12- sedimentary rock formation PDF

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EASC 101 12- Forming Sedimentary Rocks Chapters 6.1-6.4, 16.4, 7.5

Objectives:      

Define weathering, transport, and erosion Explain the difference between mechanical and chemical weathering Describe the different weathering processes Explain how sediment is transported in fluvial, aeolian, and glacial environments Explain how steam competence results in sorting Describe diagenesis and lithification of sedimentary rocks

The Rock Cycle  One type of rock can be converted to another type of rock  This unit: o Igneous to sedimentary  Weathering, erosion/transport, deposition, lithification o Sedimentary and igneous to metamorphic

Weathering  Involves the physical breakdown (disintegration) and chemical alteration (decomposition) of rock at or near Earth’s surface  Provides the raw materials for sedimentary rocks

Mechanical Weathering  Breaks the rock down into smaller and smaller pieces to increase surface area for chemical weathering  Occurs in place  Four different types: o Frost wedging, Salt crystal growth, sheeting, and biological activity

Frost Wedging  Water seeps into fractures in the rock  Water freezes and expands o Widens the fractures o Causes pieces to break off

EASC 101 Salt Crystal Growth  Salt water seeps into fractures  Water evaporates and salt crystals form  Salt crystals grow larger, forcing fractures wider

Sheeting     

Removal of overlying rock/soil reduces downward pressure Rock expands causing layered fracturing Common with granite batholiths Occurs at smaller scales due to heating and cooling and frost/salt wedging Joints: breaking of rock due to contraction o Occurs during cooling/crystallization of magma or mountain building

Biological Activity  Plant roots penetrate fractures o Growth of roots widens fractures  Burrowing animals remove rock material  Human activities include: o Road cuts, mining, and construction

Chemical Weathering  Breaks down rock components/minerals o Alters minerals o Releases components to environment  Weakens outer surface of rocks o Makes them more susceptible to mechanical weathering  Water is the most important agent  Occurs in place  Four different types: o Dissolution, oxidation, hydrolysis

Dissolution  Certain minerals dissolve in water  Example: halite NaCl o Atoms retain charge, although molecule is neutral o Na+ and Clo H20 is polar: O has a residual negative charge and H has a residual positive charge

EASC 101 o H2O disrupts attractive forces in halite crystal and ions released into solution  Most minerals are insoluble in pure water  Presence of a small amount of acid increases the corrosive capability of water o Solution containing H+ ion  Acidity measured using pH scale o -log10[H+] o Ranges from 0 to 14 o Pure water is neutral (pH of 7.0) o Strong acidity has a pH < 5.0 o Strong alkaline has a pH > 10.0  Natural acids are produced in a number of ways: o CO2 dissolved in raindrops creates carbonic acid  H2O + CO2 → H+ + HCO3 o CO2 in soil increases acidity of acid rain that soaks in o Decaying organisms release organic acids o Sulfuric acid produced by chemical weathering of pyrite and other sulfide minerals  4 FeS2 + 15 O2 + 14 H2O → 4 Fe(OH)3- + 8 H2SO4  Calcite (makes up limestone and is a common cement in other sedimentary rocks) is easily dissolved in acidic solutions o CaCO3 + (H+ + HCO3+) → Ca2+ + 2 HCO3o Calcite + carbonic acid → calcium ion + bicarbonate ion o Forms caves, dissolves limestone buildings and monuments  Ions dissolved in water remain in underground water supply o Hard water  Active ions react with soap, making an insoluble material  Soap now does a poor job removing dirt because can’t engage in chemical solution reaction  Occurs when e lost from one element during a reaction  Presence of water speeds up oxidation reactions  Rusting of Fe-rich minerals o Olivine, pyroxene, hornblende, biotite o 4 Fe + 3O2 → 2 Fe2O3 o Iron + oxygen → hematite  Occurs only after Fe freed from crystal structure via hydrolysis

Oxidation  Decomposition of sulfide minerals e.g. pyrite FeS2 o y...