Stratigraphy and Radiometric Dating at John Day Fossil

Stratigraphy and radiometric dating

April teaches high school science and holds a master's degree in education. Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon. Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time. Now imagine that you come upon a formation like this: What do you think of it? How do you study it? How can you make any conclusions about rock layers that make such a crazy arrangement? Geologists establish the age of rocks in two ways:

Radiometric Dating and the Geological Time Scale

numerical dating and relative dating. Numerical dating determines the actual ages of rocks through the study of radioactive decay. Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another. Relative dating requires an extensive knowledge of stratigraphic succession, a fancy term for the way rock strata are built up and changed by geologic processes. In order to establish relative dates, geologists must make an initial assumption about the way rock strata are formed.

Of course, it only applies to sedimentary rocks. Recall that sedimentary rock is composed of. Sediments, which are deposited and compacted in one place over time. As you can imagine, regular sediments, like sand, silt, and clay, tend to accumulate over a wide area with a generally consistent thickness. Once we assume that all rock layers were originally horizontal, we can make another assumption:

Radiometric dating Learning Geology

that the oldest rock layers are furthest toward the bottom, and the youngest rock layers are closest to the top. This rule is called the Law of Superposition. Again, it's pretty obvious if you think about it. Say you have a layer of mud accumulating at the bottom of a lake. Then the lake dries up, and a forest grows in.

More sediment accumulates from the leaf litter and waste of the forest, until you have a second layer. The forest layer is younger than the mud layer, right? And, the mud layer is older than the forest layer. When scientists look at sedimentary rock strata, they essentially see a timeline stretching backwards through history. The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago.

How do we use the Law of Superposition to establish relative dates? Let's look at these rock strata here: We have five layers total. Let's say we find out, through numerical dating, that the rock layer shown above is 75 million years old. We're not so sure about the next layer down, but the one below it is 655 million years old.

Can we tell how old this middle layer is?

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