Geologic Time

Relative Age

Absolute Age

Geochronology (absolute age)

Radiometric Dating

• radioactive decay
  
  - example-alpha decay
  

---

---

- atomic mass
- atomic number
- isotope
- - decay constant
- half-life

---

Figure 2.7, p.31

---

• Selecting a dating method
  
  - chemical composition
  - closed system
  - half life
  

• Rubidium-Strontium
  - 87 Rb - 86 Sr 1/2 life 48,800 myrs.
  - Rb found in rocks that contain potassium (K) minerals (mica & potassium feldspar).
  - Used to date igneous and metamorphic rocks
  

• Uranium-Lead
  
  - 235 U - 207 Pb 1/2 life 704 myrs.
  - 238 U - 206 Pb 1/2 life 4468 myrs.
  - 207 Pb / 206 Pb
  - Concordant date
  - Uranium is concentrated in zircon which is found in granite.
  

• Potassium-Argon
  
  - 40 K - 40 Ar 1/2 life 1251 myrs.
  - Used to date minerals that contain potassium (mica, potassium feldspar).
  - Glauconite-sedimentary rocks
  - Argon lost at >125 C, can be used to date metamorphic events
  

• Carbon-14 method
  
  - 14 C produced by cosmic rays
  - 14 C - 14 N 1/2 life 5730 years
  - Used to date organic material < 50,000 years
  

Age of the Earth

• oldest material 4.1- 4.2 byrs.
  
  - zircon in W. Australia sandstone
  
• meteorites 4.6 byrs.
  
• moon 3.3-4.6 byrs
  

---

Next Lecture

Back to Lecture Presentations Main Page

Back to Historical Geology Main Page

last updated 2-23-01

All original material copyright (c) 1998 by Annabelle Foos. All rights reserved.