1. The Rock and Fossil Record
Chapter 6
The Rock and Fossil Record

2. I. The Principle of Uniformitarianism
Many people believed in catastrophism, or sudden events called catastrophes, changing earth’s landscape
In 1788, James Hutton wrote Theory of the Earth, in which he developed the thought of uniformitarianism
- Processes that we observe today remain uniform, or do not change over time

3. II. Modern Geology
Scientists today realize that neither uniformitarianism nor catastrophism accounts for all geologic change throughout Earth’s history
- Most geologic change occurs gradually
- however, catastrophes have also occurred

4. III. Paleontology
The science involved with the study of past life
Data collected comes from fossils
Invertebrate paleontologists study animals without backbones
Vertebrate paleontologists study animals with backbones
Paleobotanists study fossils of plants

5. IV. Relative Dating
A. The Principle of Superposition
Younger rocks lie above older rocks in undisturbed sequences
Some rocks however, are disturbed by forces within the Earth

6. B. The Geologic Column
An ideal sequence of rock layers
Contains all the known fossils and rock formations on Earth

7. C. Events That Disturb Rock Layers
Scientists assume that sediment has always been deposited in horizontal layers
If rock layers are not horizontal, something must have disturbed them after they formed
Disruptions create features that cut across existing rock

8. 1. Faults are breaks in the Earth’s crust - Blocks slide relative to one another

9. 2. Intrusions occur when molten rock forms in the Earth’s interior and squeezes into existing rock and cools

10. 3. Folding occurs when rock layers bend and buckle from Earth’s internal forces

11. 4. Tilting occurs when internal forces in the Earth slant rock layers

12. D. Gaps in the Record – Unconformities
Missing rock layers create breaks in the geologic column called unconformities
Geologists must question whether the “missing layer” was never present or whether it was somehow removed

13. E. Types of Unconformities
1. Disconformities
Younger layers at the top of the sequence are removed by erosion and are deposited elsewhere
Deposition resumes at a later time
Represents thousands to millions of years of missing time

14. 2. Nonconformities
Horizontal sedimentary rock layers lie on top of an eroded surface of older intrusive or metamorphic rock

15. 3. Angular Unconformities
Found between horizontal layers of sedimentary rock and layers of rock that have been tilted or folded

16. V. Absolute Dating
A. Radioactive decay
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons

17. Isotopes of radioactive elements are used to determine the absolute ages of fossils and rocks
Unstable, or radioactive, isotopes tend to break down into stable elements by radioactive decay
Radioactive decay occurs at a steady rate so scientists can use unstable isotopes to determine an object’s age

18. B. Radiometric Dating
Unstable isotopes, or parent isotopes, decay and form stable isotopes, or daughter isotopes
Unstable isotopes have known half-lifes, the time it takes for half of the parent isotopes to decay to daughter isotopes

19. Scientists determine the age of substances by calculating the ratio of parent isotopes to daughter isotopes
Example – if a rock contains an element with a half-life of 10,000 years, it will have half parent material and half daughter material when it is 10,000 years old
10,000 years
20,000 years
30,000 years
40,000 years
0 years

20. C. Types of Radiometric Dating
1. Potassium-Argon Method
Used to date rocks older than 100,000 years
Potassium-40 has a half-life of 1.3 billion years and decays to argon

21. 2. Uranium-Lead Method
Used to date rocks older than 10 million years
Uranium-238 has a half-life of 4.5 billion years and decays to lead-206 in a series of steps

22. 3. Rubidium-Strontium Method
Used to date rocks older than 10 million years
Radidium-87 has a half-life of 49 billion years and decays to Strontium-87

23. 4. Carbon-14 Method
Used to date things that lived within the last 50,000 years
Carbon is found in constant ratios of carbon-14, a radioactive isotope, and carbon-12

24. Carbon-14 has a half-life of 5,730 years
Theses isotopes combine with oxygen to form carbon dioxide (CO2)
Carbon dioxide is taken in by plants for photosynthesis as long as it is alive

25. Animals eat these plants and thus take in carbon dioxide as well
When a plant or animal dies, it no longer takes in carbon dioxide and the amount of carbon-14 begins to decrease

26. VI. Fossils
A. Fossilized Organisms
1. Fossils in Rocks
Most often preserved in sedimentary rock
Hard parts of organisms are more resistant to decay

27. 2. Fossils in Amber
Hardened tree sap
Best preserves insects

28. 3. Petrifaction
Process in which minerals replace an organism’s tissues
Example: petrified wood

29. 4. Frozen Fossils
Cold temperatures slow down decay
Many frozen fossils are preserved from the last ice age

30. B. Other Types of Fossils 1
B. Other Types of Fossils 1. Trace Fossils Example: tracks - Footprints are filled with sediment and preserved in rock

31. 2. Molds and casts

32. C. Using Fossils to Interpret the Past
1. The information in the Fossil Record
Fossil record is incomplete because most organisms never became fossils
- More is known about organisms that had hard body parts and organisms that lived in environments that favored fossilization

33. 2. History of Environmental Change
Marine fossils help scientists reconstruct ancient coastlines
Scientists can tell whether the climate in an area was cooler or wetter than it is at present

34. 3. History of Changing Organisms
Older rock layers contain organisms that often differ from the organisms found in younger rock layers

35. D. Using Fossils to Date Rocks
Certain types of fossils appear only in certain layers of rock
Scientists can determine the time span organisms lived by dating rock layers above and below these fossils

36. 1. Index Fossils
Fossils of organisms that lived during a relatively short time span found in layers throughout the world
Ammonites and trilobites are index fossils that lived 208 and 400 million years-ago respectfully
When scientists find these fossils, they know how old the rock is around these fossils

37. III. Geologic Time Scale
Scientists study 4.6 billion years of Earth’s history
Geologists have created a geologic time scale that divides Earth’s history into distinct intervals of time

38. A. Divisions of Time
Represent shorter intervals in which visible changes have taken place on Earth
Largest divisions of geologic time are eons
- Hadean eon, Archean eon, Proterozoic eon, and Phanerozoic eon

39. The Phanerozoic eon has three eras
- Paleozoic era, Mesozoic era, and Cenozoic era
Eras are broken down into periods which are broken down into epochs

40. 1. The Paleozoic Era – Old Life
542 to 251 million years ago
First era well represented by fossils
Marine life flourished
Land plants, amphibians, reptiles, and insects appeared at the end of the Paleozoic era

41. 2. The Mesozoic Era – The Age of Reptiles
251 million years ago
Dinosaurs inhabited the land
Birds appeared at the end of the Mesozoic era and small mammals appeared

42. 3. The Cenozoic Era – The Age of Mammals
66.5 million years ago and continues to the present
Unique traits, such as regulating body temperature internally and bearing young that develop inside the mother, may have helped mammals survive the environmental changes that probably caused the extinction of the dinosaurs

43. B. The Appearance and Disappearance of Species
An increase in the number of species on Earth is a result of either a relatively sudden increase or decrease in competitions
A decrease in the number of species on Earth is a result of extinction