[Source: Carl Zimmer, National Geographic, September 2001] Radioactive isotopes decay at a predictable rate that can be measured.
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The radioactive decay rates are measured with a spectrometer.
Sophisticated mass spectrometers can accurately measure the radioactive decay rates of several kinds of isotopes.
It dates a mineral by measuring the ratio of amount of potassium and argon produced by the decay of potassium.
Carbon is useful in measuring minerals, shell, bone, wood, teeth and charcoal between 0 and 40,000 years old.
Once a hominid fossil has been discovered it can be dated by two main ways: 1) by analyzing the volcanic ash around the fossils; 2) by analyzing the other fossils found around the newly discovered hominid bones. If the ash is older than 1.6 million it can be dated with the potassium-argon method, which dates items by measuring chemical decay.
Each volcanic eruption has a unique "fingerprint" and sometimes they scatter ash over a vast area.[Source: Time magazine, December 18, 2007] Sometimes geologists can help date objects by measuring the amount of rain wear in cracks, alluvial deposits by streams and glaciers and layers of silt in lakes.Lichenometry is useful in measuring lichens from 100 and 9,000 years old.After another 5,730 years half of the remaining amount of carbon 14 decays to by half again so that only one quarter of the original amount is left, and so on.Stable atoms remain stable over time while isotopes decay.In analyzing the other fossils found around the newly discovered hominid bones.