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This is made up of numerous regional tree-ring chronologies, particularly in the medieval and post-medieval periods, for which the laboratory now has more than 200 reference chronologies from many areas. By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.There are over 130 radiocarbon dating laboratories around the world producing radiocarbon assays for the scientific community. A series of movie clips walks you through the process.A short but clear explanation about radioactive isotopes commonly used for determining ages of rocks (with graphics) and putting numbers on the geologic time scale, extending it back before the occurance of abundant index fossils.This is a relatively new method intended to to improve the precision of uranium and thorium istopy methods.

A specialized form of cross-dating, using animal and plant fossils, is known as biostratigraphy.

The most compelling argument for an age of the earth of 4.5 billion years are the large number of independent tests that have been used to confirm this date.

These tests have been performed on what are thought to be the earth's oldest surviving rocks, meteorites, and moon rocks.

Many of these links also appear where appropriate below.

James Hutton and William Smith advanced the concept of geologic time and strengthened the belief in an ancient world.

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