbles: hard to get moving, an hard to keep moving

51 Hjulstrom Curve Sand Sand: easy to get moving, a fairly easy to keep moving

52 Hjulstrom Curve Silt and Clay Silt and Clay: hard to get moving, but very easy to keep moving

53 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.

54 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

55 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). SiO 4 is removed from solution by organisms to form silica “ooze” which lithifies to chert (arrowheads). Mg++ and SO 4-- 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.

56 Formation of sedimentary rocks Cementation of grains by CaCO 3 or SiO 2. 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 - CaCO 3, usually removed from water by a biological process (e.g. corals and sea-shells). –Evaporites - NaCl and CaSO 4 from evaporation of seawater in enclosed basins (e.g. Utah's Great Salt Lake and the Mid-East's Dead Sea).

57 Shale Formation

58 Bioclastic Limestone

59 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).

60 Coccoliths – another type of organisms with a CaCO 3 test

61 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

62 Diatoms – organisms with silica tests

63 Radiolaria – another silica test

64 Chert arrowhead

65 Chert beds, now deformed Near Golden Gate Bridge

66 Evaporites Owens Valley, CA Calcite, halite and gypsum are common precipitates from dried lakes in arid environments. the white is halite, the red are bacteria that love salt

67 Martin G. Miller Evaporites in Death Valley


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