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Potassium-40 on the other hand has a half like of 1.25 billion years and is common in rocks and minerals.

This makes it ideal for dating much older rocks and fossils.

Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils.

If the fossil you are trying to date occurs alongside one of these index fossils, then the fossil you are dating must fall into the age range of the index fossil. In a hypothetical example, a rock formation contains fossils of a type of brachiopod known to occur between 410 and 420 million years.

Studying the layers of rock or strata can also be useful. If a layer of rock containing the fossil is higher up in the sequence that another layer, you know that layer must be younger in age. This can often be complicated by the fact that geological forces can cause faulting and tilting of rocks.

Absolute dating is used to determine a precise age of a rock or fossil through radiometric dating methods.

So, often layers of volcanic rocks above and below the layers containing fossils can be dated to provide a date range for the fossil containing rocks.

The atoms in some chemical elements have different forms, called isotopes.

Scientists have used different methods to determine how the Isua Greenstone Belt formed.

Carbon-14, the radioactive isotope of carbon used in carbon dating has a half-life of 5730 years, so it decays too fast.

It can only be used to date fossils younger than about 75,000 years.

The same rock formation also contains a type of trilobite that was known to live 415 to 425 million years ago.

Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of 415 to 420 million years.

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