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5 Key Principles of Relative Dating in Geology

Melissa graduated from NC State University with a Bachelor's Degree in geology in 2015 and currently works as a geotechnical lab technician.

How Do Geologists Interpret the Geological History of an Area?

Figuring out the geologic history of an area seems like a daunting task, but there are several strategies that geologists use to figure out which rocks are older than other rocks, and what geologic processes occurred in a particular order. Geologists can numerically date certain rocks by using the radioactive decay of elements trapped in rocks or minerals to figure out their exact age. However, these radioactive isotopes aren't always present in a rock, so geologists must use context clues to build a calendar (called a geologic timescale) of when each rock layer in a formation was created. Relative dating uses a series of 5 principles (listed in the following paragraphs) that help geologists compare the ages of different layers of rock and create a geologic timescale for an area.

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Principle 1: Sediments Are Deposited in Horizontal Layers

Most sediments that you see in rock formations are deposited in horizontal layers originally, due to the effect of gravity. If the layers you see are no longer horizontal, the layers were probably affected by an event of some kind after they formed. There are a few exceptions to this rule: wind-blown sand dunes may accumulate sand on their sides after the wind has transported the sediment, and the undersea slopes of a delta will have sediment rolling downhill.

These sediments near Las Vegas were deposited horizontally and have remained that way over millions of years.

These sediments near Las Vegas were deposited horizontally and have remained that way over millions of years.

Principle 2: Units of a Younger Relative Age Are Usually on Top of Older Units

For relative dating of rock units, keep in mind that when a layer of sediment is deposited, the unit that it is covering must be older. Otherwise, there would be nothing to cover! There is a rare exception to this rule, in areas where tectonic forces were so strong that the bedding is overturned, but this can be detected by looking at folding over a larger region.

The Grand Canyon is a great place to see many different rock units showing change over time. At the bottom of the Grand Canyon, there are rock units dated to 2.5 billion years ago; these rocks came from marine sediments deposited at the base of a mountain belt. As you go up the Grand Canyon, the rock layers become younger and younger, until you get to the top, a sandstone and shale layer formed in the Mesozoic era at about 200 million years ago.

The Grand Canyon contains a very important record of ancient geological processes. It represents billions of years of sediment deposition and has fossils ranging from Precambrian algae to the wing imprints of giant dragonflies from the Paleozoic era.

The Grand Canyon contains a very important record of ancient geological processes. It represents billions of years of sediment deposition and has fossils ranging from Precambrian algae to the wing imprints of giant dragonflies from the Paleozoic era.

Principle 3: A Younger Sediment or Rock Can Contain Pieces of an Older Rock

When a rock or deposit forms, it can contain pieces, or clasts, of older rock layers. For example, say you have some granite bedrock being exposed to weathering in a fast-moving river until it breaks into pieces. Those pieces are then carried by the current downstream, where they are deposited and become part of a new layer of sedimentary rock. Those pieces of granite could not exist in the sedimentary rock without the granite existing first. The presence of clasts in an older rock inside of a younger rock still shows their relative ages, even if you cannot see where the two units make contact.

The conglomerate in this core sample has older clasts surrounded by a younger matrix of silt, sand, and clay.

The conglomerate in this core sample has older clasts surrounded by a younger matrix of silt, sand, and clay.

Principle 4: Younger Rocks or Features Can Cut Across Older Ones

Rocks can be cut across by other features, but the rocks had to be present already in order to be altered. For example, the San Andreas Fault is younger than the rocks it cuts through, and a hydrothermal vein carrying mineral deposits and searing its way through a layer of limestone must be younger than the limestone.

The striking veins of gold through the rock below were created when a hot aqueous solution carrying various elements flowed through fissures in the rock and deposited gold onto the sides of the fissures as it went. These veins of gold are therefore younger than the surrounding quartz.

The quartz had to be in place in order for the gold to be deposited into it.

The quartz had to be in place in order for the gold to be deposited into it.

Principle 5: Younger Rocks May Cause Changes When They Contact Older Rocks

Magma can contact preexisting rocks when it erupts onto the surface of the Earth or solidifies at depth. When the magma touches the preexisting rocks, it can bake the adjacent rock with its heat or chemically change nearby rocks through the migration of fluids from the magma. Looking at these signs will tell you that the magma is younger than the rock it altered.

The purplish areas on this rock from the New Jersey Palisades are areas of contact metamorphism. Compared to the rock around them, they are more crumbly due to their exposure to intense 1000 degree heat.

The purplish areas on this rock from the New Jersey Palisades are areas of contact metamorphism. Compared to the rock around them, they are more crumbly due to their exposure to intense 1000 degree heat.

Question: Which order did these rock units form in?

Now that you have these relative dating principles in mind, can you figure out the order in which these rock units formed? Answer this question in the comments!

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© 2019 Melissa Clason