Absolute and relative dating in geology
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 can you make any conclusions about rock layers that make such a crazy arrangement?
Numerical dating determines the actual ages of rocks through the study of radioactive decay.Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact.We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock.What could a geologist say about that section of rock?Following the Principle of Original Horizontality, he could say that whatever forces caused the deformation, like an earthquake, must have occurred after the formation of all the rock strata.As a member, you'll also get unlimited access to over 70,000 lessons in math, English, science, history, and more.
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Geologists find the cross-cutting principle especially useful for establishing the relative ages of faults and igneous intrusions in sedimentary rocks.
Sometimes, geologists find strange things inside the strata, like chunks of metamorphic or igneous rock.
Let's say, in this set of rock strata, that we found a single intrusion of igneous rock punching through the sedimentary layers.
We could assume that this igneous intrusion must have happened after the formation of the strata.
How do we use the Law of Superposition to establish relative dates?