Geologic dating principles

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.

Long before geologists tried to quantify the age of the Earth they developed techniques to determine which geologic events preceded another, what are termed "relative age” relationships.

These techniques were first articulated by Nicolas Steno, a Dane living in the Medici court of Italy in the 17th C.

Chart of a few different isotope half lifes: In reality, geologists tend to mix and match relative and absolute age dates to piece together a geologic history.

If a rock has been partially melted, or otherwise metamorphosed, that causes complications for radiometric (absolute) age dating as well.

It's a measure of time that is based on a fundamental physical process: the Earth orbiting the Sun.

That can lead to the fact that there is a basic physical process behind radioactive decay as well.

Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.It’s based either on fossils which are recognized to represent a particular interval of time, or on radioactive decay of specific isotopes. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times.After all, a dinosaur wouldn’t be caught dead next to a trilobite.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.

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