1. Geologic Time

2. The Rock Record Rocks record geological events and changing life forms
Uniformitarianism: (James Hutton) the forces and processes that we observe today have been at work for all of earth’s history
Earth is a lot older than originally thought!

3. Relative Dating
Tells us the sequence of events, not the actual time the events occurred
Law of Superposition: in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it
Original Horizontality: layers of sediment are deposited in a horizontal position

5. Relative Dating
Cross-Cutting: when faults cut through rock layers, or magma intrudes, the intrusion or fault is younger than the rock affected
Inclusions: Pieces of one rock are contained within another; the rock containing inclusions is younger

7. Relative Dating
Unconformities: a long period during which deposition stops, erosion removes previously formed rocks, then deposition resumes
3 Types: angular unconformity, discomformity, and nonconformity
Correlation: matching up rocks of similar age in different parts of the earth

8. Relative Dating
Angular unconformity: an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers
Nonconformity exist when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock
Disconformity: unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

9. A = Angular unconformity
B= Nonconformity
C=Disconformity

11. Correlation

12. Fossil Record Fossils are the remains or traces of prehistoric life
Fossil type depends on conditions at death and how it was buried
Fossils are typically found in sedimentary rock
Unaltered Remains: bones, teeth or shells of an animal remain unchanged

13. Fossil Record Altered Remains: most likely event
Petrified: “turned into stone”
Molds: the structure is buried and dissolved. Reflects only shape and surface markings
Casts: If the hollow spaces are filled with mineral matter
Trace fossils: indirect evidence; tracks

14. Fossils and Correlation
Fossils succeed one another in a definite order; any time period can be recognized by the fossil content of the rock
Index Fossils: fossils used as time indicators
Sometimes groups of fossils are used as the index
Also used to interpret the environment

15. Fossils and Correlation
Conditions favoring preservation:
Rapid burial
Possession of hard body parts (skeleton)

16. Types of Fossilization
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17. Overlapping Ranges of Fossils
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18. Dating with Radioactivity
Basic Atomic Structures
Atomic number is the number of protons in the atom’s nucleus
Mass number is the number of protons plus the number of neutrons in an atom’s nucleus.
Isotopes are atoms of the same element containing a different number of neutrons

19. Dating with Radioactivity
Radioactivity is the spontaneous decay of certain unstable atomic nuclei.
Particles and energy are released from the nucleus
A more stable daughter is the result

20. Common Types of Radioactive Decay
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21. Dating with Radioactivity
Half-Life
A half-life is the amount of time necessary for one-half of the nuclei in a sample to decay to a stable isotope.

22. The Half-Life Decay Curve
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23. Dating with Radioactivity
Radiometric Dating
Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs
Radiometric dating is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes

24. Dating with Radioactivity
As a radioactive isotope decays, atoms of the daughter product are formed and accumulate
An accurate radiometric date can be obtained only if the mineral remained in a closed system since its formation.

25. Radioactive Isotopes Frequently Used in Radiometric Dating
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26. Dating with Radioactivity
Dating with Carbon-14
Radiocarbon dating is the method for determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample
When an organism dies, the amount of carbon-14 it contains gradually decreases as it decays. By comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determined

27. The Geologic Time Scale
Structure of the Time Scale
Based on their interpretations of the rock record, geologists have divided Earth’s 4.56-billion-year history into units that represent specific amounts of time.
Taken together, these time spans make up the geologic time scale

28. The Geologic Time Scale
Structure of the Time Scale
Eons represent the greatest expanses of time.
Eons are divided into eras.
Each era is subdivided into periods.
Finally, periods are divided into smaller units called epochs

29. The Geologic Time Scale
Structure of the Time Scale
There are three eras within the Phanerozoic eon:
Paleozoic, which means “ancient life,”
Mesozoic, which means “middle life,”
Cenozoic, which means “recent life.”

30. The Geologic Time Scale
Each period within an era is characterized by somewhat less profound changes in life forms as compared with the changes that occur during an era
The periods of the Cenozoic era are divided into still smaller units called epochs, during which even less profound changes in life forms occur.

31. The Geologic Time Scale
Precambrian Time
During Precambrian time, there were fewer life forms. These life forms are more difficult to identify and the rocks have been disturbed often

32. The Geologic Time Scale
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