SCI 102 - Chapter Six and Seven Outline - Rocks PDF

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Lecture notes and study guide for chapter six and chapter seven for Professor DeLeonardo's Earth science course....

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SCI 102 (Prof. DeLeonardo) Chapter 6 (6.1-6.3) and Chapter 7 (Sections 7.1-7.5): Rocks Chapter 6 What is weathering? o Weathering involves the physical breakdown and chemical alteration of rock at or near Earth’s surface o Two types:  Mechanical weathering—physical forces breaking rocks into smaller pieces  Chemical weathering—chemical transformation of rock into new compounds » Both work simultaneously and reinforce each other Give two examples of Chemical Weathering of rock: o The Most Important Agent Is Water • Responsible for transport of ions and molecules involved in chemical processes o Processes of Chemical Weathering = dissolution, oxidation, hydrolysis o Dissolution • Certain minerals dissolve in water » Halite is one of the most water-soluble minerals • A small amount of acid in water increases the corrosive force of water, causing dissolution » Carbonic acid is created when carbon dioxide dissolves in raindrops » Calcite is easily attacked by weakly acidic solutions » This process is responsible for the formation of limestone caverns o Oxidation • Essentially the rusting of iron-rich minerals • Oxygen combines with iron to form iron oxide • Process is slow in dry environments • Water increases the speed of the reaction • Important in decomposing ferromagnesium minerals like olivine, pyroxene, hornblende, and biotite • Oxidation can only occur after iron has been freed from the silicate structure by hydrolysis o Hydrolysis • The reaction of any substance with water • A hydrogen ion attacks and replaces another ion • Silicates primarily decompose by hydrolysis » Clay minerals are the most abundant product of weathering » Clay minerals are very stable under surface conditions • Acid greatly accelerates hydrolysis o Spheroidal Weathering • Weathering attacks edges from two sides and corners from three sides • Sharp edges gradually wear down and become rounded • Granite, for example: » Crystalline rock with joints » Water penetrates joints » H+ replaces K+ in the feldspars, disrupts crystalline structure

SCI 102 (Prof. DeLeonardo) Chapter 6 (6.1-6.3) and Chapter 7 (Sections 7.1-7.5): Rocks Give two examples of Mechanical Weathering of rock: o Mechanical weathering, by breaking rock into smaller and smaller pieces, increases surface area for chemical weathering attack  Frost wedging » Two different methods:  Water works its way into cracks in rocks and the freezing enlarges the cracks in the rocks  Lenses of ice in soil grow larger as they attract liquid water from surrounding areas  Salt Crystal Growth » Sea spray or salty groundwater penetrates crevices and pore spaces in rocks » As the water evaporates, salt crystals form and enlarge the crevices  Sheeting/Unloading » Large masses of igneous rock are exposed by erosion and concentric slabs break loose due to release of confining pressure » An exfoliation dome is formed after continued weathering causes slabs to separate and spall off • Biological ACTIVITY » Plant roots grow into fractures in a rock, causing the cracks to expand (root wedging) » Burrowing animals break down rocks by moving fresh material to the surface, enhancing physical and chemical weathering » Human impacts (rock blasting) is very noticeable- can produce effects much like unloading Chapter 7 Explain how sediment is transported: o Sediments and sedimentary rocks cover approximately 75% of land and virtually ALL of the ocean basins o Sedimentary rocks are products of mechanical and chemical weathering  Sediments and soluble constituents are typically transported downslope by gravity (mass wasting)  The sediments are then deposited and subsequently buried  As deposition continues, the sediments are lithified into sedimentary rocks o Dust, water, glaciers, weathering, dissolved in water Where is sediment eventually deposited? (3 main places) o An environment of deposition or a sedimentary environment is a geographic setting where sediment is accumulating  Sites are characterized by particular combinations of geologic processes and environmental conditions o Determines the nature of the sediments that accumulate (grain size, grain shape, etc.)  Three broad categories of sedimentary environments » Continental  Dominated by stream erosion and deposition = Streams are the dominant agent of landscape alteration

SCI 102 (Prof. DeLeonardo) Chapter 6 (6.1-6.3) and Chapter 7 (Sections 7.1-7.5): Rocks  Glacial = Deposits are typically unsorted mixtures of sediments that range from clay to boulder-sized  Wind (eolian) = Well-sorted, fine sediments

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Marine  Shallow marine (to about 200 m)  Borders the world’s continents  Receives huge quantities of terrestrial sediments  Warm seas with minimal terrestrial sediments have carbonaterich muds and debris from coral reefs  Deep marine (seaward of continental shelves)  Primarily fine sediments that accumulate on the ocean floor  Turbidity currents—submarine landslides—are the exception Transitional  The shoreline is the transition zone between marine and continental environments  Examples include: Beaches, Tidal flats, Spits, bars, and barrier islands, Lagoons, Deltas

Process of Forming a Sedimentary Rock o Diagenesis: chemical, physical, and biological changes that take place after sediments are deposited  Occurs within the upper few kilometers of Earth’s crust  Examples: » Recrystallization of more stable minerals from lass stable ones (e.g., aragonite to calcite) » Formation of coal! o Compaction: as sediments are buried, the weight of the overlying material compresses the deeper sediments o Cementation: involves the crystallization of minerals among the individual sediment grains o Lithification: unconsolidated sediments are transformed into solid sedimentary rocks  Compaction—as sediments are buried, the weight of the overlying material compresses the deeper sediments  Cementation—involves the crystallization of minerals among the individual sediment grains 3 Types of Sedimentary rocks (describe each and give example of a rock type) 1. Detrital sedimentary rocks: form from sediments that have been weathered and transported  Chief constituents of detrital rocks include clay minerals, quartz, feldspars, and micas  Particle size is used to distinguish among the various rock types » It also presents important information about the environment of deposition  Common detrital sedimentary rocks include » Shale, sandstone, conglomerate, and breccia

SCI 102 (Prof. DeLeonardo) Chapter 6 (6.1-6.3) and Chapter 7 (Sections 7.1-7.5): Rocks 2. Chemical sedimentary rocks: form from precipitated material that was once in solution  Precipitation of material occurs by: » Inorganic processes: evaporation or chemical activity » Organic processes from water-dwelling organisms form biochemical sedimentary rocks  Chemical sedimentary rocks include: » Limestone, chert, rock salt 3. Organic sedimentary rocks: form from the carbon-rich remains of organisms  Coal is different from other sedimentary rocks » Occasionally, plant structures (leaves, bark, and wood) are identifiable in coal Give an example of how a sedimentary rock can be a “rock record” or evidence of an environment and/or ecosystem in Earth’s Geologic History: o Sedimentary structures provide additional information for interpreting Earth’s history o Types of sedimentary structures  The layers of the sedimentary rocks are called strata or beds » Single most common and characteristic feature of sedimentary rocks  Bedding planes separate strata  Cross-bedding occurs when the layers in the sedimentary rocks are inclined » Characteristic of sand dunes, deltas, and some stream deposits o Example: Grand Canyon...