1. Dating Rocks
Ch. 29

2. I. Relative Dating

3. Think About It . . . How old is Earth?
What are some tools or methods that scientists could use to figure out the age of Earth?

4. Relative Age of Rocks
Relative Dating: Order rocks deposited tells you relative age of rocks (order from old to young)

5. Geologic Cross-Section Maps: show the structure of rocks underground.

6. Determining Relative Age
Can determine the relative age of rocks by using 5 geologic principles:

7. 1. Law of Superposition:
In sedimentary rock, the oldest layer is on bottom & youngest is on top.

8. Canyonlands, Utah

9. 2. Law of Original Horizontality:
Sediments deposited in flat, horizontal layers. If rock layers are folded or at an angle, they were disturbed AFTER originally deposited

10. Raplee Monocline, Utah

11. 3. Unconformity:
Surface where a break in deposition of sediments occurred or period of erosion.

12. Eastern Grand Canyon

13. 4. Law of Cross Cutting Relationships:
Fault lines or igneous rock intrusions are younger than the rock it cuts across.

14. Hance Rapids, Grand Canyon

15. 5. Principle of Inclusion:
Rock material trapped inside igneous intrusion or sedimentary rock layer is older than rock that surrounds it.

16. Lake Pleasant

17. But What If You Want to Know the Exact Age?
Geologists often also need to know the exact age of a rock or fossil.
Finding the exact age of an object is called absolute dating.
Remember “absolute” means exact or definite. O

18. II. Absolute Dating

19. Write the following:
Absolute Dating - Identifies the actual age of an object or the date of an event.

20. Radiometric dating – a method of dating geological or archeological specimens by determining the relative proportions of particular radioactive isotopes present in a sample.

21. Half-life – the time that it takes for half of the atoms to decay to a stable end product.

22. Isotope - two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, radioactive form of an element.

23. Stop writing and pay attention! 

24. Some isotopes used for dating:

25. How does this work?

26. Atoms Everything is made of atoms.
The defining characteristic of an atom is the number of protons.
The number of neutrons can vary from one carbon atom to another or from one hydrogen atom to another. These are called isotopes.
Over time, some of these atoms may change into a different type of atom.
The original atom is called the “parent.”
The new form of the atom is called the “daughter.”

27. Parents and Daughters
Half of the parent atoms turn into daughter atoms once the object has reached a certain age.
Like how every year on your b-day you turn a different age.

28. Parents and Daughters
Scientists can learn the age of a rock by counting the number of parent and daughter atoms.

29. Half-Life
A half-life is the time needed for one-half of the parent atoms to turn into daughter atoms.
Different atoms have different half-lives.
Some have half-lives are more than 4 billion years; others have half-lives of only 6,000 years.
Scientists figure out which atom would be best to use to determine the exact age of a rock.

30. Radiometric dating
Radiometric dating uses the half-life of atoms to figure out the age of the rock layers the atoms are in.
Determines Absolute Age

31. Age of Earth
Scientists have used absolute dating to figure out the age of the earth.
By counting the parent and daughter atoms in some very old rocks, scientists say that the solar system (including Earth) is about 4.6 billion years old!
(4,600,000,000 years old!)
That’s a lot of birthdays!
Before this method was invented, people believed Earth was only 6,000 yrs old.

32. Parent: 16  8  4  2 Daughter: 0  8  12  14
In this example, each time this happened, 10,000 years had gone by.
So if it happened 3 times, then the object is:
3 x 10,000 = 30,000 yrs old

33. Do you know the difference?
What method is used to figure out if a rock is older or younger than other rocks?
RELATIVE DATING
2) What methods has helped scientists determine the exact age of Earth?
ABSOLUTE DATING