1. Geologic Time
Chapter 9

2. Relative Dating
Determining the age of a rock in relation to one another.
Does not give exact age
Use a collection of strategies to determine relative age of a rock

3. Relative Ages of Events
Original Horizontality - Most sediments are deposited in horizontal layers.
If they are not horizontal some event must have affected the layers after they formed.

4. Law of Superposition
When a layer is deposited, any rock unit that it rests on must be older than that layer.
Ex- Stack of papers, the oldest paper is the one on the bottom.

5. Younger Units Deposited on Older Units
Tan sediment deposited over older rock
Red layers deposited over tan
NOTES
Can use analogies, like yesterdays mail is below today's in the stack on your desk
Third layer is youngest and is on top
09.01.a

6. Law of Embedded Fragments
As a new rock forms, older fragments of rock can be embedded into it.
So…Embedded fragments are OLDER than the rock in which they are embedded.
Ex: Chocolate chip cookie

7. Younger Sediment or Rock Can Contain Pieces of Older Rock
Breccia
Determine which rock is younger in each image
Gray layers
Gray granite
Conglomerate
INSTRUCTIONS TO STUDENTS
Determine which rock is younger in each image
EXPLANATION
First photograph: tilted quartzite beds at bottom weathered into clasts, which were incorporated into overlying breccia/conglomerate
Second photograph: lower dark basalt contributed clasts to overlying tan, volcanic-ash-rich conglomerate
Third photograph: light-colored granite contains dark-colored pieces of older metamorphic rocks that fell into granitic magma
Dark basalt
Dark metamorphic rocks
09.01.a

8. Igneous Intrusions There are several types of igneous intrusions.
Sill- horizontal intrusion
Dike- vertical intrusion
Batholith-large scale intrusions

9. Law of Crosscutting
When igneous intrusions or faults cut through a rock layer.
The intrusion is younger than the rock that it cuts through.
Ex. Toothpick in a sandwich

10. Younger Rock or Feature Can Crosscut an Older Rock or Feature
Limestone
Determine which rock or feature is younger in each image
Fractures
Tan dikes
INSTRUCTIONS TO STUDENTS
Determine which rock or feature is younger in each image
EXPLANATION
First photograph: fractures cut across (crosscut) the limestone layers (Little Colorado River, AZ)
Second photograph: Dark dike crosscut red sedimentary rocks (Grand Canyon, AZ)
Third photograph: light-colored dikes of granite cross cut darker igneous rocks
NOTES
First photograph: Little Colorado River, AZ
Second photograph: Grand Canyon, AZ
Third photograph: Mt. Lemon, southern Arizona
Red layers
Dark igneous rock
Dark dike
09.01.a

11. Unconformity A buried erosion surface over a layered surface.
You cannot account for the missing layers.
General term for missing layers.

12. What Does an Unconformity Represent?
Limestone folded and eroded
Gray limestone deposited under water
Conglomerate deposited on top of eroded surface forming an unconformity
Unconformity
09.04.a

13. Nonconformity 1st type of an unconformity.
Occur over rocks that are not layered. Sedimentary over ig. or meta

14. Disconformity 2nd type of unconformity
When an unconformity occurs over horizontal sedimentary layers and new layers form on top.

15. Absolute Dating
Relative dating does not tell you the actual age of the rock.
Absolute dating uses radioactive decay to determine the actual age of the rock.

16. Historical Methods
Tree rings- each ring usually represents a single year.
Size and color depend on environmental Conditions.
Different trees have similar rings for same years.
Used to date back to 2000 B.C.

17. Varves Varves- sediment that is deposited on a yearly cycle.
Each annual varve is distinctive.
Varves in one lake can be correlated to varves in other lakes.

18. Radioactive decay
Some particles are unstable and break down emitting subatomic particles.
Every radioactive element has a specific rate at which it breaks down.
That rate can determine the age of the substance.

19. Types of Decay Radioactive isotopes emit or capture tiny particles.
Three types of decay
Alpha- positive particle
Beta- negative e- emitted
Electron capture- p+ captures e- and becomes a neutron.

20. Isotopes Isotope’s atomic number changes and becomes a new element.
Parent isotope- original element
Daughter isotope- product of decay.
Ex- uranium-238 lead 206

21. Half-life
Half-life – The amount of time it takes for ½ of a sample of a radioactive isotope to decay. (1/2 of the radioactive atoms)

22. How Does Radioactive Decay Occur?
Before decay, unstable parent atoms
Half the parent atoms decayed to daughter atoms (time = half life)
After a second half life, only ¼ parent atoms remain
EXPLANATION
Starting atoms are called parent atoms or parent isotope; parent isotope will decay into different isotopes called the daughter product
At later time, half the parent atoms (green) have decayed to daughter product (purple)
Amount of time it take for half the parent atoms to decay is called a half life
After a time equal to another half life, half the remaining parent atoms have decayed to daughter product
NOTES
You may ask students to calculate how the numbers of parent and daughter atoms will change after even more half lives.
If a large enough class, have all students stand up and flip a coin; heads stays up, tails sit down; count number of students remaining and then keep repeating process; demonstrates half life, randomness of decay, and exponential nature of radioactive decay.
Before Any Decay
After One Half-Life
After Two Half-Lives
Atoms of Parent
1,000
500
250
Atoms of Daughter
750
Example for 1000 atoms

23. Radiocarbon Dating uses carbon-14 carbon-14 is radioactive
half-life is 5370 yrs
Produced naturally from reaction between N-14 and cosmic rays
Only works on organic material and only on sample that are under 70,000 years old.

24. Uranium-lead dating
Used to date very old rocks because uranium has a half-life of 4.5 billion years.
Does not work on rocks younger than 10 million years old.

25. Rubidium-Strontium Half-life of rubidium is 47 billion years.
Only works with extremely old rocks.
Used mainly on igneous rocks.

26. Potassium-Argon Dating
Used to date metamorphic and sedimentary rocks as well as igneous.
Can date rocks as young as 50,000 years old.

27. Fossils
Remains, traces, or imprints of a plant or animal that are preserved in a rock.
Occurs several ways.

28. Original Remains The original unchanged remains of plants or animals.
Rare!
Example: Woolly mammoths found frozen in permafrost
Example: prehistoric insects trapped in resin (a sticky sap that oozes from trees).
Resin hardens into amber

29. Replaced Remains
Remains are slowly replaced, molecule by molecule, by rock-forming minerals.
Occurs in bones, teeth and shells.
Circulating groundwater removes the original organic material and replaces them with minerals from the water.
Petrified wood is an example

30. Molds and Casts
After an organism (leaf, insect) gets buried in the mud or sediments, its hard body parts become a fossil and the mud/sediment becomes rock.
When the body part dissolves out of the rock, a hollow depression results called a mold.
Minerals may seep into the mold and fill it forming a cast of the original fossil.

31. Trace Fossils
Any impression left in the rock by an animal such as trails, footprints, tracks, burrows and bite marks.

32. Carbonaceous Films
A trace of a fossil that is a thin carbon film resembling a silhouette.
The remains are affected by high temperature and pressure causing the tissues of the animals and plants to undergo chemical changes.

33. How Are Fossils Preserved?
Shells/hard parts
Bones
Replacement
EXPLANATION
Hard parts are easiest thing to preserve: like stems of crinoids
Bones are hard parts: most bones are found as fragments rather than complete skeletons
Replacement: fossil preservation when silica, pyrite, or some other material replaces original material (petrified wood)
Cast or mold: Animal leaves a cavity in the rock that mimics its shape; if cavity is unfilled it is a mold
Thin carbon film: when buried, carbon-rich plants become thin films of carbon (300-million-year-old fern)
Impressions: fish and other soft creatures preserved as impressions, especially in quiet water
Amber: tree sap can trap insects
Constructed feature: some fossils do not preserve the actual organism, but do preserve something the organism constructed, like this stromatolite
NOTES
Mold photo: Wall of Geology Building, UTEP
Cast or mold
Thin carbon film
Impressions
Amber
Constructed feature
09.05.a

34. Using Index Fossils
An index fossil is the remains of an organism that lived and died within a particular time segment of Earth’s history.
4 characteristics of an index fossil.
Unique so they are easily identified
Abundant
Found over a great distance
Organism can only have existed for a brief period.

35. Fossils as Environmental Indicators
Fossils tell you the climate. Since animals can only exist at certain temperatures, fossils tell you the climate.

36. Matching Key Beds
Key Bed – a single rock layer that is unique, easily recognizable and widespread
Ex. volcanic eruptions

37. Stratigraphic Matching
Matching layers of rock.
Ex. A layer of limestone sandwiched between conglomerate rocks.