1. Time and Geology Physical Geology 12/e, Chapter 8

2. Define the following terms: Geology textbook page 194-204
Geologic Time Terms:
Definition
Quick Sketch
Uniformitarianism
Numerical age
Relative time
Original Horizontality
Superposition
Lateral Continuity
Cross-Cutting Relationships
Inclusions
Unconformity
Disconformity
Angular Unconformity
Nonconformity
Correlation

3. Zircon Article Read the article
On the same paper as your terms, tell at least 5 facts from the reading that explain or describe earth’s geologic history.
Whether finished or not all work is due today.

4. Relative vs. Numerical Age
Relative age - the order of events or objects, from first (oldest) to last (youngest)
Determined by applying simple principles, including original horizontality, superposition, lateral continuity, cross-cutting relationships, inclusions, unconformities, and correlation of rock units and fossils
Numerical age/ Absolute Age - the age of events or objects, expressed as a number or numbers
Determined using radiometric dating (determining how much radioactive decay of a specific element has occurred since a rock formed or an event occurred)

5. The Law of Superposition
In any undisturbed sequence of strata, the oldest layer is at the bottom of the sequence, and the youngest layer is at the top of the sequence.

6. The Cross-Cutting Law
Any feature that cuts across a body of sediment or rock is younger than the body of sediment or rock that it cuts across.
NOTE:
• A fracture is a crack in rock.
• A fault is a fracture along which movement has occurred.

7. Law of Inclusion
If one rock body contains fragments of another rock body it must be younger than the fragments of rock it contains. OR…The inclusions are older than the rocks which contain them.

8. Principles associated with Relative Dating
The comparing of rock units to decipher their age relative to one another
Principle of Original Horizontality
Sedimentary.
If layers are folded, episode of deformation must have occurred after rocks formed. Age of folding is younger than youngest deformed rocklayers are deposited in approximately horizontal sheets unit.

9. Relative Age Determination
Contacts - surfaces separating successive rock layers (beds)
Formations - bodies of rock of considerable thickness with recognizable characteristics allowing them to be distinguished from adjacent rock layers
Original horizontality - beds of sediment deposited in water are initially formed as horizontal or nearly horizontal layers

10. Relative Age Determination
Superposition - within an undisturbed sequence of sedimentary or volcanic rocks, layers get younger from bottom to top
Lateral continuity - original horizontal layer extends laterally until it tapers or thins at its edges

11. Relative Age Determination
Cross-cutting relationships - a disrupted pattern is older than the cause of the disruption
Intrusions and faults are younger than the rocks they cut through
Baked contacts - contacts between igneous intrusions and surrounding rocks, where surrounding rocks have experienced contact metamorphism
Inclusions - fragments embedded in host rock are older than the host rock

12. Unconformities
Unconformity - a surface (or contact) that represents a gap in the geologic record
Disconformity - an unconformity in which the contact representing missing rock layers separates beds that are parallel to each other
Angular unconformity - an unconformity in which the contact separates overlying younger layers from eroded tilted or folder layers

13. Unconformities
Nonconformity - an unconformity in which an erosional surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock
Plutonic and metamorphic rocks exposed by large amounts of erosion
Typically represents a large gap in the geologic record

14. Correlation
Correlation - determining the time-equivalency of rock units
Within a region, a continent, between continents
Physical continuity
Physically tracing a continuous exposure of a rock unit
Easily done in Grand Canyon
Similarity of rock types
Assumes similar sequences of rocks formed at same time
Can be inaccurate if very common rock types are involved
Correlation by fossils
Fossil species succeed one another through the layers in a predictable order (faunal succession)
Similar fossil assemblages (groups of different fossil species) used

15. Numerical Age Dating
Numerical dating - puts absolute values (e.g., millions of years) on the ages of rocks and geologic time periods
Uses radioactive decay of unstable isotopes
Only possible since radioactivity was discovered in 1896
Radioactive isotopes decay in predictable manner, depending on the half-life (time it takes for a given amount of radioactive isotope to be reduced by half)

16. Fossils Earth Science Textbook page 343-45 and Fossils Handbook page 10-15
Direct fossils
Indirect fossils
Petrified
Replacement
Mold
Cast
Carbonization
Impression
Amber
Tracks
Coprolites

17. Petrified
Replacement

18. Mold & Cast
Carbonization

19. Impression
Amber

20. Tracks
Coprolites

21. Conditions for Fossil Preservation (Earth Science Textbook page 345)
A) B)

22. Combining Relative and Numerical Ages
Radiometric dating gives numerical time brackets for events with known relative ages
Individual layers may be dated directly
Radiometric dating of units above and below brackets age of units in between
Geologic Time Scale
Divided into four Eons
Hadean, Archean, Proterozoic, Phanerozoic
Precambrian (all time prior to Phanerozoic) represents 87% of geologic time)

23. Geologic Time Scale Standard geologic time scale
Worldwide relative time scale
Subdivides geologic time based on fossil assemblages
Divided into eons, eras, periods, and epochs
Precambrian - vast amount of time prior to the Paleozoic era; few fossils preserved
Paleozoic era - “old life”
appearance of complex life; many fossils

24. Geologic Time Scale Mesozoic era - "middle life"
Dinosaurs abundant on land
Period ended by mass extinction
Cenozoic era - "new life"
Mammals and birds abundant
We are currently in the Recent (Holocene) Epoch of the Quaternary Period of the Cenozoic Era
Most recent ice ages occurred during the Pleistocene Epoch of the Quaternary Period

25. Age of the Earth
Numerical dating gives absolute age for Earth of about 4.56 billion years
Oldest age obtained for meteorites, believed to have been unchanged since the formation of the solar system
Earth and rest of solar system very likely formed at this time
Geologic (deep) time is vast
A long human lifetime (100 years) represents only about % of geologic time

26. Geologic Time Pre-Post Test Practice
Each person needs a scratch piece of paper
Number 1-20
Answers only
Work as a group/table partner

27. Geologic Time Review
Test Review

30. Divisions of Geologic Time
Eras are subdivided into periods...periods are subdivided into epochs.
Era
Period
Epoch
E + P = EP

31. Today…
Today we are in the Holocene Epoch of the Quaternary Period of the Cenozoic Era.
Which unit is the largest?
Which unit is the smallest?

32. Geologic Time How do geologists determine how old rocks are?
How do geologists determine how old rocks are?
1. Relative dating -- determine whether the rock is older or younger than other rocks
2. Absolute dating -- use radiometric dating techniques to determine how long ago the rock formed in the exact number of years
*Not all rocks can be dated absolutely, so combinations of techniques are used.

33. Relative Age Dating

34. The Law of Superposition
In any undisturbed sequence of strata, the oldest layer is at the bottom of the sequence, and the youngest layer is at the top of the sequence.

35. The Cross-Cutting Law
Any feature that cuts across a body of sediment or rock is younger than the body of sediment or rock that it cuts across.
NOTE:
• A fracture is a crack in rock.
• A fault is a fracture along which movement has occurred.

36. Law of Inclusion
If one rock body contains fragments of another rock body it must be younger than the fragments of rock it contains. OR…The inclusions are older than the rocks which contain them.

37. 2 A D C B E
Superposition/Cross-Cutting

38. 3 A B D E C
Superposition/Cross Cutting

39. 4 H H G G F C E F E D M B A C D C A B M

40. Numerical Age Dating
Numerical dating - puts absolute values (e.g., millions of years) on the ages of rocks and geologic time periods
Radioactive isotopes decay in predictable manner, depending on the half-life (time it takes for a given amount of radioactive isotope to be reduced by half)

41. Correlation
Correlation - determining the time-equivalency of rock units
Within a region, a continent, between continents
Physical continuity
Similarity of rock types
Correlation by fossils

42. End of Chapter 8