How can we know how old fossils are?

[Rhondazvous]

Rifht. It's used for dating the fossils (organic marerial) in rock? The next question is what is meant by decay? I'd think this is different from the physical wear and tear of water on rocks. I think I'll leave t hat question to those who know more than I do.

Decay, in this context, means that the nuclei of some of the atoms are unstable and will spontaneously change into another type of atom while emitting an alpha particle (two neutrons and two protons) or an electron.

This spontaneous change happens at a regular rate that is determined by the characteristics of the nucleus.

But all nuclei are surrounded by a cloud of electrons that, at ordinary temperatures (say, below that of the surface of the sun) shield the nucleus from outside influences. This is helped by the fact that the nucleus is *very* small. If an atom were as big across as a football field, the nucleus would be smaller than a tenth of an inch across. So those electrons are a pretty good shield.

Physical wear and tear removes whole atoms but leaves the nuclei the same. Similarly, chemical reactions affect the electrons but not the nucleus.

That is why radioactive dating can be done: to influence the rate of decay of the nucleus takes very high temperatures of other conditions that will kill off any living thing. Such conditions haven't existed on the Earth since it was formed (yes, they would have left traces if they had occurred).

So, to date a rock requires knowing the radioactive nuclei that were in the rock originally (this can be determined by the crystal structure) and what sort of atoms the change is into (the daughter nuclei). it is then, basically a count of how many daughter nuclei there are and how many 'parent' nuclei have decayed. Since the rate of decay is known (it can be measured today), the age is then straightforward to determine.

Are there technical issues to resolve? Of course. For example, carbon-14 has a relatively fast rate of decay, so it can only be used for fairly recent things (the last 50,000 years or so---which can be extended to 100,000 years with a loss of accuracy). So no dinosaur bones will be dated by C14 dating: all you will pick up is the background amount of C14 and not that from the dinos. Also, things that live in the deep sea tend to have already decayed carbon around, so cannot be dated by this method.

As another example, in Potassium-Argon dating, the daughter nucleus is a gas (argon). If the rock is too porous, this gas can leak out and give an incorrectly *young* age for the rock.

I was beginning to understand until you mentioned daughter nuclei. That sounds like mitosis. but how do non biotic atoms have daughter nuclei?