Post on 10-May-2015
Chapter 12
Geologic Time
Chapter 12
Outline• Geologic time: perspective & a bit of history
• Dating geologic materials-General: relative & absolute dating-Relative dating:
-7 Principals & their application to a geologic history-Fossil successions
• Gaps in the geologic record (unconformity)-3 types of unconformities-Stratigraphic correlation & the global geologic column
• Numerical (absolute) dating-Radioactive decay-Meaning of a radiometric date-Other numerical dating methods-Dating the geologic column, geologic time scale, & age of Earth
Chapter 12
Chapter 12
Geologic Time
Chapter 12
Geologic Time
• Understanding time permits assigning ages to…
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Geologic Time• Prior to late 1600s, geologic time was thought to =
historical time. • Archbishop James Ussher, Ireland, 1654.
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Geologic Time• Scientists began to find clues to an ancient Earth.
• Nicolaus Steno (1638–1686) – Danish physician.
fossilsharktooth
Chapter 12
Outline• Geologic time: perspective & a bit of history
• Dating geologic materials-General: relative & absolute dating-Relative dating:
-7 Principals & their application to a geologic history-Fossil successions
• Gaps in the geologic record (unconformity)-3 types of unconformities-Stratigraphic correlation & the global geologic column
• Numerical (absolute) dating-Radioactive decay-Meaning of a radiometric date-Other numerical dating methods-Dating the geologic column, geologic time scale, & age of Earth
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Chapter 12
Geologic Time2 ways to date geological materials:
1. Relative age –
2. Numerical (absolute) age –
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Relative vs. Absolute
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Relative Age
• Logical tools are useful for defining relative age.• Principles of:
1. Uniformitarianism
2. Superposition
3. Original horizontality
4. Original continuity
5. Cross-cutting relations
6. Inclusions
7. Baked contacts.
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Geologic Time1. Uniformitarianism – “The present is key to the past”.
•
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Defining Relative Age
2. Superposition.
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Relative Age
3 & 4. Horizontality and continuity.
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Relative Age5. Cross-cutting relations.
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Relative Age6. Inclusions – a rock fragment within another.
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Relative Age
7. Baked contacts.
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Relative Age• Determining relative ages empowers geologists to
unravel complicated geologic histories.
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Geologic History
• Deposition of horizontal strata below sea level in order 1-> 8 (old to young). *Horizontality & continuity*
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Geologic History
• Igneous intrusion of a sill. *baked contact*
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• Intrusion solidified into sill
• Tectonic compression
Geologic History
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• Compression results: • Folding (inference: layers had to exist to be folded).• Uplift (above sea level) & erosion.
• Intrusion of a pluton. *baked contact/cross-cutting*
Geologic History
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• Extension -> normal faulting.• Faulting cross-cuts pluton & rock layers.
Geologic History
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• Dike intrusion.• Dike cross-cuts everything (even normal fault).
Geologic History
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• Erosion to present landscape.• Removed volcano and cuts down the dike top.
Geologic History
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Geologic History• Relative ages help to unravel a complicated history.• Those rules permit one to decipher this diagram!
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Geologic History
• Test yourself at home:
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Fossil Succession
• Fossils (organism traces) can be preserved in sedimentary rocks.
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Fossil Succession• Species evolve, exist, and then go extinct.
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• Fossil range – first to last appearance.
• Permit correlation of strata.
Fossil Succession
Chapter 12
Outline• Geologic time: perspective & a bit of history
• Dating geologic materials-General: relative & absolute dating-Relative dating:
-7 Principals & their application to a geologic history-Fossil successions
• Gaps in the geologic record (unconformity)-3 types of unconformities-Stratigraphic correlation & the global geologic column
• Numerical (absolute) dating-Radioactive decay-Meaning of a radiometric date-Other numerical dating methods-Dating the geologic column, geologic time scale, & age of Earth
Chapter 12
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Unconformity
• An unconformity is• Causes:
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Unconformities3 Types:
1. Disconformity –
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DisconformitiesDisconformities
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Unconformities3 Types:
2. Nonconformity –
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NonconformityNonconformity
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Unconformities3 Types:
3. Angular unconformity – represents a big gap in time
Horizontal rocks deposited, then deformed
Then eroded
Then sediments horizontally deposited on erosion surface
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Types of Unconformity
This animation shows the stages in the development of three main types of unconformity in cross-section, and explains how an incomplete succession of strata provides a record of Earth history. View 1 shows a disconformity, View 2 shows a nonconformity and View 3 shows an angular unconformity. For more information, see Section 12.5 Unconformities: Gaps in the Record starting on p.423 and Figure 12.9 in your textbook.
Types of Unconformity
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Unconformities• Earth history is in
strata.• Missing strata =
missing history
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Stratigraphic Correlation
• In 1793, William “Strata” Smith noted strata could be matched across distances.
• Similar rock types in a similar order• Rock layers contained same distinctive fossils• He made the 1st geologic map of the UK
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Stratigraphic Correlation• Stratigraphic columns depict strata in a region.
• Drawn to portray relative thickness• Rock types depicted by fill patterns• Divided into formations (mapable rock units)• Formations separated by contacts
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Stratigraphic Correlation• National Parks of Arizona & Utah.
• Formations can be traced long distances • Overlap in rock type sequences• Overlapping rock columns are used to build a composite
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The Geologic Column• A composite global stratigraphic column exists.• Constructed from incomplete sections across the globe• It brackets almost all earth history
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Outline• Geologic time: perspective & a bit of history
• Dating geologic materials-General: relative & absolute dating-Relative dating:
-7 Principals & their application to a geologic history-Fossil successions
• Gaps in the geologic record (unconformity)-3 types of unconformities-Stratigraphic correlation & the global geologic column
• Numerical (absolute) dating-Radioactive decay-Meaning of a radiometric date-Other numerical dating methods-Dating the geologic column, geologic time scale, & age of Earth
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Numerical (Absolute) Dating
• Based on radioactive decay of atoms in minerals.• Radioactive decay proceeds at a known fixed rate• Radioactive elements act as internal clocks• Numerical dating is called geochronology
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Radioactive DecayIsotopes Atoms with same # of protons, different # of neutrons
Have similar but different mass numbers
Some are Stable – never change
Some are Unstable (radioactive) –spontaneously change to something else (decay) at a fixed rate
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Radioactive Decay• Decay process has 2 main components:
• Parent – isotope that decays• Daughter – decay product isotope
• Decay process can: • Have 1 step (parent>daughter)• Have many steps (parent>daughter>etc)
• Decay product is unstable and hence also decays• Eventually proceeds to a stable endpoint
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Radioactive Decay Time• Half-life (t½) – time for ½ unstable parent to decay.
• t½ is unique for each isotope
• After one t½ -1/2 original parent remains
• After three t½ -1/8 original parent remains
• Parent disappears (nonlinear) daughter accumulates
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Radiometric Dating• Mineral age can be determined by:• Measuring parent/daughter isotope ratio• Calculating time
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What Is a Radiometric Date?
• Time since a mineral began to retain all parent & daughter isotopes. • Requires cooling below “closure (blocking) temperature.”
• Daugther retained only below closure T• Daughter leaks out above closure T• Thus, if rock is reheated above closure T, the
radiometric clock can be reset to zero
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Other Numerical Ages
• Numerical ages are possible without isotopes. • Growth rings – annual layers from trees or shells • Rhythmic layering – annual layers in seds or ice
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Other Numerical Ages• Magnetostratigraphy –magnetic signatures in strata are
compared to global reference column
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Other Numerical Ages• Decay process can cause scars (tracks) in minerals.
• Decay by fission (explosion) produces scar (track)• Daughter isn’t another isotope, it’s a damage zone
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Dating the Geologic Column• Use geochronology to: • Date specific strata OR• Bracket those that cant be dated directly
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The Geologic Time Scale
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Age of the Earth
• Oldest rocks are 3.96 Ga.• Zircon minerals in some sandstones are 4.1-4.2 Ga.• Earth is ~4.57 Ga based on correlation with…
• Meteorites, moon rocks.