1. Geologic Time
Who is Stan Hatfield and Ken Pinzke

2. 12.1 Discovering Earth’s History
Rocks Record Earth History
12.1 Discovering Earth’s History
 Rocks record geological events and changing life forms of the past.
 We have learned that Earth is much older than anyone had previously imagined and that its surface and interior have been changed by the same geological processes that continue today.

3. Time Time is measured by change Age of the Earth: 4.6 billion years
Deep time, millions & billions of years, is difficult to comprehend
James Hutton ( ) postulated a very old Earth

5. Geologic Time Scale
Age correlations using fossils are used to construct the geologic time column
Names derived from locations where rocks of that age are well exposed
Different units are separated by unconformities of changes in fossils
Originally developed from superposition of rock formations in Europe

6. 12.1 Discovering Earth’s History
A Brief History of Geology
12.1 Discovering Earth’s History
Uniformitarianism means that the forces and processes that we observe today have been at work for a very long time.
By studying rocks, especially sedimentary rocks, geologists can start to understand the past.

7. Sequences of rock contain gaps
Geologic time is continuous
The rock record at a given location may not record all of geologic time

8. 12.1 Discovering Earth’s History
Relative Dating—Key Principles
12.1 Discovering Earth’s History
 Relative dating tells us the sequence in which events occurred, not how long ago they occurred.
 Stratigraphy
• The study of layered rocks in the Earth’s crust.

9. Relative Ages Rules of Stratigraphy:
Principle of Stratigraphic Superposition
Law of Original Horizontality
Faunal Succession
Crosscutting Relationships
Inclusions

10. 12.1 Discovering Earth’s History
Relative Dating—Key Principles
12.1 Discovering Earth’s History
 Law of Superposition
• The law of superposition states that in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it.

11. Ordering the Grand Canyon’s History
Makes no sense without caption in book

12. 12.1 Discovering Earth’s History
Relative Dating—Key Principles
12.1 Discovering Earth’s History
 Principle of Original Horizontality
• The principle of original horizontality means that layers of sediment are generally deposited in a horizontal position.
If you see rock layers that are flat, it means that they have not been disturbed and are still in their original position.
If rock layers are bent or tilted, this means they must have been moved into this position after they were deposited

13. Disturbed Rock Layers
Makes no sense without caption in book

14. Principle of Faunal Succession
Based on the observation that sedimentary rock strata contain fossilized flora and fauna (plants and animals) and that these succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances

16. 12.1 Discovering Earth’s History
Relative Dating—Key Principles
12.1 Discovering Earth’s History
 Principle of Cross-Cutting Relationships
• The principle of cross-cutting relationships states that when a fault cuts through rock layers, or when magma intrudes other rocks and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected.
 Inclusions
• Inclusions are rocks contained within other rocks.
• Rocks containing inclusions are younger than the inclusions they contain.

17. Applying Cross-Cutting Relationships
Cross-cutting relationships are an important principle used in relative dating. An intrusive rock body is younger than the rock it intrudes. A fault is younger than the rock layers it cuts.
Makes no sense without caption in book

18. Formation of Inclusions
This shows a mass of igneous rock formed from magma that intruded an older rock body
Makes no sense without caption in book
This shows the older rock erodes and exposes the igneous rock to weathering
Sedimentary rock layers form on top of the weathered igneous rocks

19. 12.1 Discovering Earth’s History
Relative Dating—Key Principles
12.1 Discovering Earth’s History
 Unconformities
• An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.
• An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred.

20. Formation of an Angular Conformity
Represents an extended period during which deformation and erosion occurred.
Makes no sense without caption in book

21. 12.1 Discovering Earth’s History
Relative Dating—Key Principles
12.1 Discovering Earth’s History
 Unconformities
• A nonconformity is when the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks.
• A disconformity is when two sedimentary rock layers are separated by an erosional surface.

23. A Record of Uplift, Erosion, and Deposition in the Grand Canyon
Makes no sense without caption in book

24. 12.1 Discovering Earth’s History
Correlation of Rock Layers
12.1 Discovering Earth’s History
Correlation is establishing the equivalence of rocks of similar age in different areas.
By correlating rocks from one place to another, its possible to create a more complex view of the geologic history of a region.

25. Correlation of Strata at Three Locations
No single location contains the entire sequence
Makes no sense without caption in book

26. 12.2 Fossils: Evidence of Past Life
Fossil Formation
12.2 Fossils: Evidence of Past Life
 Fossils are the remains or traces of prehistoric life. They are important components of sediment and sedimentary rocks.
 The type of fossil that is formed is determined by the conditions under which an organism died and how it was buried.
 Unaltered Remains
• Some remains of organisms—such as teeth, bones, and shells—may not have been altered, or may have changed hardly at all over time.

27. 12.2 Fossils: Evidence of Past Life
Fossil Formation
12.2 Fossils: Evidence of Past Life
 Altered Remains
• The remains of an organism are likely to be changed over time.
• Fossils often become petrified or turned to stone.
• Molds and casts are another common type of fossil.
• Carbonization is particularly effective in preserving leaves and delicate animals. It occurs when an organism is buried under fine sediment.
Insects are relatively rare fossils, some have been preserved in amber

28. 12.2 Fossils: Evidence of Past Life
Fossil Formation
12.2 Fossils: Evidence of Past Life
 Indirect Evidence
• Trace fossils are indirect evidence of prehistoric life.
Ex: Tracks,footprints, burrows
 Conditions Favoring Preservation
• Two conditions are important for preservation: rapid burial and the possession of hard parts.

29. Types of Fossilization
Bee preserved as thin carbon layer
impression
Dinosaur footprint
Insect in amber
Makes no sense without caption in book
Natural casts of shelled organisms called ammonites
Petrified wood in AZ

30. 12.2 Fossils: Evidence of Past Life
Fossils and Correlation
12.2 Fossils: Evidence of Past Life
 The principle of fossil succession states that fossil organisms succeed one another in a definite and determinable order. Therefore, any time period can be recognized by its fossil content.
 Index fossils are widespread geographically, are limited to a short span of geologic time, and occur in large numbers.

31. 12.2 Fossils: Evidence of Past Life
Fossil Formation
12.2 Fossils: Evidence of Past Life
 Interpreting Environments
• Fossils can also be used to interpret and describe ancient environments.

32. Overlapping Ranges of Fossils
Overlapping ranges of fossils help date rocks more exactly than using a single fossil. The fossils contain in unit A all have overlapping age ranges in time 4. The fossils in rock unit B have overlapping time ranges in time 2
Makes no sense without caption in book

33. 12.3 Dating with Radioactivity
Basic Atomic Structures
12.3 Dating with Radioactivity
 Orbiting the nucleus are electrons, which are negative electrical charges.
 Atomic number is the number of protons in the atom’s nucleus.
 Mass number is the number of protons plus the number of neutrons in an atom’s nucleus.

34. 12.3 Dating with Radioactivity
Radioactivity is the spontaneous decay of certain unstable atomic nuclei.
Reactions in the nucleus change the number of protons
Reaction products are released as heat
Parent: unstable or radioactive isotope of an element
Daughter Products: isotopes that result from the decay of the parent
Ratio of parent to daughter determines age

35. Common Types of Radioactive Decay
Makes no sense without caption in book

36. 12.3 Dating with Radioactivity
Half-Life
12.3 Dating with Radioactivity
 A half-life is the amount of time necessary for one-half of the nuclei in a sample to decay to a stable isotope.

37. The Half-Life Decay Curve
Makes no sense without caption in book

39. 12.3 Dating with Radioactivity
Radiometric Dating
12.3 Dating with Radioactivity
 Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs.
 Radiometric dating is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.

40. 12.3 Dating with Radioactivity
Radiometric Dating
12.3 Dating with Radioactivity
 As a radioactive isotope decays, atoms of the daughter product are formed and accumulate.
 An accurate radiometric date can be obtained only if the mineral remained in a closed system during the entire period since its formation.

41. Radioactive Isotopes Frequently Used in Radiometric Dating
Makes no sense without caption in book

42. 12.3 Dating with Radioactivity
Dating with Carbon-14
12.3 Dating with Radioactivity
 Radiocarbon dating is the method for determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample.
 When an organism dies, the amount of carbon-14 it contains gradually decreases as it decays. By comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determined.

43. Carbon-14 radiometric clock

44. 12.3 Dating with Radioactivity
Importance of Radiometric Dating
12.3 Dating with Radioactivity
 Radiometric dating has supported the ideas of James Hutton, Charles Darwin, and others who inferred that geologic time must be immense.

45. Problems with Radiometric Dating
Radioactive elements are found in trace concentrations
Precise measurements of concentration
Isotopes may move over time
Weathering, metamorphism or other alterations disrupt the clock
Some rocks don’t contain easily dated minerals

46. 12.4 The Geologic Time Scale
Difficulties With the Geologic Time Scale
12.4 The Geologic Time Scale
 A sedimentary rock may contain particles that contain radioactive isotopes, but these particles are not the same age as the rock in which they occur.
 The age of a particular mineral in a metamorphic rock does not necessarily represent the time when the rock was first formed. Instead, the date may indicate when the rock was metamorphosed.

47. So, how do we use
radiometric dating
techniques?

48. Using Radiometric Methods to Help Date Sedimentary Rocks
Numerical dates for sedimentary rock layers are usually determined by examining their relationship to igneous rocks.
Makes no sense without caption in book

49. We combine Absolute (radiometric) dating techniques with Relative dating techniques.
Ex. Principle of Superposition + Radiometric ages of volcanic rocks brackets the age of fossil-bearing layers above and below.
Aint that just cool!!!