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In this lesson, you'll learn how scientists determine the absolute age of materials.Scientists who study the ancient Earth have been working for hundreds of years to build an accurate timeline of the formation of the planet and the evolution of all life. In order to build and improve this timeline, scientists must have several types of accurate methods they can use to determine the ages of materials.

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.

For example, which is older, the bricks in a building or the building itself?

Are there repairs or cracks in the sidewalk that came after the sidewalk was built?

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.On the other hand, the half-life of the isotope potassium 40 as it decays to argon is 1.26 billion years.So carbon 14 is used to date materials that aren’t that old geologically, say in the tens of thousands of years, while potassium-argon dating can be used to determine the ages of much older materials, in the millions and billions year range.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.To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.