geologic timescale

2. PLEDGE OF LEARNING
1. I can describe how layers of the
rocks (stratified rocks) are formed;
2. I can describe the different methods
(relative and absolute dating) to
determine the age of stratified
rocks;

3. PLEDGE OF LEARNING
3. I can explain how relative and
absolute dating were used to
determine and identify the
subdivisions of the geologic time
scale;

4. PLEDGE OF LEARNING
4. I can describe how marker fossils (also
known as guide fossils) are used to
define and identify subdivisions of the
geologic time scale; and
5. I can describe how the Earth’s history can
be interpreted from the geologic time
scale.

5. Stratification of Rocks

6. Stratification
• Crustal movement, displacement of
soils, and distortion of terrains lead to
layering of rocks
• Sedimentary rocks form as
sediments are deposited on the
bottom of a body of water

7. Nicholas Steno (1638-1686)
• In late 17th century, he introduced
the principle of geologic timescale
• Each layer of the rock could
represent a “slice” of time.

8. How do geologists determine
how old rocks are?

9. Dating Methods
Relative Dating
Absolute Dating

10. Relative dating
• This method does not provide actual
numerical dates for the rocks but all
are just estimates based on the
profile of the strata

11. PRINCIPLE OF RELATIVE DATING

12. Principle of Superposition
• Rock layer above is younger
than the ones below it. (Oldest
on bottom, youngest on top)

14. Principle of Original Horizontality
• Sedimentary layers are deposited in
approximately horizontal sheets.
• If layers are folded, episode of
deformation must have occurred after
rocks formed. Age of folding is
younger than youngest deformed rock
unit.

16. Principle of Crosscutting Relationships
• Any feature (e.g. fault or intrusion)
that cuts across rocks is younger than
the youngest rock that is cut.

17. Relative Age Dating

18. Illustration of Relative Age Principles
Superposition
Cross Cutting
RelationsOriginal
Horizontality

19. Absolute dating
• use radiometric dating
techniques to determine how
long ago the rock formed in
the exact number of years

20. • Uses radioactive decay and
the Half-life of certain
elements
• Half-life - time it takes for one-
half of the radioactive material
to decay

21. • Half-Life: the time it takes for 50%
(1/2) of the nuclei in a radioactive
sample to decay to its stable isotope
• Multiply the number of half-lives by
the half-life time to get the age of a
fossil

22. • If the half-life of an isotope is 10,000
years and 3 half live have passed,
what is the age of the fossil?

23. Radiometric dating
Radioactive elements (isotopes) used for dating:
• Carbon (C14) - Halflive: 5730 years
• Potassium (K40) - Halflive: 1.25 billion years
• Uranium (U235) - Halflive: 0.71 billion years
• Thorium (Th 232) - Halflive: 14.1 billion years
• Mainly igneous and metamorphic rocks contain
Potassium, Uranium, Thorium
• C14 method to date charcoal, shells, other organic
materials carbon

25. 4.6 billion years old = 4,600,000,000
The Age of the Earth

26. The earth’s 4.6 billion year history
is divided into major units of time:
Cenozoic Era
Mesozoic Era
Paleozoic Era
Precambrian Eon
Phanerozoic Eon

27. Precambrian Eon
• 4.6 billion years before present to 544
million years before present
• Longest era with a sparse fossil record
• Origin of earth’s crust, first atmosphere,
and first seas

28. Precambrian Eon
• Earliest fossils of cyanobacteria use
photosynthesis to produce oxygen
• Ozone layer in the atmosphere is
formed from oxygen

30. Phanerozoic Eon

31. Paleozoic era
(The Era of Old Life)

32. Paleozoic era
• 544 million years before
present to 245 million
years before present
• Marine communities
flourish
• Early fishes develop

33. Paleozoic era
• Origin of amphibians,
insects & reptiles
• Recurring ice ages/
Appalachians mountains
form
• Spore-bearing plants
dominate

34. Paleozoic era
(continued)…
• 286 - 248 million years
before present:
Supercontinent of Pangaea
forms
• 248 million years before
present: MASS
EXTINCTION-90 % of all
known families lost!
c

35. Mesozoic Era
(The age of reptile)

36. Mesozoic Era
• 245 million years before
present - 65 million years
before present
• The age of the dinosaurs!
• Gymnosperms dominate
land plant/ origin of
angiosperms - flowering
plants

37. Mesozoic Era
• Origin of mammals & birds
• 145 million years before
present - asteroid impact?
MASS EXTINCTION
• Pangaea begins to separate/
Rocky mountains form

38. 65 million years before
present….
• ASTEROID IMPACT!
• Mass extinction of ALL
dinosaurs and many
marine organisms
• End of the Mesozoic era

39. Cenozoic Era
• 65 million years before
present -today
• Present era we live in
• Continued evolution and
adaptations of flowering
plants, insects, birds,
mammals

40. Cenozoic Era
(The age of mammals)
“Era of recent life”

41. Cenozoic Era
• Mammals dominant
• Major crustal
movements & mountain
building (Alps &
Himalayan mountains
form)