1. Lecture Outlines Physical Geology, 14/e
Plummer, Carlson & Hammersley
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2. Time and Geology Physical Geology 14/e, Chapter 8
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3. “The present is the key to the past”
Age of earth
Prior to the 19th century, accepted age of Earth was based on religious beliefs
Biblical: ~6,000 years for Western culture
Chinese/Hindu: Old beyond comprehension
James Hutton: “Father of Geology”
realized geologic processes require vast amounts of time
Charles Lyell
popularized Hutton’s concepts in book Principles of Geology
“The present is the key to the past”
James Hutton
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4. Relative vs. numerical age
Relative age - the order of events or objects, from oldest to youngest
original horizontality
superposition,
lateral continuity,
cross-cutting relationships,
inclusions,
unconformities,
correlation of rock units and fossils
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5. Relative vs. numerical age
Numerical age - the age of events or objects, expressed as a number or numbers
determined using radiometric dating (determining how much radioactive decay of a specific element has occurred since a rock formed or an event occurred)
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6. Relative age determination
Contacts - surfaces separating successive rock layers (beds)
Formations - bodies of rock of considerable thickness with recognizable characteristics allowing them to be distinguished from adjacent rock layers
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7. Relative age determination
Original horizontality - beds of sediment deposited in water are initially formed as horizontal or nearly horizontal layers
Superposition - within an undisturbed sequence of sedimentary or volcanic rocks, layers get younger from bottom to top
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8. Relative age determination
Lateral continuity - original horizontal layer extends laterally until it tapers or thins at its edges
Cross-cutting relationships - a disrupted pattern is older than the cause of the disruption
Intrusions and faults are younger than the rocks they cut through
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9. Relative age determination
Baked contacts - contacts between igneous intrusions and surrounding rocks, where surrounding rocks have experienced contact metamorphism
Inclusions - fragments embedded in host rock are older than the host rock
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10. unconformities
Unconformity: a surface (or contact) that represents a gap in the geologic record
Disconformity - an unconformity in which the contact representing missing rock layers separates beds that are parallel to each other
Angular unconformity - an unconformity in which the contact separates overlying younger layers from eroded tilted or folder layers
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11. unconformities
Nonconformity - an unconformity in which an erosional surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock
plutonic and metamorphic rocks exposed by large amounts of erosion
typically represents a large gap in the geologic record
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12. correlation The determination of the time-equivalency of rock units
Physical continuity—physically tracing a continuous exposure of a rock unit
Similarity of rock types
assumes similar sequences of rocks formed at same time
can be inaccurate if very common rock types are involved
Correlation by fossils
fossil species succeed one another through the layers in a predictable order
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13. Geologic time scale Worldwide relative time scale
Subdivides geologic time based on fossil assemblages
Divided into eons, eras, periods, and epochs
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14. Geologic time scale Precambrian Paleozoic era - Old Life
represents 87% of geologic time
Paleozoic era - Old Life
appearance of complex life
Mesozoic era - Middle Life
dinosaurs abundant
era ended by mass extinction
Cenozoic era - New Life
mammals and birds abundant
most recent ice ages occurred during the Pleistocene Epoch of the Quaternary Period
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15. Numerical age dating
Numerical dating - puts absolute values (e.g., millions of years) on the ages of rocks and geologic time periods
uses radioactive decay of unstable isotopes
only possible since radioactivity was discovered in 1896
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16. Numerical Age Dating
Half-life - time it takes for a given amount of radioactive isotope to be reduced by half
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17. Combining relative and Numerical ages
Radiometric dating gives numerical time brackets for events with known relative ages
individual layers may be dated directly
radiometric dating of units above and below brackets age of units in between
Geologic Time Scale
divided into four Eons
Hadean
Archean
Proterozoic
Phanerozoic
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18. Age of earth
Numerical dating gives age for Earth of ~ 4.56 billion years
oldest age obtained for meteorites, believed to have been unchanged since the formation of the solar system
Earth and rest of solar system very likely formed at this time
Geologic (deep) time is vast
a long human lifetime represents only about % of geologic time
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19. End of Chapter 8
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