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The following summaries offer a quick introduction to some of the dating techniques researchers have been using to explore and reconstruct our planet's past, from 4.5 billion years ago to the present.Kate Wong RADIOACTIVE ISOTOPES The premise behind techniques involving the use of radioactive isotopes is straightforward.Still others, like amino acid racemization, show promise but have not yet taken wing.Now, nearly 100 years after Boltwood's groundbreaking work, it is estimated that Earth formed at least twice as long ago as he had claimed.The decay of argon 40 to argon 39, for instance, played a vital role in underscoring the significance of two ancient human skulls unearthed in the Republic of Georgia last summer. Swisher III of the Berkeley Geochronology Center and his colleagues reported, are more than 1.7 million years old, and as such represent the first humans to leave Africa to colonize the rest of the world.Argon dating can also be used to date materials as young as 10,000 years and as old as billions of years.
Although visual inspection of the rocks, fossils and archaeological remains used to reconstruct our planet's past provides critical information, only by ascertaining their ages can researchers put this data into a meaningful context.As a result, different isotopes are better suited to dating different items.In the case of carbon 14, for example, the half life is only 5,730 years.Many crystals, including diamond, quartz and feldspar, accumulate and trap electric charges at a known rate over time.Heating the crystals, it turns out, liberates these electrons, emitting a measurable amount of light.
Uranium and lead isotopes take us back farther still.