Geology by Chernicoff/Whitney

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Transcript Geology by Chernicoff/Whitney

Geology G100
Quick review for Test #2
Dr. Isiorho, IPFW
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Sedimentation and Sedimentary Rocks
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What is a sedimentary rock?
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Sediments
Sediments are loose fragments of solid materials- preexisting rocks, remains of organisms, and precipitation of
minerals
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Sediment textures- size, shape, and arrangement
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Sorting
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are determined by transportation and depositional processes
selecting particle based on size, shape and density
Shape
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angular or irregular grains become rounded
longer travels lead to more rounding of grains
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From Sediments to Sedimentary
Rocks
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Pressure, heat, and underground circulating
water produce changes in rocks- known as
Diagenesis
Lithification- conversion of loose sediments
into solid sedimentary rocks
Compaction- weight of overlying materials
 Cementation- mineral precipitated in pore spaces
 Recrystallization- less stable minerals change to
new stable minerals
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Sedimentary Structures
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Physical features that reflect condition of deposition
(how and where?)
Beddings (stratification)
 sediments in distinct layers- separate depositional environments
Graded beddings
 deposition occurs in relatively quiet waters
Cross bedding
 sedimentary layers at an angle to underlying layers
Ripple marks
 small surface ridges- produced by water or wind
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Symmetrical & Asymmetrical
Mud crack
 muddy sediments that dry and contracts
Bioturbation
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No drawing provided 
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Classification of Sedimentary Rocks
Detrital and Chemical
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Detrital- based on grain size
 Mudstone- clay and silt size- constitute > 50% of all
detrital sedimentary rocks
 Shale- clay and silt size particles- parallel layers- fissility
 Siltstone- silt size particles
 Sandstone- quartz arenite, arkose (with ~ 25% feldspar),
graywacke (lithic, dark fragments & fines)
 Breccia- angular gravel size particles
 Conglomerate- rounded gravel size particles
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Sed. rock Classification contd.
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Chemical- organic and inorganic
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Organic- derived from living organism/biogenic
 Limestone and chert- composed of skeletal remains of
animals
 coal- carbon rich remains of terrestrial plants
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Inorganic Sedimentary Rocks
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Direct precipitation from water
 e.g. Limestone, chert
Evaporation of saline water
 evaporite- gypsum, halite, dolostone
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Sedimentary Environments
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Continental
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Transitional- coastal- along ocean shores
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estuaries and deltas
Marine
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rivers, lakes, caves, desert, glaciers- mostly detrital
shallow-above continental shelf (< 200m (700’))
deep- beyond the continental shelf
Sedimentary Facies- sediments deposits at the same time but in
different environments as a horizontal continuum of distinct rock
type
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Metamorphic Rocks
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Altered rocks
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Definitions
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Metamorphic rock is formed when existing
rocks change due to subjection to pressure and
or temperature
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Any rock can undergo metamorphism
Metamorphism is the process by which heat,
pressure, and chemical reactions deep within
the earth alter the mineral content and or
structure of existing rock without melting it
down
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What Drives Metamorphism
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Heat
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Pressure
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Lithostatic (confining)- rock becomes smaller and denser
Directed- minerals become aligned- Foliation
Circulating Fluids
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Accelerate pace of chemical reactions
Ions in water- change mineral composition
Parent Rocks
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Original rock’s composition will affect the outcome of
metamorphism
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Types of Metamorphism
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Contact
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Heat is the dominant factor
Area affected generally smaller than regional metarmorphism
Regional are two types with extensive coverage
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Burial- occurs in deep sedimentary basins- no plate tectonics
involved
Dynamothermal- occurs where converging plates squeeze a rock
caught between them
Others
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Hydrothermal- involves hot water from magma
Fault-zone- rocks grinding past one another
Shock- meteorites strike
Pyrometamorphism- lightning
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Metamorphic Rock Types
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Foliated- based on type of foliation
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Non-foliated- based on mineral composition
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Slate- fine grain
Phyllite- fine grain with sheen
Schist- has ‘split’ appearance
Gneiss- layers/bands of minerals
Marble
Quartzite
Hornsfel
Mixed Rock
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Migmatite- indicates partial melting
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Metamorphism
Temperature & Pressure
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Information about degree to which a metamorphic
rock differs from its parent material
Metamorphic Grade
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low (200-400) slate
high (500-800) gneiss
Index minerals/metamorphic Zones are used to
determine metamorphic condition of temperature and
pressure
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Chlorite, muscovite-low grade (low P/T)
Garnet, staurolite- intermediate
Sillimanite- high grade (high P/T)
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How old is the Rock?
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How can we tell the age of rocks?
Geochronology
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Geochronology
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Geochronology is the study of time in relation
to earth’s existence
Relative Dating
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Determines how old a rock is in relation to its
surrounding
Numerical Dating (Absolute Age?)
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Determines actual age in years
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Relative Dating
Relies on Key Principles such as
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Uniformitarianism- the present is key to the past
Original horizontality
 Sediments deposited in horizontal layers
Superposition
 Youngest rocks are on top (assuming no tectonic activity)
Cross-cutting relationships
 Cut layer is older than ‘cutting’ rock
Faunal succession
 Organisms succeed one another in recognizable reproducible
pattern
Unconformity
 Represents a break (gap) in the rock record
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Numerical Age
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Isotope Dating relies on the rate of decay of
radioactive isotopes within a rock
Radioactive isotopes have nuclei that spontaneously
decay emitting or capturing a variety of subatomic
particles
Decaying radioactive isotope- parent isotopes decay
to form daughter isotopes
Half-life- is the time it takes for half the atoms of
parent isotope to decay
Some radioactive isotopes with daughter products
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U-238 => Pb-206; K-40 => Ar-40; C-14 => N-14
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Factors Affecting Isotope Dating
Results
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Isotope dating is more useful for igneous rocks
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Rock/Mineral must be a closed system
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Atoms of parent and daughter are still present in rock/mineral being dated
Condition of parent Material
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Clock is set when igneous rock crystallizes locking the radioactive
isotopes within its crystal lattice
Fracture, weathering and migrating ground water
Age of Substance
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Enough measurable daughter isotope, use appropriate radioactive isotope
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Other Numerical Dating
Techniques
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Fission Track
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Dendrochronology (Tree-Ring dating)
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Paired layers of sediments
Lichenometry
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Annual growth rings
Varve- deposited layers of lake-bottom
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High speed particles emitted during radiation may pass through crystal leaving
‘tears’ within the crystal- the older the rock, the more fission tracks
Lichens grow at a fairly constant rate
Cosmogenic isotopes
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Used in dating land features
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Geologic Time Scale
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Contrasting several dating techniques chronicling
Earth’s history to produce a geologic Time Scale
Geologic Time Scale- divided into Eons, Eras,
Periods, and Epoches
Phanerozoic Eon (evidence of life began) divided into
three eras
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Paleozoic (ancient life) dominated by marine invertebrates
Mesozoic (middle life) dominated by reptiles
Cenozoic (recent life) dominated by mammals
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The Earth moves
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It’s not an earthquake…but the earth materials
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Mass Movement
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Process that transports Earth’s materials
downslope by the pull of gravity
Friction, strength, and cohesiveness of materials
resist mass movement
 Angle of slope (sloppiness), water content, lack of
vegetation, and biological disturbances enhance
mass wasting
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Causes of Mass Movement
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Steepness of Slope
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Composition of Material either promotes or resists
mass wasting
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Faulting, folding, river cut, glacial, coastal wave create
steep slope
Solid /Unconsolidated
Vegetation- lack of which promotes mass wasting
Water Content- increases weight of material and
reduces friction between planes of weakness
Human/Other Disturbances
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Triggers for Mass Movement
Events
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Natural Triggers
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Climatic- torrential rains and snow melt
Geologic- earthquakes and volcanic eruptions
Human-Induced Triggers
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Oversteeping of slopes- excavation
Overloading- excess water, building, and other construction
Deforestation/overgrazing of vegetation
Loud noise- trains, aircrafts, blasting
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Mass Wasting Types
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Classification is based on composition and velocity
Creep- slowest form
Slides- move along a plane of weakness
Slumps- move along concave slip surfaces
Flows- rocks and soils have with excess water
Falls- fastest type
Landslide is a general term for downslope movement
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Reducing Mass Movement
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Avoiding
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Predicting mass movement
 Terrain analysis, field visit, eye witness/recorded accounts
Vegetation- over grazing, harvesting
Preventing
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Develop Prevention Plan
Enhance Forces that Resist or Reduce forces of mass wasting
Structural Approach- reduce slope
Non-Structural Approach- tree, chemical stability
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Study for Test #2
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Use the class notes/textbook and the links
provided in the syllabus.
It’s an open book test and the “Honor System”
prevails…no help from any one, no
collaboration
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Some key words for Test #2
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Some key words for test #2
Cementation, crystallization, Compaction
Transportation of sediments results in…..
Rock salt, sandstone, siltstone, coal, arkose, graywacke
Quartzite, marble, slate, schist, migmatite, order of
metamorphism
Types of metamorphism, parent materials of some
metamorphic rocks
Relative age and principles of Superposition, original
horizontality, faunal succession, cross-cutting, unconformities,
radiometric dating, half life..
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