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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?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.Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.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.In a way this field, called geochronology, is some of the purest detective work earth scientists do.There are two basic approaches: relative age dating, and absolute age dating.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.
According to the Utah Geological Survey, nuclear decay of radioactive elements works like a clock in allowing scientists to determine an absolute age.
During radioactive decay, the parent isotope transforms into a stable daughter isotope.
What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.
You might have noticed that many of the oldest age dates come from a mineral called zircon.