How do we know the age of the earth?

Factual Questions
1

Hi guys,
I know we use some kind of radiometric dating, but what isotope of what material? Links would be great. I hate to seem lazy, but everytime I look for this kind of info on the net I get a bunch of creationist sites and very little information I can actually use.

2

IANAGeologist
IANAArchiologist
IANASpecialist
ICVWBTAOMA

But I’ll try…

Most rocks contain minerals and many minerals contain trace amounts of radioactive material. It decays so slowly it’s completely harmless though.

U-235 (Uranium) decays into Pb-207 (lead) with a half life of 73 million years. Rb-87 (evil rubidium) decays into Rb-85
(goody two shoes rubidium) at a half life of 48.8 BILLION years. These are two of the most commons elements which geologists use to date rocks.

They say the Earth is 4.6 billion years old. I have no idea how they came up with that number though. The earliest rock they’ve found, IIRC, is around 3.5 billion years old. The earliest rock they could ever hope to find is after the earth cooled down and there WERE rocks to find. So how do they determine how long it took the earth to be created and then cool down? Beats me.

3

Simple. Look in the Bible. You’ll find that the Earth was Created on the Fourth of July, 4004 BC. :stuck_out_tongue: :wink:

4

Cut it open and count the rings?

5

I believe the 4.6 billion year figure comes from rock found on the moon. I understand the basic nature of radiometric dating, I just need specifics as too how we know the initial content of a certain isotope in the material.

6

There is some discussion of this in the Talk.Origins archive. The short answer is radiometric dating of lead isotopes found in meteorites. They also debunk some of the creationist dating techniques from those sites you’re finding.

7

No need to cut it open, just use a boring machine.

8

I believe that the primary one used for the Earth is U-238, with a half-life of around four and a half gigayears. This is convenient, because you’ll get the most precise results from isotopic dating if you use an element with half-life close to the age of the thing you’re measuring. I seem to recall that an isotope of thorium is used, also, but I can’t remember which one or how long.

I should also note that anything with a half-life of 48.8 billion years isn’t going to be particularly evil. Longer half-lives means less radioactivity.