Power point packet notes for students

1. CH. 11 – GEOLOGIC TIME

2. Geologic Time
Earth = 4.6 billion years old
Oldest rocks = 3.2 billion years
old
Question: How can we be sure
Earth is older than its oldest rx?

3. Base Units of Time
Humans = 1 year
Earth = 1 million years

4. Types of Geologic Dating
1) Relative Dating
- chronological order (1st, 2nd, 3rd,
etc.)
Ex: Rock layers are ______than
Grand Canyon itself

5. Types of Geologic Dating
2) Numerical Dating
- assigning an age in years to a
rock or geologic event
Ex: Grand Canyon is 10 million
yrs old

6. Methods of Relative Dating
1) Superposition – horizontal
layers (sed. rx & lava flows)
2) Original Horizontality
- layers tilted after deposition

7. Methods of Relative Dating
3) Cross-Cutting Relations
- a feature which cuts across rx
is younger than the rx it cuts
Ex: dike, fault, canyons

8. Methods of Relative Dating
4) Inclusions
- fragment contained w/in a rock
is ________ than the rock it’s in

9. Inclusions
Large-scale: Rock fragment of
sandstone in a batholith
- (batholith/sandstone) is older
Ex: Xenolith (“xeno” = stranger)
Piece of country rock found w/in
igneous rock

10. Inclusions
Small-scale:
Gravel in a conglomerate is
_______ than the
conglomerate

11. Methods of Relative Dating
5) Unconformities - “missing
time”
- gap of time in rock record at
that location
- usually buried erosional surface

12. Unconformities
Major John Wesley Powell
- “missing time” is like pages torn
from history book

13. Unconformities
Three types:
a) Angular unconformity
- layers on bottom tilted at an
angle to layers on
top

14. Types of Unconformities
b) Nonconformity
- crystalline rx on bottom overlain
by sedimentary rx
- crystalline rx = intrusive
igneous or metamorphic

15. The great unconformity

16. Types of Unconformities
c) Disconformity
- layers above & below
unconformity are parallel
- identified based on
fossil content or
channel cuts

17. Methods of Relative Dating
6) Fossil Succession
- plants & animals have changed
through time
- same sequence of major life
groups around world

18. Uses of Fossil Succession
a) Geologic Time Scale
- any time period can be
identified based on its fossil
content
- know eras and major life forms
- know boundary criteria

19. Geologic Time Scale
Paleozoic Era (“ancient life”)
“Age of Invertebrates”
- ended with largest mass
extinction in Earth’s history
(over 90% organisms were
extinct by 245 mya)

20. Geologic Time Scale
Mesozoic Era (“middle life”)
“Age of the Dinosaurs”
- ended with extinction of
dinosaurs (66 mya)

21. Geologic Time Scale
Cenozoic Era (“recent life”)
“Age of Mammals”
Will the Cenozoic Era end?

22. Uses of Fossil Succession
b) Correlation – matching rx of
similar ages in different regions

23. Methods of Correlation
i) Index fossil
Requirements:
a) widespread geographically
b) existed for a short period of
geologic time

24. Methods of Correlation
ii) Fossil Assemblages
*Overlapping geologic ranges
- narrows down time frame

25. Fossil Assemblages
Fossil A = Devonian-Tertiary
Fossil B = Cambrian-Permian
Fossil C = Pennsylvanian -
Jurassic
Age of rock:

26. Numerical Dating
Assigns an age in years to rx or
geologic events
One method is radiometric dating
Radioactivity = spontaneous
change w/in nucleus of atom

27. Radioactivity
Isotope – variable forms of same
element due to different
number of neutrons in atoms
Ex: Carbon
Atomic Number = 6
Atomic Mass = 12.011

28. Radioactivity
Unstable isotopes are
radioactive
- nucleus emits or captures
atomic particles

29. Radioactivity Vocabulary
1) Parent element = unstable
atom
2) Daughter product = atom it
turns into

30. Daughter Product
Belongs to a different element
than parent element
- some daughter products are
also unstable (radioactive)
- can take several steps to reach
a stable atom

31. Radioactivity Vocabulary
3) Half-life = amount of time for
one-half parent element to
change into daughter product
*Rate of decay
NOTE: decay doesn’t imply we
have less matter (atoms)

32. Half-life
Each parent element has a
unique half-life
Each half-life is a constant
- unaffected by changes in
temperature or pressure

33. Review of Radiometric Dating
1) Half-life is constant for each
radioactive isotope
Ability to use radiometric dating
depends on:
i) parent element available
ii) age of rock

34. Examples:
Parent Daughter Half-life
238U 206Pb 4,500 my
235U 207Pb 713 my
40K 40Ar 1,300 my
14C 14N 5,730 yrs

35. Half-life diagram

36. 14C
Produced in upper atmosphere
Used to measure ages of organic
remains
Effective back to 75,000 yrs.

37. Review of Radiometric Dating
2)*IMPORTANT to have a closed
system for an accurate date!
Radiometric dating is most
accurate for igneous rx
- clock starts “ticking” as soon as
magma crystallizes

38. Review of Radiometric Dating
Sedimentary rx – age obtained is
for source area of sediment
Sedimentary rock has to be
(younger, older) than this age

39. Review of Radiometric Dating
Metamorphic rx
- chance of losing atoms out of
system is greater
- partial melting can allow atoms
to escape (esp. Ar40)

40. Metamorphic Rx
If daughter product escapes
rock, its radiometric age will be
(older, younger) than it really is.

41. Geologic Time
Earth = 4.6 billion years old
Oldest rocks = 3.2 billion years
old
How can we be sure Earth is
older than its oldest rx?