1. Geologic time

2. Geologic Time Geologic time scale Relative dating principles
Absolute dating w/ radioactivity
Correlation of rock layers

3. Relative Geologic Time Scale
The relative geologic time scale has a sequence of
eons
eras
periods
epochs
but no numbers indicating how long ago each of these times occurred

4. 0-66.4 m.y.a 66.4-245 m.y.a 245-570 m.y.a Geologic time scale Eons
Phanerozoic - last 570 million years
Cenozoic - recent life
Mesozoic - age of middle life
Paleozoic - ancient life
Precambrian - from birth of Earth up to before
complex life forms developed
Eras
m.y.a
(mammals, humans)
m.y.a
(dinosaurs, 1st scrawny mammals)
m.y.a
(fish, trilobites, clams, corals, ferns )
(algae, bacteria, some fossils without shells like jellyfish)

6. “Telling time geologically”
Relative Dating: Key principles
“Telling time geologically”
Earth’s history concealed in rocks
Goal of geology: unraveling Earth’s history
Principle time keeping devices:
Relative dating - putting rocks/events in proper order
Absolute dating - determining event’s actual time

7. Relative Dating: Key principles
A bed of rock is older than that above, younger than that below
Law of superposition
Principle of original horizontality
Principle of cross-cutting relationships
Inclusions
Unconformities

9. Relative Dating: Key principles
Law of superposition
Principle of original horizontality
Principle of cross-cutting relationships
Inclusions
Unconformities
A bed of rock is older than that above, younger than that below
Sedimentary layers deposited flat

10. Relative Dating: Key principles
Law of superposition
Principle of original horizontality
Principle of cross-cutting relationships
Inclusions
Unconformities
A bed of rock is older than that above, younger than that below
Sedimentary layers deposited flat
Faults & dikes are younger than beds they cross

11. Relative Dating: Key principles
Inclusions
Unconformities
Fragments of one rock enclosed in another

12. Fragments of one rock enclosed in another

13. Relative Dating: Key principles
Inclusions
Unconformities
Fragments of one rock enclosed in another
Break in time in deposition
• An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.
(Conformable layers: deposited w/out interruption)

14. Unconformities 3 types Angular unconformity Disconformity
Relative Dating: Key principles
3 types
Angular unconformity
Disconformity
Nonconformity
• An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred.
• A disconformity is when two sedimentary rock layers are separated by an erosional surface.
• A nonconformity is when the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks.

15. Unconformities 3 types Angular unconformity Disconformity
Relative Dating: Key principles
3 types
Angular unconformity
Disconformity
Nonconformity
Tilted/folded sedimentary rocks overlain by younger, more flat layers

16. Recipe for an angular unconformity
Deposition
Folding/Uplift
Erosion
Subsidence/more erosion

17. Unconformities 3 types Angular unconformity Disconformity
Relative Dating: Key principles
3 types
Angular unconformity
Disconformity
Nonconformity
Layers on either side of unconformity are parallel

18. Unconformities 3 types Angular unconformity Disconformity
Relative Dating: Key principles
3 types
Angular unconformity
Disconformity
Nonconformity
Interface between sedimentary layers and metamorphic or igneous rock

19. Unconformity Types
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21. Relative Dating Principles
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22. Physical criteria Fossils Correlation of rock layers
Matching up rocks from different places
that are similar in age
Physical criteria
Fossils

23. position of a bed in sequence of beds distinct/uncommon minerals
Correlation of rock layers
Physical criteria (OK for short distances)
position of a bed in sequence of beds
distinct/uncommon minerals

24. “principle of fossil succession”
Correlation of rock layers
Fossils
fossil organisms succeed each other in
systematic fashion
any time period can be determined by
fossil content
“principle of fossil succession”

25. “Index fossils” geographically wide-spread short span of time
Correlation of rock layers
“Index fossils”
geographically wide-spread
short span of time
T&L Figure 8.10

27. Overlapping Ranges of Fossils
Makes no sense without caption in book

28. correlated sequence 1 Kansas 2 Indiana 3 Ohio 4 Pennsylvania Mostly
nonmarine
limestone
correlated sequence

30. Chemistry review electrons nucleus Absolute dating w/ radioactivity
Atom
model
electrons
# protrons = atomic #,
defines the element
nucleus
# neutrons can vary:
“isotopes”
protrons
neutrons

31. # of atoms Parent Daughter Now 1000 0 Next year 500 500
Absolute dating w/ radioactivity
Example: an isotope with a half life of 1 year
# of atoms
Parent Daughter
Now
Next year
2 yrs from now
3 yrs from now
10 yrs from now

32. Radioactive Decay
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33. Absolute dating w/ radioactivity