Radioactive isotopes in dating the past

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These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing.

As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils.

The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks.

Just as when they were deposited, the strata are mostly horizontal (principle of original horizontality).

Thus, any deformations of strata (Figures 2 and 3) must have occurred after the rock was deposited.

Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited.

This is the principle of cross-cutting relationships.

The principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it (Figures 1 and 2).

Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top.

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