How does potassium 40 dating work
Dating is done by reheating the rock and measuring the escaping gas.
The date received from this test is for the last time that the object was heated.
I understand how radioactive dating works, but something about it concerns me. If we have a rock and assume that it was 100% carbon-14 at formation, and we now measure it to be 25% carbon-14 and 75% nitrogen-14 (I know nitrogen is a gas, but bear with me), then we can calculate that the rock has been around long enough to pass through 2 half-lives (2 x 5,730 years = 11,460 years). If, in fact, the rock was 50% carbon-14 and 50% nitrogen-14 at its formation, then it would actually be only 5,730 years old (only half the originally calculated age).
This measurement seems to hinge on the fact that we know that the rock was originally 100% carbon-14.
Common dates tested are the firing of ceramics (archaeology), and the setting of rocks (geology).
Thermoluminesence testing also dates items to the last time they were heated.
In the case of living organisms, there is a constant ratio of Carbon-14 to Carbon-12 in the environement, and this is because carbon-14 is constantly created anew by cosmic radiation impacting on nitrogen-14 (I can imagine there might be variations to take into account as historic cosmic ratiation levels would vary).
After yet another 5,730 years only one-eighth will be left.
About 40,000 years ago is the oldest the technique is reliable.
Radiocarbon is also less useful for historic sites or recent sites.
Other radiometric dating techniques are available for earlier periods.
One of the most widely used is potassium-argon dating (K-Ar dating).