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How do we know what happened first?
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James Hutton (1726-1797) “Father of Modern Geology”
native of Edinburgh, Scotland educated as a medical doctor in Leiden (1749) passionate about scientific inquiry
Historical Developments
“Theory of the Earth” -- processes are slow; take a long time
Charles Lyell (1795-1875)• Scotsman who attended Oxford University • father was an avid naturalist • rebelled against prevailing thought of
“catastrophism”/”Neptunism”. “Principles of Geology” -- popularized Hutton’s views
idea of “uniformitarianism” -- same processes operating today occurred in the past
….the present is the key to the past….
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study of timing of geologic events and processes is geochronology
Relative timeorder of events or objects from first (oldest) to last (youngest)she is older than he is; she was born first and he was born last
age of events or objects expressed numericallyshe is twenty-one and he is nineteen
Absolute time
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Relative Time- “this rock is older than that”
Principles Used to Determine Relative Age Unconformities Correlation The Standard Geologic Time Scale Index Fossils
Absolute Time- “this rock is 28 million years old”
Principles of radioactive decay
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apply simple concepts to determine…
• original horizontality
• superposition
• lateral continuity
• cross-cutting relationships
• inclusions
• unconformities
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original horizontalityall beds originally deposited in water formed in horizontal layers
sediments will settleto bottomand blanketthe sea floor
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Superposition: within a sequence of undisturbed sedimentary or volcanic rocks, layers become younger, upward
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Lateral Continuity: original sedimentary layers extend laterally until it thins out at edges
rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous.
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cross-cutting relationships
a disrupted pattern is older thanthe cause of the disruptione.g. an intrusion is youngerthan the rocks it intrudes
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inclusions
fragments of other rocks contained in a body of rockmust be older than the host rock
e.g. 1) xenoliths in granite are olderthan granite and2) pieces of rock inconglomerate are older than conglomerate
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unconformities
A gap in the geologic record -- “gap” may be an amount of time or amount of missing section
conformity
• relatively continuous deposition
• deposition of a sequence of parallel layers
• contacts between formations do not represent significant gaps in time
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from: http://www.elohi.com/photo/grandcanyon
conformity
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different types of unconformities
1. angular unconformity
• contact separates overlying younger layers from tilted older layers• sequence of layers is not parallel
• contacts between formations may represent significant amounts of time
angular unconformity
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angular unconformityfrom: http://www.uakron.edu/envstudies/parks/rmgcan2.html
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angular unconformity
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different types of unconformities
2. disconformity
• contact separates beds (formations) that are parallel
• sequence of layers is parallel
• contacts between formations may represent significant amounts of time• missing time is difficult to recognize
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different types of unconformities
3. nonconformity
• strata deposited on older crystalline (metamorphic/igneous) rock• erosion surface on igneous/metamorphic rock covered bysedimentary rocks
• large gap in geologic record
nonconformity
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Unconformity Types Using Grand Canyon as Example
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faunal succession (correlation by fossils)
How is this done?
index fossil short-lived organism; points to narrow range of geologic time
fossil species succeed one another through the layers in a predictable order
fossil assemblage group of fossilsassociatedtogether
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