How does Radioactivity determine the age of ancient rocks??
There’s a radioactive form of Carbon, C-14, which is very rare, but, since it occurs in the atmosphere, is present in all living, carbon-based lifeforms. The ratio of C-14 to other Carbon is constant in living things, equal to the ratio in the atmosphere. Once something dies, however, the C-14 is no longer replenished, and begins to decay at a very stable rate. As there is less and less C-14, the fossil must be older. This method cannot date rocks per se, but anything that has Carbon in it and is not so old that there is no traceable amount of C-14 left.
