Unlike people, you can’t really guess the age of a rock from looking at it.
Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.
What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.
This process lead to a system of time containing eons, eras, periods, and epochs all determined by their position in the rock record.
For example, rocks of the Phanerozoic eon are found on top of rocks from the Proterozoic eons therefore rocks of the Phanerozoic are younger than rocks of the Proterozoic.
The narrower a range of time that an animal lived, the better it is as an index of a specific time.
No bones about it, fossils are important age markers.
The most obvious feature of sedimentary rock is its layering.
This feature is produced by changes in deposition over time.That’s because zircon is super tough – it resists weathering. Each radioactive isotope works best for particular applications.The half-life of carbon 14, for example, is 5,730 years.Pretty obvious that the dike came after the rocks it cuts through, right?With absolute age dating, you get a real age in actual years.With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old?