What is Relative Age? - Definition & Effect

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• 0:00 A Look at Relative Age
• 0:50 Principle of Relative Age
• 5:07 Effect of Relative Age
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Lesson Transcript
Instructor: Kimberly Schulte

Kimberly has taught at the university level for over 17 years.

How do geologists determine the age of different rock layers or fossils without the aid of modern equipment? Learn how geologists use rock layers to determine relative ages.

A Look at Relative Age

Geologic time extends back 4.6 billion years. Because the earth is so old and rocks formed millions of years ago, geologists needed a way to date rocks and rock units called strata. Today there are two common practices for dating rocks and strata. The first is called absolute dating, where geologists use radioactive decay to determine the actual age of a rock. The second is by using relative dating techniques.

Let's say you are a geologist who is tasked with dating the rocks found in the Grand Canyon, and you must do so in the canyon without the aid of any laboratory equipment. How would you do it? What can you deduce from the rocks? Geologists use what they see and some simple strategies to relative date the rock layers found in the Grand Canyon.

Principles of Relative Age

Relative dating doesn't really give us an actual 'age,' but it does put things in sequential order. This allows geologists to determine the age of a rock or strata relative to another rock or strata. So, instead of saying when something happened, it puts events in the order they happened. So, basically it's the difference between saying 'I'm 25 years old, and my sibling is 20 years old' and 'I'm older than my sibling.' Geologists use a variety of techniques, or principles, to determine the relative age of a rock unit.

Principle of Original Horizontality

Principle of original horizontality: This principle simply states that deposition of rocks occurs horizontally or nearly horizontally. Deposition means the process of adding new rock. Most deposition occurs as the ocean deposits sediment flat and horizontally on existing rock, either on the ocean floor or on the continent. Sediments can be deposited on an incline, but this doesn't happen very often. Once the rocks are deposited flat, forces can act upon them to tilt or fold them.

Law of Superposition

Law of superposition: This is one of the most basic techniques of relative dating geologists use. This principle says that the oldest rock layer is always on the bottom and layers above it get progressively younger. To understand this better, consider a four-layer cake. The oldest layer would be the bottom layer. It wouldn't be possible for the other layers to be added without the bottom layer first. This same process applies with each layer to the cake added. You can't start with the top layer and add the ones under it since there would be nothing to build on.

Principle of Lateral Continuity

Principle of later continuity: The principle of lateral continuity says that rocks continue in all lateral directions, even if you can't see them on the surface. This is because the rocks were deposited laterally in continuous flat layers and can be covered with vegetation or eroded in some places. So, when a valley or canyon cuts through these rock layers, the principle of lateral continuity says that rocks on either side of the canyon were once joined and continuous.

Principle of Cross-Cutting Relations

Principle of cross-cutting relations: The principle is another example of which came first. Instead of using layers, it involves features that cut through the rock, like a fault or a dike. In this case, the principle states that the item doing the cutting is younger than what is being cut. If we return to our example of the layered cake, the cake has to be there first before you can cut with a knife.

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