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Version: Earth

Geologic Age

Age of the Earth

Information you need to complete this is in the Introduction

It may be helpful to complete the relative age and radiometric dating version before doing this part.


The Age of the Earth

The determination of the Earth's age is a rather difficult puzzle, since models indicate none of the original crust should still exist. The best we can hope for is to set a lower limit on the age by finding the oldest rocks possible: if the crust is recycled or metamorphosed, Earth must be older than the oldest rock. However, we can set some other limits using the theories of the formation of the Moon and Solar system. A combination of knowing the age of rocks on Earth, the Moon and asteroids (pieces of which fall to Earth as meteorites) will allow us to set a minimum and maximum age.

Table 3.1 has data for several samples, including the collection site, the method used to date the sample, and the age yielded by that method.  In some cases, multiple people did the same or multiple tests on the same set of samples.  All results are listed, but not their errors (for more details, see Dalrymple “The Age of the Earth” 1991) Data from Dalrymple "The Age of the Earth" 1991 and the Hawai'i Center for Volcanology

 
Table 3.1: Rock Samples

#

Rock type and location

Method

Age (Gyr)

1

Qorqut granites from Godthaab, Greenland

Rb-Sr

2.53

Rb-Sr

2.52

Pb-Pb

2.58

U-Pb

2.53

2

Amitsoq gneisses from Godthaab, Greenland

U-Pb

3.60

Pb-Pb

3.56

3

Amitsoq gneisses from Isua, Greenland

Pb-Pb

3.74

Rb-Sr

3.64

4

Uivak gneisses from Saglek and Hebron, Labrador

U-Pb

3.76

Rb-Sr

3.55

Rb-Sr

3.66

Rb-Sr

3.61

Sm-Nd

3.56

5

Sacred Heart Granite from Morton, Minnesota

Pb-Pb

2.6

Rb-Sr

2.64

Rb-Sr

2.32

6

Puritan Quartz Monzonite from Watersmeet, Mich

Rb-Sr

2.65

7

Older Granitoid gneisses from Morton Minn

U-Pb

3.59

U-Pb

3.54

U-Pb

3.66

Rb-Sr

3.48

8

Older Granitoid gneisses from Watersmeet, Mi

U-Pb

3.62

9

Basalt from Mauna Kea, Hawai’i

K-Ar

375,000 years

10

Charcoal from same area as 9, Mauna Kea, Hawai’i

C-14

4500 years

11

Basalt from the 1669 eruption of Mt Etna, Sicily

K-Ar

0

12

Basalt from the 1792 eruption of Mt Etna, Sicily

K-Ar

150,000 years

13

North Star Basalt, Marble Bar, Western Australia

Rb-Sr

3.57

Sm-Nd

3.56

Sm-Nd

3.56

14

Sample 10062 Mare Tranquillitatis basalt, Apollo 11 landing site

Ar-Ar

3.78

Ar-Ar

3.79

Sm-Nd

3.88

Rb-Sr

3.92

15

Sample 76535, troctolite, southern highlands, Apollo 17

Sm-Nd

4.26

Rb-Sr

4.51

K-Ar

4.27

Ar-Ar

4.16

Ar-Ar

4.19

Ar-Ar

4.20

Ar-Ar

4.19

16

Sample 78236, norite, southern highlands, Apollo 17

Sm-Nd

4.34

Sm-Nd

4.43

Rb-Sr

4.29

Ar-Ar

4.36

17

Allende Chrondrite meteorite fragment, Pueblito de Allende, Mexico (impact Feb 8, 1969)

Ar-Ar

4.52

Ar-Ar 

4.53

Ar-Ar 

4.48

Ar-Ar 

4.55

Ar-Ar 

4.55

Ar-Ar 

4.57

 Ar-Ar

4.50

 Ar-Ar

4.56

18

Weekeroo Station iron meteorite, found 1924, Mannahill S. Australia

Rb-Sr

4.39

Ar-Ar

4.54

Using appropriate relative dating methods, determine whether the samples are useful or not useful in determining the age of the Earth. Place a check in the second column of table 3.2 if the sample will be useful.  Explain your reasoning in the third column for all the samples.

 
Table 3.2: Usefulness of Samples in Table 3.1

Sample

Useful?

Explanation

1

 

 

2

 

 

3

 

 

4

 

 

5

 

 

6

 

 

7

 

 

8

 

 

9

 

 

10

 

 

11

 

 

12

 

 

13

 

 

14

 

 

15

 

 

16

 

 

17

 

 

18

 

 

Record the numbers of the useful samples in table 3.3, below(the table is longer than necessary).  Look at the radiometric method used for the useful samples, and determine if the method(s) used is/are valid.  Calculate the average age of the sample for all the valid methods, and explain your reasoning for the ones you used.  If you exclude one of the ages but not the entire sample, be sure to include why in your explanation. You may also want to mark the samples as being from Earth, the Moon or meteorites.

Using table 3.3, determine the age of the Earth. Explain your reasoning, show your work, and give your final answer below. Be sure to include the minimum and maximum age.

Table 3.3: Average Ages of sample useful for determining the age of the Earth

Number

Average age (Gyr)

Explanation

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Last modified: 2/19/08

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