| |  | Last lecture we looked at how the rock record was subdivided into the relative time units of the Geologic Time scale. | 
 |  | Today we will look at how absolute dates were added to the Geologic Time scale | 
 |  | There has been a long debate over the age of the Earth. | 
 |  | The age of the earth is key to evalutating geologic and biologic processes. | 
 |  | Uniformitarianism and Evolution require a large age of the Earth. | 
 | Measuring the age of the Earth | 
 |  | extrapolating age by using generations listed in Genesis | 
 |  | Bishop Jame Ussher (1665) | 
 |  | El Moro Nation Monument, New Mexico | 
 |  | Canyon De Chelly, Arizona | 
 |  | Salt concentration in the oceans | 
 |  | John Joly (1899), 90 million years | 
 |  | John Phillips (1860), 96 Million years | 
 |  | C.D Walcott (1893), 27.6 Million years | 
 |  | Lord Kelvin (1862), 20 to 40 million years | 
 |  | Radioactive Dating Methods | 
 |  | Radioactivity discovered in 1890s | 
 |  | Radioactive elements decay to non-radioactive forms at a constant rate | 
 |  | Use uniformitarianism (present is key to past) | 
 |  | Measure ratio of parent to daughter | 
 |  | Half-lifes and Applicable Range | 
 |  | 
	
		| Parent | Daughter | Half-life | Applicable Range |  
		| Rubidium-87 | Strontium | 47,000 million years | > 10 million years |  
		| Thorium-232 | Lead-208 | 13,900 million years | > 5 million years |  
		| Uranium-238 | Lead-206 | 4,500 million years | > 1 million years |  
		| Uranium-235 | Lead-207 | 713 million years | > 1 million years |  
		| Potassium | Argon | 1,300 million years | >50,000 years |  
		| Carbon-14 | Nitrogen-14 | 5,568 years | < 50,000 years |  | 
 |  | Oldest Earth rocks are 3.8 billion years old | 
 |  | Meteorites on earth are generally around 4.6 billion years old | 
 |  | Moon rocks are between 4.6 and 3 billion years old | 
 |  | Age of the Earth is thought to be 4.6 billion years old | 
 
						 |