1. Earth: An Introduction to Physical Geology, 10e
Tarbuck & Lutgens
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2. Geologic Time Earth, 10e - Chapter 9
Stan Hatfield
Southwestern Illinois College
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3. © 2011 Pearson Education, Inc.
Relative Dating
Law of superposition
Developed by Nicolaus Steno in 1669
In an undeformed sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom.
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4. Superposition Is Well Illustrated by the Strata in the Grand Canyon
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Relative Dating
Principle of original horizontality
Layers of sediment are generally deposited in a horizontal position.
Rock layers that are flat have not been disturbed.
Principle of cross-cutting relationships
Younger features cut across older features.
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6. Cross-Cutting Relationships
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Relative Dating
Inclusions
An inclusion is a piece of rock that is enclosed within another rock.
The rock containing the inclusion is younger.
Unconformity
An unconformity is a break in the rock record produced by erosion and/or nondeposition of rock units.
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Relative Dating
Unconformity
Types of unconformities
Angular unconformity—tilted rocks are overlain by flat-lying rocks
Disconformity—strata on either side of the unconformity are parallel
Nonconformity—metamorphic or igneous rocks in contact with sedimentary strata
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Formation
of an
Angular
Unconformity
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10. Unconformities in the Grand Canyon
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11. Fossils—Evidence of Past Life
Fossils are traces or remains of prehistoric life that are now preserved in rock.
Fossils are generally found in sedimentary rock (rarely in metamorphic and never in igneous rock).
Paleontology is the study of fossils.
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12. Fossils—Evidence of Past Life
Geologically fossils are important because they:
Aid in interpretation of the geologic past
Serve as important time indicators
Allow for correlation of rocks from different places
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13. Fossils—Evidence of Past Life
Types of fossils
The remains of relatively recent organisms—teeth, bones, etc.
Entire animals, flesh included
Given enough time, remains may be petrified (literally “turned into stone”).
Molds and casts
Carbonization
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14. Fossils—Evidence of Past Life
Types of fossils
Others
Tracks
Burrows
Coprolites (fossil dung)
Gastroliths (polished stomach stones)
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15. Fossils—Evidence of Past Life
Conditions favoring preservation
Rapid burial
Possession of hard parts (skeleton, shell, etc.)
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16. Fossils and Correlation
Matching of rocks of similar ages in different regions is known as correlation.
Correlation often relies upon fossils.
William Smith (in the late 1700s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content.
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17. Fossils and Correlation
Principle of fossil succession—fossil organisms succeed one another in a definite and determinable order. Therefore, any time period can be recognized by its fossil content.
An index fossil is a geographically widespread fossil that is limited to a short span of geologic time.
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18. Dating Rocks Using Overlapping Fossil Ranges
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19. Dating with Radioactivity
Reviewing basic atomic structure
Nucleus
Protons are positively-charged particles with mass.
Neutrons are neutral particles with mass.
Electrons are negatively-charged particles that orbit the nucleus.
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20. Dating with Radioactivity
Reviewing basic atomic structure
Atomic number
Element’s identifying number
Equal to the number of protons
Mass number
Sum of the number of protons and neutrons
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21. Dating with Radioactivity
Reviewing basic atomic structure
Isotope
Variant of the same parent atom
Differs in the number of neutrons
Results in a different mass number than the parent atom
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22. Dating with Radioactivity
Spontaneous changes (decay) in the structure of atomic nuclei
Types of radioactive decay
Alpha emission
Emission of two protons and two neutrons (an alpha particle)
Mass number is reduced by 4, and the atomic number is lowered by 2.
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23. Dating with Radioactivity
Types of radioactive decay
Beta emission
An electron (beta particle) is ejected from the nucleus.
Mass number remains unchanged and the atomic number increases by 1.
Electron capture
An electron is captured by the nucleus and combines with a proton to form a neutron.
Mass number remains unchanged and the atomic number decreases by 1.
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24. Dating with Radioactivity
Parent—an unstable radioactive isotope
Daughter product—the isotopes resulting from the decay of a parent
Half-life—the time required for one-half of the radioactive nuclei in a sample to decay
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25. Dating with Radioactivity
Radiometric dating
The percentage of radioactive atoms that decay during one half-life is always the same (50%).
However, the actual number of atoms that decay continually decreases.
Comparing the ratio of parent to daughter yields the age of the sample.
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26. Radioactive-Decay Curve
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27. Isotopes Commonly Used in Radiometric Dating
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28. Dating with Radioactivity
Radiometric dating
Sources of error
A closed system is required.
To avoid potential problems, only fresh, unweathered rock samples should be used.
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29. Dating with Radioactivity
Dating with carbon-14 (radiocarbon dating)
Half-life = 5730 years.
Used to date very recent events
Carbon-14 is produced in the upper atmosphere.
Useful tool for anthropologists, archaeologists, and geologists who study very recent Earth history
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30. Dating with Radioactivity
Importance of radiometric dating
Radiometric dating is a complex procedure that requires precise measurement.
Rocks from several localities have been dated at more than 3 billion years.
Confirms the idea that geologic time is immense
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31. The Geologic Time Scale
The geologic time scale is a “calendar” of Earth history.
Subdivides geologic history into units
Originally created using relative dates
Structure of the geologic time scale
An eon is the greatest expanse of time.
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32. The Geologic Time Scale
Structure of the geologic time scale
Names of the eons
Phanerozoic (“visible life”)—the most recent eon, which began about 540 million years ago
Proterozoic
Archean
Hadean—the oldest eon
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33. The Geologic Time Scale
Structure of the geologic time scale
Era—subdivision of an eon
Eras of the Phanerozoic eon
Cenozoic (“recent life”)
Mesozoic (“middle life”)
Paleozoic (“ancient life”)
Eras are subdivided into periods.
Periods are subdivided into epochs.
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34. The Geologic Time Scale
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35. The Geologic Time Scale
Precambrian time
Nearly 4 billion years prior to the Cambrian period
Not divided into smaller time units because the events of Precambrian history are not known in great enough detail
First abundant fossil evidence does not appear until the beginning of the Cambrian period.
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36. The Geologic Time Scale
Difficulties in dating the geologic time scale
Not all rocks can be dated by radiometric methods.
Grains comprising detrital sedimentary rocks are not the same age as the rock in which they formed.
The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed.
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37. The Geologic Time Scale
Difficulties in dating the geologic time scale
Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth’s history and to arrive at ages.
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38. Dating Sedimentary Rocks
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End of Chapter 9
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