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.
Geologic Time Scale
Rock ages, both absolute and relative, are useful because the rocks represent events in Earth's history such as the age of fossils or major geologic events like meteors and volcanic eruptions. And when we put both absolute and relative time together, we create a geologic time scale that puts all these events in perspective. On this scale, periods, eons, epochs, etc. are layered in their relative order from oldest at the bottom to most recent at the top, and we also see how much absolute time each one spans.
The geologic time scale of Earth combines both absolute and relative time.
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For example, if we look at the scale, we see that the Paleozoic Era comes before the Mesozoic Era, relatively speaking, and that it spans about 290 million years, starting about 542 million years ago and ending about 251 million years ago.
Lesson Summary
Both relative and absolute time are important ways we describe events in Earth's history. Absolute time, also called chronometric time, gives us distinct measurements and points of reference, such as 65 million years ago or 5 pm. We get these rock dates and ages with radiometric dating techniques that tell us how old rocks are based on their rate of decay.
Relative time, also called chronostratic time tells us when events occurred relative to each other. For example, a rock layer that is below another one is older, and we know this even without knowing how old the rocks are simply by their position relative to each other. Just like we know that dinner comes after lunch even though we may not know what time it occurs.
When we put both absolute and relative time together, we create a geologic time scale. Rocks relate to events in Earth's history, and we can use them to put together a timeline that shows us both the order of events as well as when and for how long they occurred.
