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Ch 16 1. 2. 3. 4. 5. Weathering, Erosion and mass wasting Rock Cycle, weathering, and erosion Mechanical weathering Chemical Weathering Soil Mass wasting Rocks are a mix of minerals A rock is classified by its: 1) TEXTURE Size of mineral crystals 2) COMPOSITION Minerals present 3) Process of formation What type of rocks are made from previously weathered rocks? a. b. c. d. Igneous Metamorphic Sedimentary B and C There are 3 types of rocks: divided according to process of formation Igneous: crystals solidify from melted rocks Metamorphic: reactions of minerals from pre-existing rocks changes in temperature or pressure Sedimentary: derived from physical disintegration or chemical dissolution of pre-existing rocks Ch 16 Weathering, erosion, and mass wasting Weathering: Changes that occur in sediments and rocks when exposed at the earth’s surface – Changes are physical and chemical – Atmosphere, hydrosphere, and biosphere all contribute Weathering, erosion and soil Weathering is extremely boring from a spectator point of view But the product of weathering and erosion are spectacular Tim Davis/Photo Researchers Weathering is critical for LIFE ON EARTH Only small fraction of Earth’s land surface has bedrock Most of Earth’s land surface is covered by SOIL the product of weathering What is soil made of? a. b. c. d. Mineral matter, from weathered rocks Gases, from the atmosphere Water All of the above Weathering is a fundamental component of the ROCK CYCLE Weathering and Only small fraction of erosion Earth’s land surface has bedrock Rocks Sediments Lithification and diagensis Weathering and erosion operate together EROSION: incorporation and transport of material by water, wind, ice, etc weathered rock, sediment, or soil Factors Controlling Rates of Weathering SLOW Rock type Minerals present Rock structure High SiO2 climate Cold dry Vegetation and animals Soil limited Massive None Bare rock FAST Low SiO2 and carbonates fractures warm wet extensive soil cover Joint-controlled Weathering Jeff Foott/DRK Fig. 6.11 How do you calculate a weathering or erosion rate Distance = rate Only small fraction of * time Earth’s land surface 200 km = 100 km/hour *erosion 2 hours has bedrock Rate = distance / time If I run a 10 km in 0.75 hours, at what rate am I running? weathering How do you calculate an erosion rate Only small fraction of Earth’s land surface has bedrock weathering erosion Rate = distance / time ? = 20 meters / 5 m.y. Rate = 4 meters per m.y. Time = distance / rate Weathering and erosion operate together Weathering: Changes that occur in sediments and rocks when exposed at the earth’s surface EROSION: incorporation and transport of material by water, wind, ice, etc typically weathered rock Two main types of weathering Mechanical weathering: physical forces break rock into smaller pieces no change in chemical composition Chemical weathering: chemical transformation of rock into one or more new components Mechanical weathering 1. Frost — water expands by 9% when it freezes: freezing expands cracks in rocks 2. Thermal expansion — differential thermal expansion of minerals creates stress in rocks 3. Organic activity — tree roots, animal burrowing, insects, micro-organisms 4. Unloading — decrease in pressure, rocks expand Boulder Fractured by Frost Action Michael Hambrey Fig. 6.13 Role of Organisms in Weathering Peter Kresam Fig. 6.12 Mechanical Weathering Changes the Surface to Volume Ratio Fig. 6.5 Chemical weathering • minerals formed in the earth’s interior are not stable at the surface. • For silicate minerals, stability varies with Si02 content – High Si02 most stable – Low Si02 least stable When you get to the beach……. nothing left but quartz Chemical weathering 1. Solution or Dissolution 2. Oxidation 3. Hydrolysis 1. Weathering by solution (i.e., in water) •Breakup of minerals into ions in solution •NaCl (halite) is simple example •calcite (limestone) common carbonate= CaCO3 CaCO3 + H2CO3 = Ca2+ + 2HCO3- Rainwater is acidic (pH = ?) In soil, organic activity introduces additional acids Fig. 6.6 Limestone (CaCO3) weathers faster than granite due to dissolution Weathered Limestone Ric Ergenbright Fig. 6.10 2. Oxidation provide Color to the Desert Landscape 4Fe + 3O2 2Fe2O3 Betty Crowell Fig. 6.9 What type of weathering produces clay minerals? a. mechanical b. oxidation c. Dissolution d. hydrolysis 3. Hydrolysis Hydrolysis: reaction between mineral elements and the hydrogen ion (H+) of water •Primary mechanism for weathering silicates •acids in water are critical Analogy of hydrolysis: making coffee fresh grounds + water = coffee + residue (a solution) K-feldspar + water = K+ + kaolinite (a clay mineral) Weathering and Making coffee Fig. 6.4 Weathering and soil Soil: Mixture of altered mineral matter, organic matter, air, and soil water Regolith: a layer of broken pieces of rock and slightly altered rock Bedrock: unaltered rock of any kind Soil Profile: vertical differences in soil properties called horizons There are many, many different types of soils why? Called “parent material” Fig. 6.17 Weathering and Soil Formation is complex Fig. 6.16 Soil loss/erosion intermittent example, dust bowl 1930’s MASS WASTING: The work of gravity Which location is at greatest risk for debris flows? A. Ridge top. B. Open land. C. Bottom of a steep mountain channel. D. Middle of a broad mountain valley. Gros Ventre Slide, Wyoming Mt Huascaran, Peru (before 1970) Mt Huascaran, Peru (after 1970) Towns buried by debris avalanche Map showing alpine debris flows trigger by July 25, 1999 thunderstorm, central Front Range, Colorado (Source: Jonathan W. Godt and Jeffrey A. Coe U.S. Geological Survey OpenFile Report 03-050) I-70, Western Colorado, May 2003 Carabelleda, Venezuela, December 1999 Mass wasting and landform development • Mass wasting = downslope movement of rock, regolith, & soil under the direct influence of gravity Mass wasting and landform development • Role of mass wasting • often follows weathering • mass wasting (and running water) produce stream valleys Mass Movement Depends on Nature of Material Angle of Repose: the maximum angle at which a pile of unconsolidated particles can rest Weathered shale forms rubble at base of cliff Weathered shale forms rubble at base of cliff Angle of Repose The effect of water on mass wasting Mass Movement Depends on Water Content Surface tension in damp sand increases cohesion Mass Movement Depends on Water Content Surface tension in damp sand increases cohesion Dry sand is bound only by friction Mass Movement Depends on Water Content Surface tension in damp sand increases cohesion Dry sand is bound only by friction Saturated sand flows easily because of water in pores Controls and triggers of mass wasting • Important factors include • The role of water – Diminishes inter-particle friction – Water adds weight Classification of mass movement is based on dominant material, fluid content, and velocity of movement. Rockfall A very rapid mass movement in which newly detached blocks of rock suddenly fall from a steep slope or cliff. Rockfall <- Rockfall in Zion National Park http://www.youtube.com/watch?v=fzRhLs5GkYs Rock avalanche Types of Unconsolidated Mass Movement Unconsolidated Flows Creep Debris slump Debris flow Increased velocity Soil Creep The downhill movement of soil and other debris, typically at rates of about 1 to 10 mm/year. Evidence of Creep Debris flow A fluid mass movement of rock fragments supported by a muddy matrix. Peak speed can be >10 m/s! Debris flow Debris flow videos http://www.youtube.com/watch?v=r6Lt0oPJFwA http://www.youtube.com/watch?v=8mKC3eID074 Central Apennines, Italy Small debris flow Monterey Park, greater LA Winter 1980 Photos by Douglas M. Morton U.S. Geological Survey Debris flows can carry big rocks … as in Malibu, CA, 2004 Rocky Mountain National Park Which location is at greatest risk for debris flows? A. Ridge top. B. Open land. C. Bottom of a steep mountain channel. D. Middle of a broad mountain valley. Map showing alpine debris flows trigger by July 25, 1999 thunderstorm, central Front Range, Colorado (Source: Jonathan W. Godt and Jeffrey A. Coe U.S. Geological Survey OpenFile Report 03-050) Possible Triggers for Mass Movement • over-steepened slope: – erosion at base – volcanic ash – excavation (manmade) • increased water content: – intense rainfall – rising water table (e.g. behind dam) • earthquakes Effect of Tectonic Setting • high relief, steep slopes • fractured, tilted rocks • frequent earthquakes (triggers) Aftermath of 1999 Chi Chi earthquake, Taiwan (photo Jan 2002) Ways to Reduce Losses Due to Landslides Include: • avoid construction in areas prone to mass movement • build in a way that does not make naturally stable slope unstable • engineer water drainage to prevent strata to become water saturated and prone to fail