Formation of the Earth

Earth's age: ~4.6 billion years old
oldest rocks ~3.8 billion years old

Q: How do we know these ages?

The Earth's age has been revised over the years:

"The world was created in 3963 B.C."
                    - Philip Melanchthon 1497-1560
 

"Heaven and earth, centre and circumference, were created together, in the same instant, and clouds of water . . .  This work took place and man was created . . . on the 17th of September 3928 B.C. at 9 o'clock in the morning.
                    - Dr. John Lightfoot, Vice-chancellor of the University of Cambridge, 1642

"In the beginning God created Heaven and Earth, Gen. I, V. 1., which beginning of time, according to our Chronologie, fell upon the entrance of night preceding the twenty-third day of Octob. in the year of the Julian Calendar, 710 [i.e., 4004 B.C.]"
                    - Archbishop James Ussher of Armagh 1658
This isn't to say that early attempts to estimate the age of the Earth were wrong.  Early attempts didn't know many of the things that we currently understand about the Earth.

1785: James Hutton, Scottish geologist, Theory of the Earth maintained present is the key to the past (uniformitarianism)

Demanded that the earth was more than 6000 years old
Hutton said that processes that were occurring today have occurred in the past and will continue to occur in the future at roughly the same rate - hence his Principle of Uniformitarianism can be paraphrased as the present is the key to the past.
 
1899: Joly used the concentration of salt in sea water
1) early oceans were not salty
2) salt from the earth carried to the oceans in solution
3) present rate is previous rate (uniformitarianism)
age: ~100 million years old


1862-1897: Lord Kelvin (Thompson) estimated age of Earth based on heat

loss from an early molten Earth.
age: 20-40 million years old


Discovery of radioactivity (1895) invalidated Kelvin's estimate.

Age Dating

Absolute vs. relative

Absolute age dating is a method in which the actual age of something is determined.
Absolute methods include:

isotopic dating: using the natural, constant radioactive decay of isotopes.  For example, a given amount of 14C will decay by half every ~5568 years into 14N.  Therefore, by measuring how much 14N is present, we can estimate how long it took to form.


Relative age dating methods just give us a sense that one thing is older than another.
Relative age dating techniques include:

flora
fauna
Law of Superposition
Cross-cutting Relationships
Paleomagnetic

Accretion and Formation of the Earth