Dating old rocks
Carbon-14 has a half life of 5730 years, meaning that 5730 years after an organism dies, half of its carbon-14 atoms have decayed to nitrogen atoms.Similarly, 11460 years after an organism dies, only one quarter of its original carbon-14 atoms are still around.Geologists measure the abundance of these radioisotopes instead to date rocks.
Principle of cross-cutting relations: Any geologic feature is younger than anything else that it cuts across.
Once the organism dies, it stops replenishing its carbon supply, and the total carbon-14 content in the organism slowly disappears.
Scientists can determine how long ago an organism died by measuring how much carbon-14 is left relative to the carbon-12.
Some very straightforward principles are used to determine the age of fossils.
Students should be able to understand the principles and have that as a background so that age determinations by paleontologists and geologists don't seem like black magic. Geologists in the late 18th and early 19th century studied rock layers and the fossils in them to determine relative age.