GEOL 102 Lecture 4: Geology102Lecture4Notes

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6 Feb 2017
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Geology 102 Lecture 4 Notes
Relative and Absolute Dating Methods
Relative Dating
Stratigraphic principles are very useful for figuring out the order in which rock
layers were formed, folded, faulted, etc
Most of the geologic time scale was reconstructed this way
Lithostratigraphy
A branch of stratigraphy which uses features of rock layers to perform geologic
correlations
Steno’s 3 laws, Principle of Cross-Cutting Relationships, Principle of Included
Fragments
Cross Sections
The layering of rocks (from old to new)
Biostratigraphy
A branch of stratigraphy which uses fossils to perform geologic correlations
Utilizes index fossils to define biostratigraphic units
Index fossils
Traits of useful index fossils
Abundant
Easily Identifiable
Geographically widespread
Independent of geology
Narrow stratigraphic and temporal range
Igneous Rocks
Extrusive/ Volcanic rocks also often form in layers
Intrusive/ Plutonic rocks often follow cracks, voids, or other weakness in
pre-existing rocks
Useful for radiometric dating
Metamorphic Rocks
Are created when pre-existing rocks are exposed to heat and pressure
Unconformities
Unconformities represent missing time in the rock record
A surface between two bodies of rock formed when
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The rock that was there has eroded away
No rock formed during the period of time the surface represents
Types of unconformities
Disconformity
Occurs between parallel layers of sedimentary rock
Angular Unconformity
Occurs when horizontal sedimentary layers are deposited over the
exposed, eroded surface of previously folded/dipping sedimentary layers
Nonconformity
Occurs between metamorphic or igneous rocks (below) and sedimentary
rocks (above)
Magnetic Polarity Scale
Motion in the liquid outer core creates the Earth’s magnetic field
Poles periodically swap positions
Pattern of polarity shifts can be correlated globally
Arriving at Absolute Dates
Human History is a miniscule portion of all of Earth history
Written histories only go back so far
Dendrochronology
Trees grow in annual layers, with the oldest at the trunk’s center and the
youngest just under the bark
Environmental conditions can affect features of the growth rings
Can be very precise
Only effective back 11,000 years
Absolutes Dates>11,000 Years ago
Thermoluminescence/Optical Dating- 300-100,000 years ago
Cosmogenic Dating- 10-30 million years
Antoine-Henri Becquerel
1852-1908
French Physicist
Discovered radioactive decay in 1895
Nobel Prize in 1903 with Marie Curie and Pierre Curie
Radioactive Decay
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