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Measuring the uranium-to-lead ratios in the oldest rocks on Earth gave scientists an estimated age of the planet of 4.6 billion years.Segment from A Science Odyssey: "Origins."Geologists have calculated the age of Earth at 4.6 billion years.

Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.Uranium is present in many different rocks and minerals, usually in the form of uranium-238.Before then, the Bible had provided the only estimate for the age of the world: about 6,000 years, with Genesis as the history book.Hutton's theories were short on evidence at first, but by 1830 most scientists concurred that Noah's ark was more allegory than reality as they documented geological layering.Using fossils as guides, they began to piece together a crude history of Earth, but it was an imperfect history.

After all, the ever-changing Earth rarely left a complete geological record.

This problem is now reduced by the careful collection of samples, rigorous crosschecking and the use of newer techniques that can date minute samples.

Volcanic rocks – such as tuff and basalt – can be used in dating because they are formed at a particular moment in time, during an eruption.

Different methods have their own limitations, especially with regard to the age range they can measure and the substances they can date.

A common problem with any dating method is that a sample may be contaminated with older or younger material and give a false age.

Using relative dating the fossil is compared to something for which an age is already known.