For Geologists

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

6. 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
Law of superposition
A bed of rock is older than that above, younger than that below
Principle of original horizontality
Principle of cross-cutting relationships
Inclusions
Unconformities

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

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

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

12. Fragments of one rock
enclosed in another

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

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

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

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

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

19. Unconformity Types

21. Relative Dating Principles

22. Correlation of rock layers
Matching up rocks from different places
that are similar in age
Physical criteria
Fossils

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

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

27. Overlapping Ranges of Fossils

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

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

31. Absolute dating w/ radioactivity
Example: an isotope with a half life of 1 year
# of atoms
Parent Daughter
Now 1000 0
Next year 500 500
2 yrs from now 250 750
3 yrs from now 125 875
10 yrs from now 1 999

32. Radioactive Decay

33. Absolute dating w/ radioactivity