1. Ch 21 Fossils and the Rock Record
Page Objectives:
1. Describe the geologic time scale
2. Distinguish among the following geologic time scale divisions: eon, era, period, and epoch.
3. Master the geologic principles of original horizontality, superposition, cross-cutting relationships, inclusions, and methods for determining relative age

3. Section 21.1 The Geologic Time Scale
The rock record ***Take out ESRTs turn to pgs 8+9
The Earth’s history has been divided into time units based upon fossils contained in the rocks.

4. Section 21.1 The Geologic Time Scale
Geologic time scale: A record of Earth’s history from it’s origin over 4.6 billion years ago to the present.
The oldest part of the scale is on the bottom.
The same as the rock layers (oldest on the bottom).
Eon: Longest time unit and is measured in billions of years.

6. Section 21.1 The Geologic Time Scale
Page
There are 3 eons:
1. Archean
2. Proterozoic
3. Phanerozoic

7. Section 21.1 The Geologic Time Scale
1. Archean
Stromatolites created free oxygen and began to change the composition of our atmosphere.

8. Section 21.1 The Geologic Time Scale
2. Proterozoic

9. Section 21.1 The Geologic Time Scale
3. Phanerozoic

10. Section 21.1 The Geologic Time Scale
An Era is the next longest time period.
There are also 3
Paleozoic
(Paleo “old” zoic “life”
Mesozoic
(Meso “middle” zoic “life”)
Cenozoic
(Ceno “young’ zoic “life”)

11. Section 21.1 The Geologic Time Scale
An Era is the next longest time period.
There are also 3
Paleozoic
(Paleo “old” zoic “life”

12. Section 21.1 The Geologic Time Scale
An Era is the next longest time period.
There are also 3
Mesozoic
(Meso “middle” zoic “life”)

13. Section 21.1 The Geologic Time Scale
An Era is the next longest time period.
There are also 3
Cenozoic
(Ceno “young’ zoic “life”)

14. Section 21.1 The Geologic Time Scale
Page
The next largest unit of time is the Period.
Each Era is divided into Periods.
The Precambrian (before the Cambrian period) makes up about 90% of all geologic history.
The first 2 Eons (Archean and Proterozoic), represent this Precambrian time period.
All organisms of this time had soft bodies with no shells or skeletons. Ex Worms and sponges.

16. Section 21.1 The Geologic Time Scale
Plants and animals evolve
Paleozoic era:
Oceans fill with diversity of life
Cambrian period:
Land plants appear -->land animals
End of Paleozoic marks largest extinction ever
-90% of all marine invertebrates become extinct
Mesozoic: Amphibians left water for land
-large water reptiles emerge
-Dinosaurs emerge

17. Section 21.1 The Geologic Time Scale
Cretaceous Period:
Dinosaur pop goes down
Mammal pop goes up
Flowering plants and trees emerge.

18. Section 21.1 The Geologic Time Scale
Mesozoic ends with a large extinction event.
This sharp division in time is the K T boundary
-Dinosaurs and many other groups go extinct here
-65 million years ago

19. Section 21.1 The Geologic Time Scale
These periods are defined by the life forms that were abundant or went extinct at that time.
Time range is from 10’s of millions to 100’s of millions of years.
Many periods named after geographic locations.
Epochs: Are time divisions of a period.

20. Sect 21.2 Relative-age Dating of Rocks
James Usher (some Irish guy) at the turn of the century states: -Earth only 4,000 years old
-Based on a chronology of human history

21. Sect 21.2 Relative-age Dating of Rocks
James Huton (some Scottish guy) around 1770 states:
Principal of Uniformitarianism : The processes concerning mountain building, erosion, and sea level changes that are occurring today have been occurring since Earth formed.

22. Sect 21.2 Relative-age Dating of Rocks
Principles for determining Relative age:
Places rocks and events that formed them in order, but without exact dates.
A) There are four basic concepts for relative age dating.
1. Principle of original horizontality
2. Principle of superposition
3. Principle of cross-cutting relationships
4. Inclusions

23. Sect 21.2 Relative-age Dating of Rocks
1. Principle of original horizontality
a) Sedimentary rocks are deposited in horizontal or nearly horizontal layers

24. Sect 21.2 Relative-age Dating of Rocks
2. Principle of original Superposition
a) The oldest rocks are on the bottom and the youngest rocks are on top.
Youngest -->
Oldest layer -->

25. Sect 21.2 Relative-age Dating of Rocks
3. Principle of cross-cutting relationships
a) An intrusion or a fault is younger than the rock it cuts across

27. Sect 21.2 Relative-age Dating of Rocks
4. Inclusions
a) Rock fragments found in sedimentary rocks are older than the rock itself.

28. Sect 21.2 Relative-age Dating of Rocks
Other means of determining relative age
-Constant tectonic movement and erosion make it hard to find an undisturbed sequence of rocks.
Unconformity: A gap in the geologic rock record

29. Sect 21.2 Relative-age Dating of Rocks
***FYI
Disconformity When sedimentary rocks overlie other horizontal sedimentary rocks.
Angular Disconformity: When sedimentary rocks overlie at an angle to horizontal sedimentary rocks.
Nonconformity: Sedimentary rocks overlie non-sedimentary rocks suggesting uplift of the non-sedimentary rocks.

30. Sect 21.2 Relative-age Dating of Rocks
Correlation of rock strata
Correlation: the matching of outcrops of one geographic region to another.
-Geologists can use distinctive fossils or unique rock and mineral formations to help.

32. Sect 21.3 Absolute-age Dating of Rocks
Absolute dating: The actual age of the rock or fossil is determined. Look at your ESRTs cover for carbon
Radioactive decay: The emission of radioactive particles and the resulting change into other elements over time.
Once the emission begins the rate of decay is a constant regardless of the environment, pressure, temperature, or other physical changes.

33. Sect 21.3 Absolute-age Dating of Rocks
(parent) --> (daughter)
Original radioactive material --> New material
ex: U > Pb 206
Uranium --> Lead

34. Sect 21.3 Absolute-age Dating of Rocks
Radiometric dating: Process of calculating the ratio of parent nuclei to daughter nuclei within a given sample of a rock or a fossil.
Isotope: Atoms of the same element that have different mass numbers and the same chemical properties.

35. Sect 21.3 Absolute-age Dating of Rocks
Mass Number: Combined number of protons and neutrons in the nucleus of an atom.
Half life: Period of time it takes for a radioactive isotope, such as carbon -14, to decay to one-half of its original amount.
Page

36. Sect 21.3 Absolute-age Dating of Rocks
Carbon dating very useful for dating organic matter but is limited to about 75,000 years.
The half-lives of various elements are on the first page of your ESRT’s
Carbon -14 is 5.7 X 10 the third power. This = 5,700 years

37. Sect 21.3 Absolute-age Dating of Rocks
***Carbon dating could not be used to date dinosaur bones since they went extinct 65 million years ago.
In a similar sense Uranium with a half life of 700,000 years would not be very useful for dating objects only a couple thousand years old.

38. Sect 21.3 Absolute-age Dating of Rocks

39. Sect 21.3 Absolute-age Dating of Rocks
--->
Original parent
Daughter
First half life
Second
Third
Fourth
???
--->
--->
--->
--->
25%
50%
1/8
12.5%
1/1

40. Sect 21.3 Absolute-age Dating of Rocks
Other ways to determine age
-Tree rings
Dendrochronology: the science of comparing annual growth rings in trees to date events and changes in past environments.
-Seasonal climatic changes

41. Sect 21.3 Absolute-age Dating of Rocks
Other ways to determine age
Varves: Bands of alternating light and dark color sediments of sand, clay and silt. These are caused by seasonal changes in a lake.
Summer --> more NRG--> thin layer of light colored deposits
Winter --> Less NRG --> thick layer of dark (organic rich) deposits

42. Sect 21.3 Absolute-age Dating of Rocks
Page or glossary page
Key Bed: Sediment layer that serves as a time marker in the rock record and results from volcanic ash or meteorite impact debris that spread out and covered large areas of Earth.

43. Sect 21.4 Remains of Organisms in the Rock Record
Fossils: Are the evidence of remains of once living organisms.
Evolution: Adaptation of life forms to changing environmental conditions.
Original preservation: Parts of an organism that have not undergone any change since the organisms’ death.
Ex: insects trapped in tree sap (now amber) like in “Jurassic Park”

44. Sect 21.4 Remains of Organisms in the Rock Record
altered hard parts: The parts that have been replaced by minerals or by recrystallization.
This process is called permineralization
Index fossils: Fossils that can be used by geologists to correlate rock layers over large geographic areas, or to date a particular rock layer. The four criteria are:
1. Easily recognized
2. Abundant
Wide spread
4. Lived for a short period of time

45. Sect 21.4 Remains of Organisms in the Rock Record
Mold: is formed when the original part of an organism creates an imprint in the rock after the original part was weathered away.
Cast: When a cavity left by an organism is filled by minerals or sediments.

46. Vocabulary quiz
Define the following terms:
1. Photosynthesis
2. Absolute age
3. The principle of superposition
4. Correlation

47. 1. Which of the following represents 90% of Earth’s geologic history?
A. Cenozoic B. Mesozoic
C. Paleozoic D. Precambrian
2. Which term describes when sedimentary rocks overlie non-sedimentary rocks suggesting uplift of the non-sedimentary rocks?
A. Unconformity B. Disconformity
C. Angular Disconformity D. Nonconformity

48. 3. In each diagram, which letter represents the oldest rock?
A. A B. B
C. C D. There is no way to tell

49. 4. Which of the following is not a characteristic of an Index fossils?
A. Lived for a short period of time B. Abundant
C. Wide spread D. Rare
5. Substance X has a 100 g and a half life of 20 minutes. How many grams of substance X will be left after one hour?
Bonus:
How many years will it take to have 25% of the original carbon left?