1. Relative Age and the Laws of Geologic History
Title
GeoSleuth

2. Murder Mystery
M. d’Alessio, 2004

3. What we see in the present is a key to what happened in the past.
LLaw
Law of Uniformitarianism
What we see in the present is a key to what happened in the past.

4. PRESENT
ANCIENT
“The present is the key to the past.” James Hutton Famous Geologist (lived )
fundamentals/10c.html
PROCESS
Death Valley, CA
Copyright Marli Miller
AGI Image Bank photo ha45z2
Zion, Utah
Copyright Marli Miller
AGI Image Bank photo ha45jb
Wind and Process

5. Law of Superposition
In a horizontal layer of rock, the youngest layer is on top and the oldest layer is on the bottom.

6. Law of Superposition

7. Law of Original Horizontality
Rock layers have been laid down horizontally.
Law of Original Horizontality

8. Originally Horizontal
OLDER
YOUNGER
Timing
Grand Canyon, Arizona, Copyright Larry Fellows
Layers on top of layers

9. Watching Layers Form Larry Ridenhour, BLM/Jennifer Loomis, TERC
Watching Layers Form

10. Tilted Layers

11. USGS/Jennifer Loomis, TERCTilted limestone beds in the Mojave Desert, California
Watching Layers Tilt

12. Unconformity
An unconformity is a break in the fossil record. Layers of rock are missing due to erosion.

13. Unconformity Unconformity
Olympic Coast, Washington: 4th Beach near Kalaloch
Copyright Patti Bleifuss
Unconformity

14. Fold Fold Marin Headlands, California -- Image from Roland Bürgmann
Fold

15. USGS/Tom Grace, TERCFolded layers in the Sangre de Cristo Range of Colorado.
Watching Layers Fold

16. Law of Cross Cutting Relationships
Rock layers that cut through other layers of rock are younger than the rock layers they cut through.

17. Cross Cutting Cross Cutting MOST RECENT
Grand Canyon, Arizona -- Copyright Ramón Arrowsmith

18. Faults A fault is a break in a rock layer.
A fault is always younger than the rock layer it breaks through.

19. Fault

20. Intrusion
An intrusion is a layer of igneous rock that has melted through a rock layer.
Intrusion

21. Layer A
Layer B
Layer C
Layer D
Layer E
An intrusion is always younger than the rock that it cuts through. Intrusion H is younger than rock layers B, C, D and E. So is intrusion D. l

22. extrusion- a hardened layer of lava that has cut through a layer of rock.
An extrusion is always younger than the rock it cuts through.

23. TIMING: RELATIVE vs. ABSOLUTE
OLDER
YOUNGER
270 Million Years Old
Here is a picture from the Grand Canyon showing different layers and an unconformity. You can tell which rocks are older because the oldest rocks are on the bottom. We call this “relative dating” because it describes the relationship between the ages and positions of different layers.
Another way geologists can tell about a sequence of events is by using radiometric dating. You’ve probably heard of carbon-14 dating, but there are all sorts of elements that radioactively decay over time. Carbon is not very abundant in most rocks, so we can’t use carbon-14 dating. Instead, most rock ages are determined using the decay of Uranium, or sometimes other elements. These radiometric clocks, when properly calibrated, tell us the age in years of a given layer of rock. Oldest layers are still on the bottom, but now we know exactly HOW old.
In the murder mystery, there are two examples of absolute dating. What are they? (Wrist watch on dead body and calendar open on desk). Geologist can read the layers of rock almost as easily as a person could read the pages of a calendar.
600+ Million Years Old
Geologic Time

24. Paleontologists date rock using relative age and absolute age.
Relative age is the age of rock compared to the rock around it.
Absolute age is the exact age of rock determined by a technique called radioactive dating.
Relative Age of Rock

25. Example of Relative Age:
I am younger than my brother.
Rock layer B is older than rock layer C.
Example of Absolute Age:
I am 11 years old.
Rock layer B is 300 million years old.

26. Index Fossil
An index fossil helps to determine the relative age of rock.
To be an index fossil, an organism:
1) must have lived for a short period of time but is now extinct and
2) must have lived in many different locations.
extinct- means an organism once lived but will never live again.
Index Fossils

27. http://www. phschool. com/webcodes10/index. cfm
fossil interactive

28. How Old is That Rock? Absolute Age

29. The age of a rock in years is called its absolute age
The age of a rock in years is called its absolute age. Geologists find absolute ages by measuring the amount of certain radioactive elements in the rock.
A common "parent-daughter" combination that geologists use is radioactive uranium and non-radioactive lead. As shown in the diagram above, uranium is trapped in a newly formed rock. As the rock ages, more and more of the uranium changes into lead.

30. Murder Mystery
M. d’Alessio, 2004

31. Which is older, layer 1 or layer 4? How do you know?
What is the irregular surface of 4 called?
What happened at point 2?
How do geologists know what happened in the past?