Relative dating of rocks and events
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.Geologists establish the age of rocks in two ways: numerical dating and relative dating.Numerical dating determines the actual ages of rocks through the study of radioactive decay.Let's say we find out, through numerical dating, that the rock layer shown above is 70 million years old.We're not so sure about the next layer down, but the one below it is 100 million years old. Not exactly, but we do know that it's somewhere between 70 and 100 million years old.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.
In order to establish relative dates, geologists must make an initial assumption about the way rock strata are formed. sediments, which are deposited and compacted in one place over time.
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.
We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!
Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon.