# Relative vs. Absolute Time in Geology

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• 0:40 Absolute Time
• 1:29 Relative Time
• 2:17 Geologic Time Scale
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

You may not associate geology with time, but the former greatly depends on the latter. In this lesson we'll discuss both absolute and relative time, and how they work together to give us a detailed history of Earth.

How much of your life do you spend thinking about time? For geologists, the answer is a lot! Time comes in different forms in geology, mainly absolute and relative. They are both important in terms of Earth's history and its geological timeline, and they work together in concert to build the planet's geological record. In this lesson, we're going to discuss what each type of time is and why it is important so that you too can understand how they work to describe past events on Earth. Ready to get started?

## Absolute Time

Let's start with absolute time, also called chronometric time ('chrono' means 'time' and 'metric' means 'measure'). You can think of this type of time as how we normally view it on a day-to-day basis: specific intervals or moments measured in days, months, years, etc. For example, 60 million years is a measure of absolute time. So is 12:00 pm. These are numerical representations of time, and they give us specific points of reference.

We have a long record of events in absolute time but much of that occurred before humans were on Earth to write it down. So instead of human records, geologists use techniques such as radiometric dating. These processes involve sampling rocks and determining how old they are from their rate of decay.

## Relative Time

Since absolute time gives us points of reference, it helps calibrate relative time, also called chronostratic time ('strata' means 'layers'). Here, we are looking at events relative to other events. For example, instead of 12:00 pm we might say 'lunch time.' There is no definitive time for lunch, except that it occurs between breakfast and dinner. So relative to the other meals, it falls in the middle and is later than breakfast yet earlier than dinner.

This is useful in geology because you can age layers of rock relative to other layers. But instead of saying that one layer is x number of years old, you can simply describe it as older than the layer above it yet younger than the layer below it. Relative age starts from the bottom and works upward.

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