Index fossil relative dating

Then after another 5,000 years half of the remaining parent isotope will have decayed.While people are most familiar with carbon dating, carbon dating is rarely applicable to fossils.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.Some fossils, called index fossils, are particularly useful in correlating rocks.For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.

This matching process is called correlation, which has been an important process in constructing geological timescales.The Wheeler Formation has been previously dated to approximately 507 million year old, so we know the trilobite is also about 507 million years old.But, how can we determine how old a rock formation is, if it hasn’t previously been dated?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.

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